Powering UNI/O® Device Through SCIO Author: Chris Parris Microchi

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Powering UNI/O® Device Through SCIO Author: Chris Parris Microchi

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AN1213
Powering UNI/O® Device Through SCIO
Author: Chris Parris Microchip Technology Inc. This application note describes standard halfwave rectifier capacitor circuit added allow power extracted parasitically from SCIO signal. Guidance offered selecting capacitor value diode based application parameters such voltage serial frequency. modifications standard UNI/O protocol necessary. assumed that reader already familiar with basic terms operation UNI/O bus. Within this application note, equations shown with heavy outline around them critical equations used calculate important parameter. other equations provided show steps necessary deriving final equations. Figure shows half-wave rectifier capacitor circuit connected UNI/O serial EEPROM.
embedded systems become smaller, growing need exists minimize usage communication between devices. Microchip addressed this need developing UNI/O® bus, low-cost, easyto-implement solution requiring only single communication. standard configuration UNI/O combines serial clock, data, address, control signals onto SCIO signal. This allows UNI/O devices enhance application facing restrictions available stemming from connectors, board space, master device. some applications benefit from further reduction connections.
FIGURE
CIRCUIT EXTRACTING POWER FROM SCIO
SOT-23
11XXX
SCIO
Master
2008 Microchip Technology Inc.
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AN1213
DESCRIPTION OPERATION
circuit shown Figure allows power extracted from SCIO storing energy capacitor, This energy then used power UNI/O slave during times when master driving bus. When master drives SCIO high, diode, becomes forward-biased allows current flow through UNI/O slave, well charge capacitor. Charge will continue build until capacitor's voltage equals master's high output voltage minus voltage drop across diode. When master drives SCIO low, diode becomes reverse-biased prevents capacitor from discharging back through SCIO. this situation, well when slave driving SCIO, capacitor will discharge powering slave directly. Because UNI/O uses Manchester encoding, high signal SCIO must occur every bit. since capacitor only charged master, worstcase situation when reading data from slave. This effectively results square wave input into rectifier circuit with pulse width 4-6%, depending input jitter. This because master will only driving SCIO high during 40-60% every bits. Note that this very short period time, critical that proper components selected ensure correct operation. Figure shows example capacitor cyclically charges discharges every byte during read operation, assuming constant current consumption slave. solid line voltage capacitor, dotted line represents voltage SCIO when master driving, which only occurs during read operation.
FIGURE
EXAMPLE CAPACITOR CHARGING DISCHARGING DURING READ
VCMAX VCMIN
SELECTING RIGHT DIODE
their forward voltage drop fast reverse recovery, recommended that Schottky diode used. even after limiting only Schottky diodes, there still many different ones from which choose. When selecting diode, following parameters should considered: Reverse Leakage Current (IR) Reverse Recovery Time (TRR) Reverse Voltage (VR) Forward Current (IF) Forward Voltage (VF)
cantly affect results calculations. However, minimizing leakage current when selecting diode still recommended.
Reverse Recovery Time (TRR)
Reverse recovery time amount time necessary diode change from forward bias reverse bias. During this time, excess reverse current allowed flow backwards through diode. this application, this means capacitor will discharge back through diode during this time. However, Schottky diodes, reverse recovery time very fast, usually less than This typically results charge loss, during reverse recovery time, less than compared loss experienced when slave outputting considered negligible purposes this application note.
Reverse Leakage Current (IR)
Although Schottky diodes generally have higher reverse leakage currents than their junction counterparts, this parameter typically considered negligible purposes this application note. Even leakage currents around will signifi-
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2008 Microchip Technology Inc.
AN1213
Reverse Voltage (VR)
selected diode should able withstand, minimum, reverse voltage equal 2*VCC master. This times when capacitor fully charged driven low, will provide adequate guardband ensure diode damaged excess reverse voltage.
