What 1-Wire net? 1-Wire® net, sometimes known MicroLAN, cost base

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What 1-Wire net? 1-Wire® net, sometimes known MicroLAN, cost base

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Tech Brief MicroLAN Design Guide
What 1-Wire net?
1-Wire® net, sometimes known MicroLAN, cost based microcontroller communicating digitally over twisted pair cable with components. network defined with open drain (wired-AND) master/slave multidrop architecture that uses resistor pull-up nominal supply master. 1-Wire based system consists three main elements: master with controlling software, wiring associated connectors 1-Wire devices. 1-Wire allows tight control because node allowed speak unless requested master, communication allowed between slaves, except through master. standard microcontroller such 8051 with 1.8MHz greater clock, well using 115.2 kbps capable UART serve master net. UART supplies 1-Wire timing sending byte each 1-Wire bit, creating short long time slots encode binary 0's. this 14.4 kbps data rate (115.2 divided =14.4 kbps) address node start receiving data less than milliseconds. Since timing controlled UART, microprocessor clock speed does affect time required find read slave net. Diagram illustrates portion typical communication sequence 1Wire protocol. Software such TMEXto control monitor activity available downloading
Reset Pulse Presence Pulses
1-Wire protocol uses conventional CMOS/TTL logic levels, where 0.8V less indicates logic zero 2.2V greater represents logic one. Operation specified over supply voltage range volts. Both master slaves configured transceivers allowing data flow either direction, only direction time. Technically speaking, data transfers half-duplex sequential over single pair wires, data return, from which slaves "steal" power internal diode capacitor. Data read written least significant first. economical, DS9097 Port Adapter available interface RS232 net. Newer more versatile adapters based DS2480 Serial 1-Wire Line Driver chip provide more capability such active pull-up slew-rate control. DS2480 designed interface between RS232 1-Wire generating proper signals programmable waveforms that provide maximum performance. Regardless whether DS9097 DS2480 based adapter used, readily available, capacitance, unshielded, Category twisted pair phone wire recommended bus. previously mentioned, data 1-Wire transferred with respect time slots. example, write logic 1-Wire device, master pulls holds microseconds less. write logic zero, master pulls holds least microseconds proNext Reset Pulse
SCALE
READ WRITE DATA
Reset Sequence 8-BIT FUNCTION Command Code (Unique device selected) 8-Bits MEMORY FUNCTION Command Code
Diagram typical 1-Wire communication sequence. 1-Wire registered trademark Dallas Semiconductor. TMEX trademark Dallas Semiconductor. 092603 1/32
Tech Brief
vide timing margin worse case conditions. system clock required, each 1-Wire part self clocked internal oscillator that synchronized falling edge master. Power chip operation derived from during idle communication periods when DATA line including half wave rectifier onboard each slave. Figure whenever data line pulled high pull-up resistor diode half wave rectifier turns charges internal 800pF capacitor. When drops below voltage capacitor, diode reverse biased, isolating charge. isolated charge stored capacitor provides energy source power slave during intervals pulled low. amount charge lost during these periods proportional time low. replenished when data line again turns half wave rectifier diode. This concept "stealing" power from data line half wave rectifier referred "parasite power."
Parasite power
typ.
technical timing details available Book DS19xx iButton® Standards. Within each 1-Wire slave created stored lasered section with guaranteed unique, 64-bit serial number that acts node address. This unique address composed eight bytes divided into three main sections. Starting with LSB, first byte stores 8-bit family code that identifies device type. next bytes store customizable 48-bit individual address while last byte (MSB) contains cyclic redundancy checksum (CRC) with value based data contained first seven bytes. This allows master determine address read without error. With serial numbers available, conflicting duplicate node addresses will never problem.
Floppy Button
1-Wire devices formatted with file directory just like floppy disk. This allows files randomly accessed changed without disturbing other records. Information read written when device connected addressed master, identification badge Decoder Ring touched port somewhere along 1-Wire net. typical access port consists outer ring insulated spring loaded center conductors mounted appropriate housing. ring touches case iButton Decoder Ring connects return line while spring loaded center contact connects data line. inclusion memory 1-Wire chips allows standard information such employee name, number etc., stored within device. example, would only take about fourth available memory store equivalent business card digitized black white photo This still leaves space additional important data such medical records, credit information security level included. With such information literally hand" case Decoder Ring, reliable identification access readily available machine readable. Memory devices also useful storing "tag" information nodes sensor calibration function information.
DATA
RETURN
Figure 1-Wire parasite power circuit.
operation, master resets network holding least microseconds, (Diagram again) releasing then looking responding Presence pulse from slave connected line. Presence pulse detected, then accesses slave calling address. master issues device specific commands required, performs needed data transfers between slave. controls information transfer generating time slots examining response from slave. more detailed description communication sequence, refer that's works!" Chapter Complete
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iButton registered trademark Dallas Semiconductor.
Tech Brief
Packaged ready
Because 1-Wire only requires single wire plus return, iButtons packaged coin style battery case 16mm diameter, 5.8mm thick. This about size stack five dimes. piece stainless steel package acts both protective housing electrical connection point. case serves return contact (ground), data contact. package size allows inclusion Lithium cell provide years standby power maintain data volatile memory when connected net. variety memory configurations available, including iButtons containing memory. 1-Wire devices that require memory backup battery also available more traditional solder-mount packages.
port adapter, there active pull down transistor that either turned fully completely under control master. falling edge generated turning signals start time slot net. When switch turned off, line pulled toward supply voltage pull-up resistor. rapid response impedance active pull down transistor used generate logic zero, signal fall time will sub-microsecond range. switching occurs less time than transition takes traverse electrical length cable return, 1-Wire operating transmission line environment reflections from line disrupt communications. Normally, solution would terminate ends cable characteristic impedance with fixed resistors. These resistors would then absorb energy that otherwise would reflected impedance mismatch cause communications problems. Unfortunately, recommended cabling typical impedance which would result inability generate logic with acceptable pull-up resistor value. interesting note that since port transistor inside 1-Wire devices on-resistance, properly terminated anytime cable turned possible some 1-Wire Nets terminate using series resistor capacitor connected ground. After capacitor charges, blocks current series resistor presents load bus. During transitions however, capacitor appears short circuit resistor terminates line. general rule thumb selecting capacitor, times rise time divided cable impedance. microsecond rise time cable, this works 0.1µF. disadvantage termination dependent timing jitter generates charging discharging terminating capacitor shifting waveforms. Since possible terminate ends cable characteristic impedance, alternative control slew rate master pull down transistor. lengths meters more volt microsecond slew rate recommended. This provides zero transition
SECTION
Working Control slew rate driver
typical system using running Dallas' 1-Wire Operating System Software (TMEX) using port adapter, communication occurs time slots 8.68 microseconds under control UART. communication cycle begins when transistor master actively pulls line logic zero. This zero transition synchronizing edge 1-Wire communications. 1-wire slave holds zero appropriate, resistor returns line supply voltage after both master slave release line. search, which required identify devices bus, most critical part 1-Wire communication read data time slot, especially being transmitted. general case, there many arbitrarily placed devices bus, each seeing falling edge time slot issued master slightly different time. Because communication requires that signal travel length cable return, electrical length must less than half time interval allowed single data slot. That round trip propagation time signal must less than 4.34 microseconds (8.68 microseconds divided Devices beyond this range will seen master.
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Tech Brief
that takes about microseconds ramp 0.8V logic threshold. Because port transistors 1Wire devices only hold line down after master pulls low, they normally exhibit slew rate problems. exception when Presence pulse generated response Reset command from master since slew rate Presence pulse 1-Wire slave essentially uncontrolled. Disruptive edge rates also occur when 1-Wire device connected net.
such 2N2222 also used with minor component value changes provide recommended slew rate. Refer Appendix additional information 1-Wire waveform template.
330pF Rpullup
1-WIRE
5.1K 220pF
2N7000
Phantom Presence pulse
consists fixed collection 1-Wire slaves, slew rate problem their Presence pulse overcome generating "Phantom Presence Pulse" with master. This simply artificial slew rate controlled Presence pulse created master that starts 10mS terminates 60mS. sample pulse then generated 70mS slave node desired. This technique masks high slew rate slaves high-to-low Presence pulse transition because already logic zero when they occur. However, Phantom Presence pulse technique obviously ineffective with slaves which arrive unpredictable times bus.
Phantom Presence pulse Earlist possible Presence pulse Phantom Presence pulse
Figure controlled slew rate pull down 1Wire master. Transistor type critical.
Pulling line
Once both master slave turn off, pull-up resistor pulls data line high. capacitive load increases adding 1-Wire devices, time raise data line supply voltage also increases. This also occurs when network lengthened 50pF capacitance added meter twisted pair cable. This seen Figure number slaves increased from 300. product total capacitive load (including cable, device, stray capacitance, etc.), pull-up resistor value results time constant (RC) that exceeds time slot defined 1-Wire
1.00 2.00
Last possible Presence pulse
Sample time Presence pulse
10uS
15uS
60uS
70uS
Figure Phantom Presence pulse generated master prior possible Presence pulse from slave. must last until slowest possible Presence pulse started.
Controlling edge
Figure shows suggested slew rate control circuit. 2N7000 shown, commonly available general purpose n-channel FET, transistor characteristics critical almost general purpose ntype transistor substituted. bipolar type
Figure Loading effect increasing number 1-Wire devices using active pull-up, cable.
protocol, communication stops. Because grounding unused wires shields cable adds capacitance
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Tech Brief
which significantly increase time constant, they should left disconnected. seen Figure input capacitance 1-Wire devices contribute capacitive load network. However, parasitic power supply capacitance only exists voltage levels above 2.8V minimum. Ignoring capacitance parasitic power capacitor, pull-up resistor value, together with cable capacitance 1Wire device input capacitance represent network time constant This reasonable omission since parasite capacitance does become factor until already passed 2.2V logic threshold. network time constant determines rate which data line returns logic voltage. With requirement that equals 13.02 microseconds (the original data sample time) 1Wire voltage needs have reached 2.2V threshold logic one, value calculated follows. 13.02 (Vs/(Vs-2.2V)) 22.4µs Where pull-up supply voltage. Using recommended 1.5K minimum pull-up resistor value supply voltage, calculated 22.4 microseconds. Assuming loaded with maximum fanout calculated later section, cable capacitance alone must exceed 12nF yield network time constant more than value just calculated. Using pF/m typical cable capacitance implies that theoretical maximum cable length meters. data sample time recalculated with value 21.7 microseconds, becomes 37.4µs. This permits 22nF cable capacitance, which represents cable length that exceeds maximum allowed round trip propagation time. effect cable capacitance signal seen Figure where 1Wire slaves were addressed meter cable. meter cable added 5000pF capacitance.
1.00 2.00
meter cable meter cable
Figure Effect cable capacitance signal driving 1-Wire devices. 100m cable adds 5nF. risetime improved reducing value pull-up resistor, using lower capacitance cable, shortening cable, reducing number devices bus. pull-up resistor, however, should reduced below Reducing value pull resistor increases logic zero voltage network, reducing system noise immunity. value pull-up resistor already minimum, active pull-up substituted. This also allows longer cables decreasing network time constant. course, same rules apply active pull-up pull down, slew rate must controlled avoid operating transmission line environment. effect residual current flowing when active pull-up turns seen Figure This little concern with passive pull-up time constant inherently slow slew rate.
active pull-up
convenient source active pull-up MAX6314. This part designed MAXIM bi-directional open-drain reset 68HC11. However, contains circuitry automatically enable 20mA p-FET pull-up microseconds when data line exceeds trip voltage about 0.6V rising edge. Unfortunately, does generate logic zero output whenever supply voltage drops below reset threshold (it's intended use). 1-Wire this detrimental heavily loaded data lines cause reset, disabling communication, selection part with reset threshold suggested. possibility MAX6314US31D3-T. This part volt trip level that allows supply drop volts before
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Tech Brief
generates reset. MAX6314 comes four SOT143 package that requires little area. Although MAX6314 contains internal 4.7K pull-up resistor, recommended that external 2.2K resistor added parallel. This provides equivalent recommended minimum 1.5K pullup resistor, which results crossing trip voltage minimum time. waveform then exhibits three distinct segments. When both master 1-Wire devices release data line, starts rising rate determined value pull-up total capacitive load time constant). When passes trip threshold approximately 0.6V, 20mA p-FET turned accelerates toward supply voltage. heavily loaded, microsecond oneshot time with well below supply voltage. this occurs, 1.5K equivalent pull-up resistor continues raise voltage rate seen when first released. three these segments seen Figure This data however, taken with discrete proprietary design with MAX6314. Refer Chapter additional information active pull-ups. maximum voltage which pull-up resistor raise data line determined product pull-up resistor value idle current devices line. more devices, greater voltage drop across pull-up resistor. fanout limit particular 1-Wire reached voltage drop across pull-up resistor reduces supply voltage 2.8V. This minimum voltage that will recharge parasitic power supply 1-Wire devices. From this, maximum theoretical fanout calculated. equal supply voltage (Vs) minus 2.8V (the minimum operating voltage) divided pull-up resistor value. resultant divided 15µA, worse case device supply current. supply 1.5K minimum pull-up resistor value have following. Fanout (5-2.8)/1.5K=1.47mA/15µA=98 devices This represents theoretical maximum number 1-Wire devices that successfully communicate with master using 1.5K pull-up resistor supply over worse case conditions current
temperature. assumptions being that devices drawing maximum supply current operating 85°C environment. real world, devices will only drawing 15µA maximum supply current during System Reset Presence Detect. that time device oscillators turn times. Since time typically lasts microseconds, times represents microseconds, with worse case microseconds. Circuit design ensures that 1-Wire devices will able operate from their internal parasite power source duration this interval once fully charged. Thereafter, they will drawing maximum which permits tripling previously calculated fanout addition, most systems will operating over much narrower temperature range which allows still larger fanouts. example, typical environment, over 1-Wire devices continuous communication only 1.2V drop across pull-up resistor. This implies that typical idle current device less than when environmental supply ranges limited.
SECTION
cable Take care selecting cable
previously mentioned, 1-Wire consists three segments, master, wiring connectors 1-Wire slaves, shown electrical equivalent circuit Figure wiring between master slaves modeled inductance resistance data return lines lumped capacitance cable. Cable capacitance simply product cable length times unit capacitance. This typically 50pF/m recommended Category twisted pair cable. Similarly, line resistance represents cable length multiplied specified resistance meter single wire. 1-Wire slave modeled input capacitance (Cin), constant discharge current (Idisc), parasitic power supply circuitry (Di, Cload) operating current (Iop) during communication. idle current slave required keep interface synchronized with communication protocol. When 1-Wire port transistor impedance nominally less than which provides 0.4V logic zero with current sink. multiple 1-Wire devices residing
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Tech Brief
bus, Cin, Idisc, Cload should multiplied number devices determine
total. needs divided number devices. port transistor inside slave allows
MASTER
Rpullup
1-Wire Slave
DATA
Rdata 30pF Ccable Idisc
Ldata
LOAD 800pF
PullDown Switch
Lreturn
Rreturn
10uA
RETURN
during communication only
Figure Electrical Equivalent Circuit 1-Wire those Figure taken with meters makes this plain. short runs meters, cable selection 1-Wire less critical impedance characteristics generally insufficient have marked adverse effect bus. Even flat modular phone cable works with limited numbers 1-Wire devices. However, longer net, more pronounced cable effects become, consequently greater importance placed cable selection.