EQUATION
AVERAGE DIODE CURRENT DURING CHARGE
DCHG
Forward Voltage (VF)
When system achieves stability, voltage capacitor will oscillate between VCMAX VCMIN, described Section "Description Operation". value VCMAX determined Equation where VMOH high-level output voltage master device while sourcing average diode current level, calculated Equation
Forward Current (IF)
During both read write operations, capacitor being discharged more time each byte than being charged. Because this, more current must flow average into capacitor during charge than flowing average capacitor during discharge. Read operations worst-case, because master only charges capacitor during high time every byte. This means that large amount current must flow through diode into capacitor while being charged account loss during discharge. very important that selected diode able withstand this elevated level current. beginning command, capacitor will charged nearly VCC. However, within command, capacitor will begin discharge until system achieves point stability. This point where charge removed from capacitor during discharge equal charge added capacitor during charging. charge loss during discharge dependent upon current being consumed, ICCR, slave device follows that charge gain during charging also depends ICCR. following equation shows calculate charge current based ICCR amount time charging discharging.
EQUATION
MAXIMUM VOLTAGE
CMAX
VCMAX VCMIN values seen UNI/O slave. VCMIN must above minimum operating voltage slave device, also high enough ensure that slave's high-level output voltage (VSOH) exceeds master's high-level input voltage (VMIH). value VCMIN depends chosen size capacitor well VCMAX, calculated Section "Sizing Capacitor". Because this dependency, VCMIN affected both forward voltage drop across diode capacitor value. absolute maximum ratings UNI/O slave specify that SCIO must above slave's more than 1.0V. This means that only requirement forward voltage drop diode that less than 1.0V, which very easily Schottky diodes. However, since forward voltage also affects capacitor value, then forward voltage should still minimized much possible.
EQUATION
CAPACITOR CHARGING CURRENT
DCHG ICCR DCHG DCHG
average current, flowing through diode during capacitor charge combination current flowing into capacitor current flowing into UNI/O slave. This current value calculated using Equation
2008 Microchip Technology Inc.
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AN1213
SIZING CAPACITOR
slave's high-level output voltage does reach master's high-level input voltage (VMIH), master detect high levels correctly SCIO. Equation standard equation calculating current when charging discharging capacitor. also increase. Therefore, operating faster frequency will actually allow smaller capacitor. Once minimum capacitor value calculated using Equation VCMIN must calculated using Equation ensure that above minimum operating voltage UNI/O slave. not, then larger capacitor value must used.
EQUATION
CAPACITOR CURRENT
Applying Equation Equation discharging capacitor yields Equation which shows calculate VCMIN based particular capacitor value,
OTHER CONSIDERATIONS
Power-Up Timing
Before initiating communication with UNI/O slave, including low-to-high edge release device from standby pulse, capacitor must charged minimum operating voltage slave. amount time necessary charge capacitor depends capacitor value impedance device performing charging. pull-up resistor being used charge, Equation used calculate amount time necessary charge capacitor.
EQUATION
MINIMUM VOLTAGE
DCHG DCHG CMAX CMIN CMIN CMIN DCHG CMAX
EQUATION
CAPACITOR CHARGING
Alternatively, master output driver used charge capacitor. This will typically offer significantly faster charging time. However, charging time dependent upon master output driver impedance which varies both master device output voltage. this reason, much simpler characterize amount time needed specific application measuring charge time using final components. This measurement should guardbanded ensure robust design.
DCHG
Applying requirement that VSOH higher than VMIH Equation solving yields following equation:
EQUATION
MINIMUM VALUE
VCMIN 0.5V DCHG -VMOH 0.5V
Note that minimum capacitor value directly proportional amount time spent discharging, which inversely proportional frequency. discharge time increases, capacitor value must
Pull-Up Resistor
pull-up resistor SCIO recommended standard UNI/O configuration. This ensure idle during times when UNI/O slave being powered device driving (for example, when master held Reset). when power being extracted from SCIO parasitically, this condition will occur concern.
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2008 Microchip Technology Inc.
AN1213
When UNI/O slave driving SCIO high, path current flow created through slave output driver slave's connection. pull-up resistor used, then will provide small amount current that flows through output driver help power slave when slave driving high. This effectively raises VCMIN since capacitor having provide less current power slave. However, because current through pull-up very small, does have significant effect results calculations provided above. pull-up will also raise slave's high-level output voltage creating voltage divider with output driver. This results additional guardband which will provide more robust design. Because guardband provided, pullup resistor recommended, required. pullup value should selected same manner standard UNI/O applications typical). Otherwise, serial EEPROM will likely lose power before write cycle completed, causing data being written corrupted.