1.00 2.00
place logic zero network. Except presence detect cycle, Search, Skip Read commands, only them will conducting when addressed master. While parasitic resistance reduces zero logic level noise margin digital signals, cable capacitance together with parasitic power supply 1-Wire devices adversely affect size network. power this capacitive loading require several milliseconds charge before communication start net, especially passive (resistive) pull-up used. Also long lines with many slaves grouped end, parasitic power requirements create slope change rising edge waveform about 2.8V energy reservoirs devices filled. Once full, recovery time after each time slot will sufficient maintain charge. Figure shows effect charging parasite power capacitance different numbers 1-Wire devices meters Category cable. Notice that slope rising edge decreases crosses 2.8V threshold finally reversing direction form when loaded with more devices. These "dips" become more pronounced longer remains low. Clearly physical properties cable connecting master 1-Wire slaves strongly dominates network. Comparison waveforms shown Figure taken with meters Category cable
Figure Parasite power loading transition. Notice that becomes more pronounced with increasing number devices 100m cable. Given sufficient length, cable exhibits transmission line effects. Real cables display distinct properties resistance, capacitance inductance, which turn determined cable geometry, size spacing conductors their surrounding dielectric. These physical properties define characteristic impedance, signal bandwidth supported propagation
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Tech Brief
velocity cable. Specifically, cable resistance reduces zero logic level noise margin, although values Ohms total cable length acceptable. Cable capacitance however, which range from 30pF/m 100pF/m, loads 1-Wire driver, increasing only network time constant computed earlier, also peak current flowing cable pull down transistor turns discharges line. this transistor turns before charge stored line capacitance completely discharged, residual current left flowing line determines amplitude transient voltage spike generated product this current cable inductance. resulting voltage spike seen driver become large enough interfere with communication. effect residual current flowing cable when pull down transistor active pullup turns seen Figure Notice that each case, spike generated direction opposite rail. cable, when pull down transistor turns it's inductively
1.00 2.00
Inductive pulse pull down transistor
single wire because current flows opposite directions pair ideal case would cancel completely. Because differential inductance decreases distance between conductors reduced, adjacent preferably, twisted pair recommended. Twisted pairs help reduce unwanted coupling from nearby interference sources because currents induced wires flows opposite directions tends cancel. Another concern that recommended category unshielded twisted pair cable commonly available with multiple pairs. While capacitance between wires single pair approximately pF/m; that between wires different pairs closer pF/m. Because grounding unused wires will this pF/m 1-Wire capacitive load, unused wires shields need left unconnected both ends cable. Grounding them increase capacitive load point that pull-up cannot raise line above logic switching threshold within time slot communication stops. Refer back Section discussion network time constant "Pulling line" more information. also recommended simultaneously 1-Wire nets same cable bundle, because capacitive load varies dynamically dependent upon data pattern, which lead erratic operation.
inductive pulse active pull-up
Keep running
1000
Figure Inductively generated voltage spikes such seen here, occur long lines residual current flow when pull down pull-up circuits turn off. generated voltage spike swings negative, reverse biasing substrate 1-Wire device closest cable which clamps voltage excursion diode drop. This device will then respond master. problem differential inductance, which measured across cable input with line shorted together end. Differential inductance substantially lower than inductance
line inductance increases, product di/dt generate voltage excursions that cause errors reverse bias substrate least first 1Wire device cable. These voltages spikes generated current still flowing data return lines cable when transistor master turned before charge stored line capacitance fully discharged. obvious recommended solution maintain pull down transistor state until current line discharges. possible stretch timing, Schottky diode placed across suggested clamp inductive generated voltage overshoot. Connect diode across cable with cathode data line anode return. Only diode required each branch.
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Tech Brief
SECTION
Rewiring
DS2409 MicroLAN Coupler
Historically, 1-Wire envisioned single twisted pair routed throughout area interest with 1-Wire slaves being attached daisy chain fashion where needed. However, network heavily loaded, and/or very long, preferable even necessary, separate into sections. This added benefit providing information about physical position 1-Wire device which facilitates trouble-shooting. using section main "trunk", adding removing segment "branches" with DS2409 needed, true 1-Wire created. also possible course remove additional segments branches using MicroLAN Coupler node control. This approach reduces capacitive idle current loads master sees that trunk those segments connected activated DS2409's. However, limitations total capacitive load, idle current line length covered earlier portions this document still apply. That trunk activated sections LAN, capacitive load must enough allow pull-up raise line above logic threshold within time slot. combined idle current 1-wire devices must reduce supply voltage below 2.8V. electrical length activated cable must allow transition from master reach cable return within time slot. DS2409 MicroLAN Coupler component creating complex 1-Wire nets. contains MAIN transmission gate outputs, plus open drain CONT output transistor, each which remotely controlled master over 1-Wire network. Both MAIN outputs support "smart-on" command which generates reset/presence sequence selected output before connecting 1-Wire bus. This allows subsequent function command apply only devices just activated segment. main caution that DS2409 requires supply without which shorts 1-Wire communication possible. Since DS2409 contains
user available memory, output used label node connecting 1-Wire memory chip store required data. simple 1-Wire branch with EEPROM labels connected described shown Figure Tagging protocol information available Dallas Semiconductor site example, DS2430 EEPROM connected DS2409 output. This provides tagging information specific that particular node such location, function, etc. attached CONT output provides visual indication specific branch being addressed caused flash software extra visual impact. single DS1820 Digital Thermometer shown branch output multiple 1-Wire devices placed required.
1-WIRE GROUND
BRANCH
DATA
DS1820
MAIN
MICROLAN COUPLER
CONT
EEPROM
DATA
DS2409
DATA
DS2430A
1-WIRE GROUND
MAIN TRUNK
Figure Separating 1-Wire into branches using DS2409 MicroLAN Coupler.
avoid loading entire 1-Wire with capacitance associated with routing power along with 1-Wire DATA GND, CONT output DS2409 used operate pass gate transistor switch supply along with MAIN output switching branch. MICREL MIC94031 transistor shown Figure contains gate pull-up resistor shown, implementing circuit requires only addition single SOT-23 package. default, whenever MAIN output DS2409 turned CONT output pulled low, turning pass gate routing power
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Tech Brief
selected branch. Turning MAIN automatically turns CONT therefore pass gate off, software revisions required circuit. DS2409 available TSOC surface-mount
BRANCH
MIC94031
MAIN
SWITCHED
MICROLAN COUPLER
CONT
DS2409
DATA
DATA
hold maximum 13V, while PIO-B sinks holds 6.5V. onresistance DS2406 PIO-A output transistor about Ohms, whereas PIO-B output about Ohms. impedance both addressable switches exceeds Ohms. DS2406 available TSOC surface-mount package, single channel (PIO-A) version TO-92 package. more information DS2406 dual addressable switch which contains time user programmable EPROM, refer datasheet available Dallas Semiconductor website www.dalsemi.com.
Figure Method switching local power branch using DS2409 pass transistor. package. further information about DS2409, refer datasheet available Dallas Semiconductor website www.dalsemi.com.
Using both high switch
While DS2409 DS2406 very useful when used alone, together they form basis general purpose Pick system. Figure shows DS2409s being used select arbitrary row, while dual DS2406 low-side switch used select arbitrary column. shown, they form simple array with LEDs visually indicate specific intersection being addressed master. However, array easily expanded either direction addition more DS2409s and/or DS2406s. this manner array required size implemented limited only loading. operation, master selects output DS2409 that controls interest, column output corresponding DS2406 that intersects that required position. example, output DS2409 output DS2406 both turned position upper right hand corner selected. This connects iButton port intersection selected column master serial number 1-Wire device any) that point read. indicate which intersection being addressed, master switches selected DS2409 from output Main output. default this causes CONT turn grounding gate associated pMOS transistor turning With pass transistor power supplied selected intersection turns desired, DS2409 switched repeatedly between Main causing blink greater visual effect.
DS2406 dual addressable switch
DS2406 side addressable switch intended remote control perhaps combination with opto-coupler relay; provide visual indication operation end-of-limb means (Light-Emitting-Diode). DS2406 recommended means providing branches 1-Wire net, because branching requires high side switch. This because current flowing through on-resistance port transistor develops voltage that raises ground devices connected above ground reference. Since 1-Wire slaves will hard wired data potential data becomes negative with respect voltage ground pin. This polarity acts forward bias various junctions slaves resulting device dependent disruptions. example, when this voltage reaches approximately negative 0.3V, continuously activate (power-on-reset) attached DS2406, effectively removing from bus. DS2406 addressable switch designed perform closed-loop control open drain output transistors, PIO-A PIO-B, over twisted pair cable meters from Each output also sense report logic level port. When turned master, PIOA port connected ground sink
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Tech Brief
Main outputs DS2409s turned LEDs entire column selected DS2406 turn Alternatively, outputs DS2406s turned LEDs entire selected DS2409 turn Consequently, follows that turning column switches
illuminates entire array which serves convenient test verify that system fully functional. While iButton port shown example, blue-dot connector even solder mount devices could substituted.
MIC94031 1.5K 1.5K
4.7uF
MAIN CONT
500K
DS2409
DATA
MIC94031 1.5K 1.5K
MAIN CONT
500K
DS2409
BAT54S
DATA
DATA
BAT54S
DS2406
Figure Using DS2409s DS2406s form matrix Pick system with visual indicators.
SECTION
Policing
Protection Noise
1-Wire compatible devices contain built-in (electrostatic discharge) protection circuitry able withstand 10kV minimum Human Body Model events. However, when interfacing computer 1-Wire good engineering practice provide additional means protect system from (electromagnetic interference) damage. Economical protection provided master means diodes that short negative spikes shunt positive ones higher than power supply where they absorbed capacitors other protection devices. When selecting protection diodes surge suppressers, choose devices with minimum junction capacitance fast switching times. good examples ERA82 Schottky diode from FUJI multi-source 1N5817. BAT54x series Schottky diodes
available SOT-23 surface mount package also acceptable. Refer Figure example using BAT54S dual diode. 1-Wire EPROM parts will programmed using RS232 1-Wire interface adapter, excellent protection provided with single DS9502 protection diode. These behave like 7.5V zener diode during normal operation, however, voltage across them exceeds their trigger voltage they fold back 5.5V. exposed event beyond their ratings, DS9502 will eventually fail shorted, preventing damage protected circuitry. DS9502 available lead TSOC package, solder mount "bumped" die.
Introducing DS2480
order reduce engineering load setting 1-Wire net, Dallas Semiconductor developed DS2480 Serial 1-Wire Driver. This connects
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Tech Brief
directly UARTs RS232 systems. Adapters available using DS2480 that connect directly standard port provide 1-Wire outputs. contains programmable pull-down slew-rate control active pull-up. Other features include support data rates (default), 19.2, 57.6 115.2 Kbps programmable 1-Wire timing. DS2480 available DS9097U-x09, which RJ11 port adapter, DS1411 with DS9098 iButton socket. These true ground Crypto-capable adapters with class approval. schematic DS9097U provided Chapter
DS2409 MicroLAN Couplers separate into branches.
DS2480 based port adapter DS9097U-009, RJ11 adapter
DS1411, DS9098 socket adapter
DS2480 type adapter used, Control slew rate
Limit slew rate pull down about volts microsecond. Replace pull-up resistor with active pullup, limiting slew rate about V/mS. Keep master transistor until residual line current dissipates. 21.7 microsecond timing (2.5T), instead original 13.02 microsecond (1.5T) timing.
active pull-up
results that count
applying information presented this application note such using Category twisted pair, controlling slew rates substituting active pull-up, reliable communication over 300m cable with more than assorted 1-Wire devices demonstrated. Without slew rate control active pull-up, limit about meters with 1-Wire devices.
Keep running
later sampling
Recommendation summary operating long heavily loaded MicroLANs: Select cable
Good Better -twisted pair phone -Category twisted pair
diode clamp
Schottky diode across cable's end. Connect diode reverse biased with cathode data line anode return.
multilevel branching
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Tech Brief
CHAPTER
Hardware 1-Wire
1-Wire adapters
with UART capable 115.2 kbps serve master reading writing 1-Wire devices net. However, serial port 1Wire adapter needed interface computer RS232 levels 1-Wire bus. schematic original DS9097 port adapter developed Dallas Semiconductor economic easy interface, shown Figure adapter powered entirely from computer port. Note that circuitry makes maximum UART controlled output using only clamping level shifting diodes. Although this provides simple reliable circuit that directly couples serial port, lacks ground reference. While concern operating net, present complications during trouble shooting, oscilloscope ground cannot connected 1-Wire return. Connecting DS9097 1-Wire return ground result permanent damage adapter.
3.9V
DATALINE
recognized logic zero 0.8V, this leaves 0.5V noise margin. DS9097 comes with attachment serial port RJ11 connecting 1Wire net. same circuitry available DS9098 socket DS1413. DS9097E version DB25 case also available programming EPROM based 1-Wire products such DS198x series. provides power jack accept external auxiliary supply required.
true ground port adapter
return line must grounded some point, true ground port adapter interface required. schematic true ground adapter given Figure positive supply derived from Schottky diodes CR2, filter capacitor well-regulated volts provided dropout LP2980 voltage regulator long port provides least 5.1V. positive level both drops below this regulator drops regulation. provide negative supply required DS275 RS232 Transceiver chip from during read data idle time slots. DS275 connects directly 1-Wire converting it's CMOS/TTL levels RS232 levels required UART. surface mount components used, circuitry will comfortably 0.6" 0.9" printed circuit board. This mounted inside female RJ11 adapter.
6.2V
RETURN
1.5K
Figure DS9097 port adapter. Note that "return" ground. pull-up. Figure current limiting slew rate control provided UART RS232 requirements, while zener clamps data line 3.9V. Zener diode limits maximum voltage range 1Wire 6.2V. also restricts most negative voltage swing minus 2.3V. When positive, Schottky diode limits voltage difference between 1-Wire data return lines 0.2V, connects RXD. This bypasses passive pull-up, provides impedance path initiate time slot. 1.5K minimum value pull-up resistor generates 0.3V logic zero across on-resistance 1Wire device. Since maximum voltage still
slew rate controlled pull down
Because possible terminate cable characteristic impedance, alternative control slew rate master pull down transistor prevent adverse transmission line effects. more information this subject, refer back Chapter "Control slew rate driver." Figure transistor components associated with gate, limit slew rate about volts microsecond. This provides zero transition that takes about microseconds ramp from points. This proven adequate cable lengths exceeding meters. 2N7000 shown commonly available general
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Tech Brief
purpose n-channel FET, transistor characteristics critical almost general purpose ntype transistor substituted. bipolar type such 2N2222 also used with base resistor minor component value changes provide recommended slew rate. pull down circuit Figure Miller integrator capacitor across acts primary slew rate control element. lightly loaded LANs using about devices meters less cable, possible eliminate Sufficient slew rate control then provided operation, when switches from minus rail plus rail, signal passed gate input resistor This transition slowed delayed slightly time constant When voltage level gate reaches threshold voltage transistor begins turn effect transistor turning with across terminals results nearly linear voltage ramp from supply voltage ground. This zero transition synchronizing edge
Wire communications. remains state duration time that held plus UART. Afterward, returned supply voltage action pull-up resistor Zener diode serves several functions circuit. protects gate limiting positive voltage excursions 12V, negative ones minus 0.6V. Gate input resistor limits current through zener. capacitance also adds that helping control transistor transition. While they shown clarity reasons, good engineering practice would place reverse biased Schottky diodes across output circuit. diode should from supply voltage data line, other from data line ground. ultra fast capacitance Schottky diodes such 1N5817 ERA82 from FUJI suggested. BAT54x series available SOT-23 packages also acceptable.