EXAMPLE CALCULATIONS
following example shows equations described above select correct components. Table lists important parameters example.
TABLE
Parameter ICCR TDCHG TCHG VMOH VMIH Note
EXAMPLE PARAMETERS
Value 237.5 4.35 0.332
Units
12.51
Polling
polling feature offers simple method maximizing data throughput, requires consumption additional current. During write cycle, write operating current (ICCW) drawn operate charge pump which allows data stored array. polling adds read operating current level this, which results current draw higher than normal read operation. recommended that master power slave device during write operation driving SCIO high full write cycle time, TWC. polling necessary, procedures described previously selecting capacitor value diode performed using combined ICCR ICCW current value.
TDCHG TCHG based frequency with input jitter. Based diode selected after calculating
Note that will vary depending selected diode, VMOH VMIH will vary depending master device. this example, Equation yields average diode current Knowing this value allowed selection diode. Applying parameters Equation results minimum capacitor value 0.469 Also, Equation yields VCMIN value 2.50V, which within valid operating voltage range UNI/O slave devices.
FIGURE
OSCILLOSCOPE PLOT EXAMPLE READ OPERATION
VCMAX
VCMIN
2008 Microchip Technology Inc.
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AN1213
Figure shows oscilloscope plot middle read operation after reached stable oscillating range. cursors mark second half bit, while master charging capacitor. components used were selected described above. Note that VCMIN predicted equations. This because equations assume UNI/O slave will consume maximum specified current, ICCR, device consumed less than maximum this example.
SUMMARY
This application note offered details examples combining power SCIO over single connection UNI/O application. This provides fewer required connections, leading smaller lower costing system designs. procedures described require small amount additional effort over standard UNI/O implementation, following them will allow more robust design.
DS01213B-page
2008 Microchip Technology Inc.
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Trademarks Microchip name logo, Microchip logo, Accuron, dsPIC, KEELOQ, KEELOQ logo, MPLAB, PIC, PICmicro, PICSTART, rfPIC, SmartShunt UNI/O registered trademarks Microchip Technology Incorporated U.S.A. other countries. FilterLab, Linear Active Thermistor, MXDEV, MXLAB, SEEVAL, SmartSensor Embedded Control Solutions Company registered trademarks Microchip Technology Incorporated U.S.A. Analog-for-the-Digital Age, Application Maestro, CodeGuard, dsPICDEM, dsPICDEM.net, dsPICworks, dsSPEAK, ECAN, ECONOMONITOR, FanSense, In-Circuit Serial Programming, ICSP, ICEPIC, Mindi, MiWi, MPASM, MPLAB Certified logo, MPLIB, MPLINK, mTouch, PICkit, PICDEM, PICDEM.net, PICtail, PIC32 logo, PowerCal, PowerInfo, PowerMate, PowerTool, REAL ICE, rfLAB, Select Mode, Total Endurance, WiperLock ZENA trademarks Microchip Technology Incorporated U.S.A. other countries. SQTP service mark Microchip Technology Incorporated U.S.A. other trademarks mentioned herein property their respective companies. 2008, Microchip Technology Incorporated, Printed U.S.A., Rights Reserved. Printed recycled paper.
Microchip received ISO/TS-16949:2002 certification worldwide headquarters, design wafer fabrication facilities Chandler Tempe, Arizona; Gresham, Oregon design centers California India. Company's quality system processes procedures PIC® MCUs dsPIC® DSCs, KEELOQ® code hopping devices, Serial EEPROMs, microperipherals, nonvolatile memory analog products. addition, Microchip's quality system design manufacture development systems 9001:2000 certified.
2008 Microchip Technology Inc.
DS01213B-page
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01/02/08
DS01213B-page
2008 Microchip Technology Inc.

Powering UNI/O® Device Through SCIO Author: Chris Parris Microchi

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