LP2980
Vdrv
1.5K
DS275
DATA
330pF 2N7000 220pF
2.7K
Figure serial port 1-Wire true ground adapter. slew rate controlled pull down passive pull-up resistor.
that's works!
cycle sequence 1-Wire communication over true ground port adapter would proceed follows. Initially, would minus RS232 "mark" logic one), gate would clamped minus 0.6V reverse bias zener diode Consequently off,
will have been raised supply pull-up resistor When switches between minus plus rails, will turned ramp voltage down rate Miller integrator capacitor This zero transition paths, continues down bus, other returns UART through
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Tech Brief
signal returned UART, will used determine when reexamine registers response from 1-Wire device bus. Meanwhile, transition traveling down received sequentially attached 1-Wire slaves edge propagates past them. this signal from UART Reset pulse, lasts least microseconds then releases bus. pull-up raises line supply, approximately microseconds later UART reexamines registers Presence pulse being sent 1-Wire device. interim, internal oscillators controllers within each 1-Wire device have determined that Reset been sent appropriate time will pull low. exact time pulled duration, function individual device variation. fastest will pull first, slowest release last. When final 1-Wire device releases, pull-up resistor raises toward supply voltage rate determined value times capacitive load sees line time constant). pull-up
cannot raise above logic threshold within time slot, master will always logic zero conclude line shorted. Consequently, communication cannot occur. Assuming however that timing acceptable, UART sees proper Presence pulse indicating there 1Wire devices bus. then proceeds call serial numbers identify them. order write logic zero values onto bus, UART turns short (less than microseconds), long (greater than microseconds) time slots respectively. read, UART begins turning short time slot exactly logic being sent. this time slot remains unchanged, UART defines this reading logic one. 1-Wire slave extends time slot initiated master continuing hold line (even though UART released attempts pull data line high) UART defines this reading logic zero.
LP2980
2.7K 2.7K
3.3K
5.1K
DATA
2.7K
HCPL-2202
330pF
2N7000
HCPL-2300
220pF
Figure optically isolated port 1-Wire adapter.
optocoupler isolated adapter
some applications, safety reasons because ground loops, necessary provide galvanic isolation between computer master net. such cases, optically coupled adapter circuit Figure suggested. Optical isolation port requires special purpose optocouplers,
such high speed, input current HCPL2300 used transmit section. HPCL-2202 with totem pole output used 1-Wire RS232 channel. HCPL-2300 0.5mA input LED, output transistor with nanosecond maximum
15/32
Tech Brief
propagation time. circuit, current with resistor current source such J503 CR056 from Siliconix will provide superior performance over diversity logic levels available with RS232. constant current source used necessary recalculate value maintain 0.5mA with voltages particular RS232 port. required protect LED, which only breakdown reverse direction. operation, HCPL-2300 coupler provides necessary isolation level shifting from double rail plus minus RS232 single rail 1-Wire net. coupler connected noninverting, when minus 12V, input Consequently, coupler output transistor also pull down transistor off. With off, pull-up resistor holds supply voltage. receive optocoupler also off, output minus 12V. When changes state, HCPL-2300 input reversed biased, turning coupler output transistor off. collector then pulled high resistor This zero signal slowed delayed slightly time constant When voltage level gate reaches threshold voltage transistor begins turn effect transistor turning with Miller capacitor across terminals results nearly linear voltage ramp from supply voltage ground. When pulls data line ground, also turns HCPL-2202 coupler, causing totem-pole output stage switch plus This transition will received UART used determine when reexamine registers response from 1-Wire devices bus. data line remains ground until returns original state, along with 1-Wire slaves turn off. that time, pull-up resistors parallel with raise data line toward supply rate determined their equivalent value times capacitive load seen line time constant). This turns input causes output stage switch minus RS232 voltage level. Assuming pull-up raise above 2.2V within time slot, UART
will logic one, communication proceeds. pull-up cannot raise above logic threshold within time slot, master will always logic zero conclude line shorted. Consequently, communication stops.
active pull-up
1-Wire where minimum acceptable pullup resistor value cannot raise data line above logic threshold within time slot, active pull-up must used. Obviously, active pull-up circuit should only during defined range rising edge (zero transition). Conversely, should respond falling edge, active during logic zero time intervals. must trigger rising edge about 0.9V plus minus 0.1V provide acceptable noise margin. Preferably, once triggered will remain until line raised above specified threshold rather than time interval (one shot). This insures that data line will raised above 2.8V level required recharge parasite power capacitors regardless load. maximum current supplied should limited about 15mA. Larger currents when flowing cable inductance cause problems. Refer Chapter Section "Its cable" Figure more information effects cable inductance. Chapter Section under active pull-up," MAX6314 suggested available circuit. This part designed MAXIM bi-directional reset intended with 68HC11 microprocessor. such, important function chip monitor supply voltage assert reset (logic zero) during power-up, power-down, during supply droop. This obviously undesirable function net. However, part includes active pull-up solve time constant problem faced high capacitance bus. Coincidentally, this consists p-FET parallel with 4.7K pull-up resistor (the maximum 1-Wire value). turned microseconds when waveform reset exceeds 0.9V maximum comparator triggered one-shot. Additional control circuitry ensures that active pull-up disabled other times.
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Tech Brief
Operation MAX6314 when connected 1Wire proceeds follows. When (connected MAX6314 reset pin) pulled below 0.5V master, comparator sets internal flip-flop enabling active pull-up control circuitry. When released, internal 4.7K pull-up resistor starts raising toward (the supply voltage) rate determined it's time constant. This generates ramp that starts logic zero level ends trip voltage
Active pull-up turn
This starts second much steeper ramp that begins comparator trip voltage ends However, heavily loaded, third ramp created microsecond one-shot times out, voltage reached This ramp starts where one-shot times out, ends maximum voltage which passive pull-up raise line with idle current load 1-Wire devices bus. This third ramp will have slope similar first. characteristic waveform produced active pull-up illustrated Figure Although MAX6314 contains internal 4.7K pull-up resistor, suggested that external 2.2K resistor added parallel. This yields equivalent recommended 1.5K minimum pullup resistor, allows cross enable trip voltage active pull-up minimum time. combination MAX6314 2.2K resistor substituted Figure provide true ground port adapter with active pull-up.
ACTIVE PULL-UP REGION
Active pull-up trip point
PASSIVE PULL-UP REGIONS
Figure Characteristics active pull-up. comparator. When trip voltage active pullup enable comparator exceeded, triggers microsecond one-shot that turns 20mA p-FET
BAT54S
BAT54S
BAT54S
Female
LP2980
.1uF
ON/OFF
VOUT
10uF
10uF 33uF
2.7K
DS2502
BAT54S
4.7K
2N7002
ADAPTER
BSS84
DS2480 DS9503 DATA
DIODE
Figure schematic DS2480 based DS9097U port Adapter. order reduce engineering load setting 1-Wire net, Dallas Semiconductor developed DS2480 based series port adapters. basic schematic shown Figure used through-
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Tech Brief
series, major difference being type number 1-Wire connections output. DS2480 provides programmable slew-rate control pull-down active pull-up. Other features include support data rates (default), 19.2, 57.6 115.2 Kbps programmable 1-Wire timing. DS2480 also provides strong pull-up mode facilitate higher current requirements devices such DS1820 temperature sensor during conversion. DS2502 shown used provide adapter DS2480 available with without RJ11 port adapter DS9097U-009
DS9097U-S09 respectively. circuitry also available DS1411 with DS9098 iButton socket. adapters true ground Cryptocapable adapters with approval.
CHAPTER
Supplying power 1-wire
1-Wire chips both their power communications over same line making installation very economical. When adding power-consuming devices 1-Wire node, same line also supply power them using more techniques described here. Typical examples could simple driving operating solenoid, complex powering pressure sensor. such cases, most convenient method would transfer energy they require over same communication line. This chapter will review some methods providing power including technique accumulating level energy releasing demand high energy burst. general, solutions problem supplying power 1-Wire line fall into following four methods. Keep mind that regardless which method chosen, consideration must also given energy required, duration distance from master. Sourcing power whenever line above 3.5V. Sourcing power transferring charge capacitor through blocking diode. Sourcing power with strong pull-up during idle communication time. Alternative power source using additional wires connections.
power between
Because 1-Wire devices operate with little supply, energy available between supply levels volts tapped. This equivalent operating load shunt mode used operate clamp type loads such LEDs. This requires that total voltage drop across LED(s) least 3.5V. While possible connect shunt load permanently across bus, preferably, load would operated under master control connecting between 1Wire DATA lead output addressable switch shown Figure this mode, 1-Wire communication takes place below 3.5V power delivery occurs above that value. Whenever output DS2406 pulled voltage approximately equal 3.5V, forward voltage LED. When output turned off, voltage nominal value. Operational current supplied master which DS2480 based DS9097U port adapter normally limited about milliamps, increases about milliamps when active pull-up turns shown Figure voltage will clamped level which keeps active pull-up supplying milliamps. current limiting resistor connected between DATA line LEDs will allow active pull-up turn off.
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Tech Brief
TEXT
DATA
DATA
LEDS WITH 3.5V
Parasite power
DS2406
Figure Using power available between directly under master control.
Figure Using Schottky diode capacitor supply local power 1-Wire net.
Transfer charge capacitor through blocking diode
some applications acceptable series Schottky diode capacitor across 1Wire generate local supply point interest. Refer Figure wind speed sensor 1-Wire weather station which uses BAT54S Schottky diode .01mF ceramic capacitor power DS2423 counter uses this technique. Figure During idle communication periods when circuit `steals' power from line charge capacitor power load. This discrete implementation parasite power technique used internally 1Wire devices provide their operating power. value used depends current consumption load long voltage must held above design value. While simple economical circuit adds both leakage capacitive loads that reduce range capability 1-Wire net. This loading places upper limit capacitance value used number that placed net. Consideration must also given fact that event capacitor shorted held discharged state load, will also shorted inoperable. further communication take place until capacitor charged above volts.
Deliver energy under master control
shown Figure half wave rectifier Figure isolated between addressable switches controlled master. When input switch closed, capacitor receives charge over DATA line 1-Wire same manner circuit Figure significant advantage arrangement that when switch opened capacitor charge isolated from normal communication resumes without burden capacitive leakage loads When stored energy needed, output (power dump) switch closed capacitor discharged through load. Note that while reference made capacitor, rechargeable battery could used equally well. Important elements concept architecture low-level transfer energy from master storage element, subsequent high energy burst. Conceptionally, this somewhat similar circuitry flash camera develops energy needed fire flashbulb. Equally important isolation storage element from 1-Wire failure doesn't bring down net, complete control energy source delivery master. practical example concept using DS2406 control element pFETs switches shown Figure Notice that MICREL MIC94031 isolation switches specified four terminal devices with substrate terminal brought out. This provides correct biasing terminal under operating conditions. gate pull-up resistor shown unlabeled internal chip shown clarity. order insure that both switches turned same time
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Tech Brief
STORAGE ELEMENT
DATA
CHARGE CAP. POWER DUMP 220uF
ENERGY OUTPUT
CHARGE CAP.
POWER DUMP
Figure parasite powered 1-Wire remote highenergy source concept.
possibly bring down net, lockout circuit constructed using 74HC126 tri-state gate. design only allows alternate enabling pass gates charge discharge energy storage element shown truth table Figure both outputs DS2406 simultaneously placed same logic state, either intentionally accident, insures that neither pass gate turned operation, charged commanding output (pin DS2406 logic zero. This turns
CR2a BAT54S .1uF
connecting 1-Wire DATA line through diode which prevents from discharging back through 1-Wire net. diode were present 1-Wire device were placed when pass gate turned Presence Pulse would short discharge capacitor possibly damaging chip. initial state with charge gate discharge pass gate, held higher potential then source terminal pull-up resistor off. When master turns output off, charge stored isolated from both 1-Wire load only leakage paths exist discharge When master commands output (pin logic zero, pass gate turns discharges through load. more sophisticated implementation concept, barometric sensor constructed using DS2450 Quad 1-Wire addressable switch control element. DS2450 also reads charge level energy storage capacitor controls sample-and-hold output
CR1b BAT54S
CR2b BAT54S
ENERGY OUTPUT
DATA
MIC94031
220uF
MIC94031
200K
200K
CR1a BAT54S
74HC126 Hi-Z
DS2406
Figure isolated remote energy source based DS2406 addressable switch.
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Tech Brief
sensor. major design consideration circuit that barometer required energy source provide 10mA 22mS. schematic prototype circuit shown Figure circuit, DS2450 pins used digital outputs that control capacitor charge discharge analog switch specialized switchedcapacitor voltage regulator used place output (discharge) analog switch. charge pump, MAX684 from MAXIM, provides regulated output energy capacitor discharges down 2.7V. Surprisingly, efficiency increases input voltage drops, very useful feature when using discharging capacitor energy source. remaining pins used analog inputs which read voltage storage capacitor voltage from sample-and-hold (U7) that stores output from barometer representing current barometric pressure. circuit performed expected with values 0.22 Farad energy storage capacitor. Obviously, higher capacitance, longer takes charge longer voltage level maintained relatively constant. operation, pulling U1.7 closes analog switch allowing charge through CR1. prevents from discharging back through 1Wire net. voltage generated charging read needed U1.8 insure that sufficient
energy exists operate load. When U1.7 turned off, analog switch also turns charge stored completely isolated from net. appropriate time, U1.6 pulled which enables voltage regulator providing path discharge through barometer MPXA4115 Motorola part requires 22mS maximum turn stabilize, which time output voltage representing current atmospheric pressure stored sampling capacitor. After sample, turns minimize energy loss from storage capacitor Instead wide interval used prototype circuit sample barometer would preferable narrow pulse immediately after output settled.
additional available wires
order obtain maximum performance, Dallas Semiconductor recommends (unshielded twisted pair) routing 1-Wire net. However, since typically comes with multiple pairs, there natural inclination extra pair route power. look some cable properties will help understanding such arrangement affects performance. cable with multiple twisted pairs, average given conductor
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Tech Brief
.1uF BAT54C .1uF MAX4515 1N4148
SKIP
BAROMETER
MAX4515
DATA
220uF .22uF
MAX684
MPXA4115
SHDN Pgnd
.47uF
750K
LMC7111
1N4148
74HC125
DATA
Ain-D Ain-C Ain-B Ain-A
.036uF
DS2450
LMC7111
U1.8 READ CAP. CHARGE U1.7 CHARGE CAP. U1.5 READ SAMPLE
U1.6
POWER DUMP
U2.8
22mS SAMPLE
Figure 1-Wire barometer powered isolated regulated energy source. pair adjacent second conductor another pair half length. Typically, capacitance between conductors twisted pair approximately pF/m while that between conductors different pair runs about pF/m. Because grounding unused conductors will increase capacitive load seen master, Dallas Semiconductor recommends that unused wires shields left unconnected both ends cable. Still, within limits increased capacitive loading potential cross talk caused using extra pairs within cable, external power routed along with 1-Wire DATA pair. However, loading reduce long and/or number 1-Wire devices support. addition, current voltage variations pair carrying power induce cross talk 1-Wire DATA that disrupts communication. Since master sees less capacitive loading when routing power over flat cable where conductors maintain fixed distant separation, flat 6-conductor phone (silver satin) used about feet. Insure that DATA have maximum separation from power conductors using outer conductors next 1-Wire carry power. shown Figure wiring sequence should proceed this order; connection); 1-Wire DATA; 1-Wire GND, then external power ground outermost conductors. This arrangement helps shield critical DATA lead from additional capacitive load cross talk external power leads. Notice again that conductors prior 1-Wire DATA line shown dashed lines left uncommitted. repeatedly emphasized, connected they will substantially increase capacitive load seen DATA line. possibility 4-conductor silver satin assemble cable with
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Tech Brief
these slots RJ11 connector empty. Unfortunately, significant disadvantage flat cable that lacks noise rejection properties twisted pair cable, present significant performance problem routed near sources electrical noise.
FLAT CONDUCTOR PHONE
1-Wire DATA 1-Wire Power Power
Figure power routed over flat phone wire along with 1-Wire DATA GND, outer conductors 1-Wire minimize cross-talk capacitive loading.
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Tech Brief
CHAPTER
1-Wire Instrumentation
addition 1-Wire control chips such DS2406 DS2409, several digital functions such temperature sensors analog-to-digital (ADCs) converters available. These make possible measure wide variety physical properties over 1-Wire net. distinct advantage 1-Wire instruments that interface master same manner regardless particular property being measuring. Whether basic sensing element voltage, current, resistive capacitive based, they communicate over using 1-Wire protocol. Other methods employ variety signal conditioning circuitry such instrumentation amplifiers voltage-to-frequency converters which necessity makes their outputs different require separate cables each sensor. unique address serial number each device master interpret what parameter particular 1-Wire instrument measuring. Since science meteorology requires variety diverse sensors, several examples 1-Wire instrumentation weather station will given here. example, DS2423 Counter inputs which respond logic level changes switch closures making them suitable implement variety rate sensors. example using magnetically actuated reed switches suitable rain gauge wind speed sensor shown Figure counter indicating inch rain fallen.[1] similar circuit used 1-Wire weather station measure wind speed.[2] same circuit with Lithium backup also been used mounted wheel odometer. Conveniently, DS2423 also contains 4096 bits user accessible SRAM, which useful temporary storage, with Lithium backup, calibration, location function information.
DATA
BAT54S
DS2423
.01uF
1Meg
Figure basic DS2423 Counter circuit.
DS2438, versatile performer
Originally designed perform multiple functions battery pack, DS2438 contains ADCs temperature sensor. main performs 10-bit conversion 0-10V input, 9-bit conversion 0-5V signal, with internal multiplexer that allows read voltage power supply pin. other converter intended measure voltage developed large currents flowing across external resistor with 10-bit accuracy fullscale reading 250mV. 13-bit internal temperature sensor similar DS18B20. Additional features include Real Time Clock 40-bytes nonvolatile memory useful storing calibration, location function information.
Counting with DS2423 1-Wire Counter
figure, dual diode BAT54S serves both protect circuit from signals that below ground, with provide local source power. While DS2423 internal pull-up resistor keep input from floating, high value Ohms) make susceptible noise. prevent generating spurious counts during turn-on, minimize noise pick-up, external pull down resistor used instead. Except Lithium back-up (not shown), this counter circuit used 1-Wire rain gauge. that application, small permanent magnet moves past reed switch each time tipping bucket fills empties. This momentarily closes reed switch which increments
Measuring Humidity over 1-Wire
Humidity important factor many manufacturing operations well affecting personal comfort. With proper sensing element, measured over 1-Wire net. Honeywell sensing element specified here develops linear voltage versus relative humidity (RH) output that ratiometric supply
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Tech Brief
voltage. That when supply voltage varies, sensor output voltage follows direct proportion. This requires that voltage across sensor element well output voltage measured. addition, calculation True requires knowledge temperature sensing element. Because contains necessary measurement functions calculations, DS2438 makes ideal choice construct humidity sensor.
DATA
1-Wire Barometer Using DS2438
Barometric pressure another important meteorological parameter that measured over 1Wire using DS2438. selecting pressure sensor that contains comprehensive on-chip signal conditioning circuitry circuit Figure very straightforward. case with humidity sensing element, specified pressure sensing element ratiometric. This requires that both output voltage representing atmospheric pressure supply voltage across element known order accurately calculate barometric pressure.
BAT54S
.1uF
HIH-3610
100K
DS2438
DATA
HONEYWELL .01uF
BAT54S
.1uF
MPXA4115
MOTOROLA BAROMETER
Figure humidity sensor using DS2438.
DS2438
Figure analog output HIH-3610 humidity sensing element converted digital main input DS2438. DS2423 Counter circuit, dual-diode BAT54S serves both protect circuit from signals that below ground, with provide local source power. Notice that been increased from .01uF .1uF handle operating current required HIH-3610. network output pass filter used remove level clock feed-through from sensing element's signal conditioning circuitry. However, averaging done software, necessay omitted. operation, master first DS2438, report supply voltage level pin, which also supply voltage sensing element. Next, master read output voltage report local temperature from on-chip sensor. Finally, master calculates true relative humidity from three parameters supplied U1.[3]
Figure 1-Wire barometric pressure sensor.
typical 1-Wire instrumentation, dual-diode BAT54S serves protect circuitry from signals that below ground, with provide local source power. this case, because MPXA4115 pressure sensor require much external source power needed. Notice that external power also connected power DS2438. This allows DS2438 measure supply voltage applied pressure sensing element. Alternatively, barometer powered directly 1-Wire using concepts described Chapter However, many installations supplying external power problem barometer will mounted inside near master source power. Flexible tubing routed sample outside pressure avoid unwanted pressure changes (noise) caused opening closing doors windows elevators moving inside building.
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Tech Brief
Wind Direction Sensor Using DS2450
While original 1-Wire weather station used DS2401 Silicon Serial Number label each eight magnetic reed switches wind direction sensor, single D2450 Quad perform same functions.[2] keeping with recommendations ADI, dual-diode BAT54S serves protect circuitry from signals that below ground, with provide local source power. Note that been increased from .01uF 10uF insure
that voltage across resistor network remains relatively constant. shown Figure single DS2450 replaces eight DS2401s originally used with five resistors. wind rotates wind vane, magnet mounted rotor that tracks opens closes two) reed switches. When reed switch closes, changes voltages seen
10uF DATA
Ain-A Ain-B Ain-C Ain-D
DATA
BAT54S
DS2450
Figure wind direction sensor using DS2450 Quad ADC. input pins DS2450. example, magnet position close (North), voltage seen changes from 1/2Vcc, approximately, from 2.5V. Since sixteen positions wind vane produce unique four-bit signals from ADC, there need initialize sensor, store tagging code board required original 1-Wire weather station. only necessary indicate North, equivalently, which direction wind vane pointing. Table lists voltages seem inputs sixteen cardinal points.
POS.
Table Wind vane position versus voltage seen four DS2450 inputs.
Because reed switches closed when magnet midway between them sixteen compass points indicated with just eight reed switches. Referring schematic position Table observe that when closed Volts
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Tech Brief
applied inputs This occurs because pull-up resistors placed parallel pair connected series with form voltage divider with .66Vcc across Notice that this also occurs twice more switch positions
Measuring Solar Radiance 1-Wire
amount sunlight duration additional parameters that meteorologists interested measuring. amount measure conditions, while duration related equinoxes length day. While mounting filtering tend complex, shown following figures, electronics easily implemented using DS2438. Figure illustrates Solar radiance sensor using photodiode, while Figure uses photovoltaic cell. each case dual-diode BAT54S serves protect circuitry from signals that below ground, with provide local source power. Figure sense resistor connected series with photodiode between `current' pins. Light striking photodiode generates photo
DATA
mode. Select that acceptable current levels when exposed high noon clear day. Size resistor develop millivolts maximum using formula divided where 0.25 Volts maximum current generated. example EFA5364X from Stanley. This super-bright orange ALGaInP with peak response 609nm narrow spectral field view. 4.7K sense resistor provides acceptable outdoor performance, which increased 100K circuit only used with indoor lighting. LEDs made from other compounds will have their peak response different portion spectrum making them useful specific applications. Another approach solar radiance sensor shown Figure where suitable solar cell connected `current' inputs DS2438 through voltage divider divider necessary ensure that voltage between pins DS2438 does exceed maximum limit millivolts. Chose resistor values divider unduly load power capacity cell. form low-pass filter reduce noise sensitivity. advantage this technique ability several cells facing toward different
DATA
SOLAR CELL
BAT54S
.1uF
OP-954
DS2438
Rsens
BAT54S
.01uF
DS2438
.1uF 100K
Figure Photodiode Solar radiance sensor. currents that turn develop voltage drop across sense resistor that read ADC. commercial units, optical filters added control both wavelength bandpass which sensor responds. More sophisticated units such desirable features translucent hemisphere that collects light enable sensor view from horizon horizon. this case, sensor actually focuses inside hemisphere obtain reading. interesting variation solar radiance sensor constructed using reverse bias
Figure Solar radiance sensor using photovoltaic cell. sectors obtain horizon horizon coverage. Connect cells parallel size develop 0.25 Volts with maximum light across sensor described preceding paragraph.
MEASURING THERMOCOUPLE WITH 1-WIRE also posssible measure extreme temperatures using conventional thermocouples that directly digitized cold junction using DS2760 multi-
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Tech Brief
function 1-Wire chip. twisted pair cable 1Wire serves cover distance between (TC) master effectively replacing expensive thermocouple (TC) extension cable normally used. Because unique address multiple smart thermocouples placed where needed anywhere along greatly minimizing positioning cost installation. Although information associated with stored within chip itself ("tagging"), this same allows reference data stored master. REVIEWING THERMOCOUPLE fundamental operating principle thermocouple discovered 1821, when Thomas Seebeck discovered that dissimilar metals were joined voltage (the Seebeck Voltage) proportional temperature difference between joined open ends generated. more popular industry standards type type convention capital letters used indicate composition according American National Standards Institute (ANSI) conventions. example, type thermocouples nickel-chromium conductor constantan copper-nickel alloy) other. While full-scale output voltage falls millivolt range, type generates highest Seebeck Voltage/°C (62mV/°C @20°C) resulting output almost millivolts 900°C; more than other standard. Obviously, order measure this output voltage necessary make connections open ends wires forming thermocouple. These connections turn form second thermocouple, example nickelchromium/copper series with original `hot' junction when copper conductors used. Historically correct these `cold' junctions (one each wire) they were placed bath triple point, whereas most modern instruments electronically correct reading zero degrees. When electronic correction used, temperature cold junction measured voltage that would generated thermocouple that temperature subtracted from actual reading. voltage versus temperature transfer function thermocouple were highly linear this would that necessary correct reading.
Unfortunately, since Seebeck Voltage varies with temperature, full-scale transfer function usually fairly complex which require several piece-wise approximations maintain specified accuracy depending upon temperature range interest. this respect, type with lower Seebeck Voltage (51mV/°C @20°C) advantage over type significantly more linear over 1000°C range. in-depth information thermocouples, check reference material available manufacturers such Omega Engineering Inc.[5] While there obvious variations, typical modern electronic thermocouple consists several basic building blocks. illustrated Figure these blocks consist with secondary temperature sensor measure junction where thermocouple connecting wires join; signal conditioning block analog-to-digital converter (ADC). Usually, thermocouple connected precision noise instrumentation amplifier, which provides gain, offset impedance adjustments necessary match level signal generated input multi-bit ADC. turn converts conditioned signal from amplifier into digital format that sent microprocessor From cold junction sensor inputs, computes actual temperature seen junction thermocouple. Some custom conditioning chips such MAX6675 from Maxim AD594 from Analog Devices available that contain both instrumentation amplifier cold junction compensation circuitry particular type such These chips replace first blocks plug directly into input. DS2760 Originally designed monitor Lithium-Ion battery pack, DS2760 provides several capabilities transform simple thermocouple into smart sensor.[6] chip directly digitize millivolt level output produced between cold junctions thermocouple, while it's on-chip temperature sensor continuously monitors temperature cold junction With unique address provides label that permits
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Tech Brief
multiple units operate same twisted pair cable. contains user accessible memory
MEASURE "COLD-JUNCTION" TEMPERATURE
THERMOCOUPLE
INSTRUMENTATION AMPLIFIER
DIGITAL OUTPUT MICROPROCESSOR
GAIN
OFFSET
Figure typical electronically compensated thermocouple consists these three building blocks. storage sensor specific data such type, location date placed into service. This allows DS2760 used with type master uses stored data determine correct calculations make based type temperature cold junction reported on-chip temperature sensor. complete signal conditioning digitizing solution with thermocouple, DS2760 contains 10-bit voltage input, 13-bit temperature 12-bit plus sign current ADC. also provides 32-byte lockable EEPROM memory where pertinent user sensor documentation stored which minimize probability error mislabeling sensors. present application, thermocouple directly connected current inputs that were originally designed read voltage drop developed across milli-Ohm resistor Lithium-Ion battery pack charged discharged. With full scale range millivolts (LSB 15.625mV) converter provides resolution exceeding degree even with lower output Type thermocouple. 1-WIRE THERMOCOUPLE schematic Figure illustrates both simplicity ease with which DS2760 used convert standard thermocouple into smart sensor with multi-drop capability. circuit, Schottky diodes form half wave rectifier that provides power DS2760 `stealing' from during idle communication periods when This discrete implementation parasite power technique used internally 1-Wire devices provide their operating power. remaining Schottky diode package connected across DATA provides circuit protection restricting signal excursions that below ground about minus four tenths volt. Without this diode, negative signal excursions excess tenths volt forward bias parasitic substrate diode DS2760 chip interfere with proper functioning chip. Under master control DS2760, monitors voltage developed between cold junctions thermocouple well measuring temperature cold junction with internal temperature sensor. master uses this information calculate actual temperature junction adding optional resistor (R1), also measured. This useful trouble-shooting verify that voltage available 1-Wire within acceptable limits. When mounting thermocouple board, should connected close DS2760 practical minimal temperature difference exists between these connections chip inside DS2760 package. maintain junctions same temperature copper pour lead placement create isothermal area around point where thermocouple leads attach copper traces PCB. Keeping mind that temperature differentials generate voltage differentials over their entire length, route
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Tech Brief
traces together maintain equal numbers junctions each conductor.
DATA
BAT54S
.1uF
.1uF
DS2760
4,5,6
11,12,13
Type thermocouple
PURPLE
Figure Using DS2760 read thermocouple 1-Wire net.
TMEX 1-Wire trademarks Dallas Semiconductor.
REFERENCE Awtrey. 1999. 1-Wire Rain Gauge," Sensors:56-59. Awtrey. 1998. "The 1-Wire Weather Station," Sensors:34-40. Awtrey. 2000. 1-Wire Humidity Sensor," Sensors:62-63.
Tagging protocol downloaded "Using Thermocouples" DS2760 data sheet.
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Tech Brief
APPENDIX
Waveform template 1-Wire exceeding meters
-2.3V
+0.5
MAX. SLEW RATE NOM. SLEW RATE MIN. SLEW RATE
ACTIVE PULL-UP TURN
ACTIVE PULL-UP REGION 2.2V LOGIC LEVEL
LOGIC ZERO MAX. -0.3V MAX.
ACTIVE PULL-UP THRESHOLD
21.7 MICROSECONDS
This section defines 1-Wire template with lengths exceeding meters. Notice that zero transition shown with characteristic waveform active pull-up passive pull-up resistor problematic with cables this length. pull-up supply recommended, range from volts.
logic zero conclude shorted, communication occur.
Negative Undershoot
Signal excursions below ground must clamped less than 0.6V prevent turning parasitic diodes substrate.
falling edge (Tf)
Because controls system timing, falling edge waveform from master should monotonic. prevent signal disruptions caused unterminated transmission line effects, slew rate must controlled much longer than twice electrical length cable. This works about 1.1volts microsecond (V/µS) given conditions. shown template, this requires microseconds cross 0.8V logic zero threshold level. This slew rate provides acceptable performance with lengths meters. performs well over that range with 1-Wire loading varying from devices. communication successful, must raised voltage greater than 2.2V logic level before master samples. Beginning with TMEX v3.0, sampling occurs 21.7µS after master pulls low. pull-up fails raise voltage above logic threshold before this sampling occurs, master will always
rising edge (Tr)
number 1-Wire devices grows, time required raise line above logic threshold increases. This also occurs network lengthened 50pF capacitance added meter twisted pair cable. Because these effects, 1-Wire meters more, active pull-up must used. Obviously, active pull-up circuit should only during defined range rising edge (zero transition). Conversely, should respond falling edge, active during logic zero time intervals. must trigger rising edge about 0.9V plus minus 0.1V provide acceptable noise margin. Preferably, once triggered will remain until line raised above specified threshold rather than time interval (one-shot). This insures that data line will raised above 2.8V level required recharge parasite power capacitors regardless load. maximum current circuit supply should limited about
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Tech Brief
15mA. Larger currents flowing cable inductance cause problems.
Consult Dallas Semiconductor iButton website www.ibutton.com current product information, application notes data sheets.
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What 1-Wire net? 1-Wire® net, sometimes known MicroLAN, cost base

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