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Application Note Crystal Considerations with Dallas Real Time Clocks
www.dalsemi.com Dallas Semiconductor offers variety real time clocks (RTCs). majority these available either integrated circuits modules. Modules include real time clock integrated circuit, crystal, lithium energy source encapsulated package. This application note intended help those customers choose Dallas Semiconductor real time clock chips rather than modules therefore need attach their crystal. information contained this article will beneficial maximizing accuracy insuring proper operation Dallas real time clocks helping customer select correct crystal providing basic guidelines that should followed when placing crystal layout. This application note will also include elementary discussion effect temperature accuracy real time clocks. This application note also includes section troubleshooting oscillator problems.
CRYSTAL SELECTION
crystal-based oscillator circuit, oscillator frequency based almost entirely characteristics crystal that used. important select crystal that meets design requirements. particular, specified load capacitance critical crystal parameter that often overlooked. This parameter specifies capacitive load that must placed across crystal pins order crystal oscillate specified frequency. crystal manufacturer actually "trims" crystal oscillate nominal frequency given specified load capacitance. Note that capacitance that crystal needs "see" from oscillator circuit, capacitance crystal itself.
NOTE:
Dallas Semiconductor integrated circuits have on-chip bias stabilization networks load capacitors which eliminate need external resistors capacitors. previously stated, load capacitance that crystal "sees" capacitance oscillator circuit itself. change load capacitance oscillator circuit will therefore have affect frequency that oscillator. Likewise, using crystal that that different than actual load capacitance circuit will also affect frequency oscillator. general, using crystal with that larger than load capacitance oscillator circuit will cause oscillator faster than specified nominal frequency crystal. Conversely, using crystal with that smaller than load capacitance oscillator circuit will cause oscillator slower than specified nominal frequency crystal. majority Dallas Semiconductor RTCs have internal capacitance across crystal input pins. Recent offerings from Dallas Semiconductor have 12.5 capacitance software configurable 12.5 proper operation accuracy, crystal that meets part's requirements should used. mentioned above, using crystal with wrong will cause oscillator fast slow. Limited characterization Dallas Semiconductor confirmed this. example, limited characterization DS1485, which designed crystal, revealed that device will approximately minutes/month fast room temperature (25°C) when crystal used. device accuracy within seconds/month when crystal used.
051100
APPLICATION NOTE
Several vendors offer crystals that used with Dallas Semiconductor's real time clocks. These vendors include Epson KDS/Daiwa. Equivalent crystals from other crystal manufacturers also used. reference, Table recommended crystal characteristics.
CRYSTAL SPECIFICATIONS Table
PARAMETER Nominal Frequency Load Capacitance Temperature Turnover Point Parabolic Curvature Constant Quality Factor Series Resistance Shunt Capacitance Capacitance Ratio Drive Level SYMBOL 32.768 70,000 UNITS ppm/°C NOTES
40,000
0.042
CRYSTAL SUPPLIERS
MANUFACTURER KDS/Daiwa Crystal Corp. MODEL DT-38 DT-381 DT-26S (1TB602G00) DT-261S DT-14 DT-26S 32.768 DMX-2632.768 DS-VT-200 C-001R 12.5 C-002RX 12.5 C-002SH-32.768KAE C-002SH-32.768KAAC-004R 12.5 C-005R 12.5 MC-306 12.5 MC-405 12.5 MC-406 12.5 MMTF-32 PACKAGE Cylinder Cylinder Cylinder Cylinder Cylinder Cylinder Cylinder Cylinder Cylinder Cylinder Cylinder CylinderCylinder Cylinder Cylinder
Epson Crystal Corp.
Hooray Electronics
CRYSTAL LOAD COMPENSATION
Dallas Semiconductor does recommend using crystals that have that matches RTC's specification because this will decrease accuracy clock. find, however, that customers sometimes choose 12.5 crystals with RTCs. this situation possible improve decreased accuracy caused mismatch. This accomplished increasing load capacitance that crystal "sees" connecting capacitor parallel with crystal shown Figure rule thumb, approximate capacitor value equal specified load capacitance crystal minus (the approximate load capacitance real time clock oscillator circuit). example, crystal being used, capacitor should placed parallel with improve accuracy oscillator. crystal adjusted crystal manufacturer oscillate specified nominal frequency when load present. capacitor therefore added load oscillator circuit compensate additional load that crystal needs order oscillate specified nominal frequency.
APPLICATION NOTE
CRYSTAL CONFIGURATION WHEN CRYSTAL USED Figure
mentioned, crystal with greater than compensated with external capacitor improve accuracy. However, should noted that oscillator start-up time (the time takes during initial power oscillator stabilize) will increase increased capacitance feedback path oscillator. This capacitance decreases loop gain oscillator, which turn increases start-up time. example, limited characterization shown that start-up time crystal typically less than couple hundred milliseconds, increase seconds when crystal with capacitor parallel used.
NOISE CRYSTAL LAYOUT GUIDELINES
Since crystal inputs Dallas Semiconductor real time clocks have very high impedance (about ohms), leads crystal like very good antennae, coupling high frequency signals from rest system. signal coupled onto crystal pins, either cancel pulses. Since most signals board much higher frequency than 32.768 crystal, more likely pulses where none wanted. These noise pulses counted extra clock "ticks" make clock appear fast. very simple determine noise cause inaccuracy real time clock. following steps illustrate this done. Power system synchronize real time clock known accurate clock. Turn system power off. Wait period time (two hours, hours, etc.). longer time period, easier will measure accuracy clock. Turn system again, read clock, compare known accurate clock. Re-synchronize real time clock known accurate clock. Keep system powered wait period time equal period step Read clock after waiting above period time compare known accurate clock.
APPLICATION NOTE
using above steps, accuracy clock determined both when system powered when system powered down. clock proves inaccurate when system powered accurate when system powered down, problem most likely noise from other signals system. However, clock inaccurate both when system powered when powered down, then problem noise from system. Since possible noise coupled onto crystal pins, care must taken when placing external crystal layout. very important follow basic layout guidelines concerning placement crystal layout insure that extra clock "ticks" couple onto crystal pins. important place crystal close possible pins. Keeping trace lengths between crystal real time clock small possible reduces probability noise coupling reducing length "antennae." Keeping trace lengths small also decreases amount stray capacitance. Keep crystal bond pads trace width pins small possible. larger these bond pads traces are, more likely that noise couple from adjacent signals. possible, place guard ring (tied ground) around crystal. This helps isolate crystal from noise coupled from adjacent signals. Figure illustration using guard ring around crystal. insure that signals other layers directly below crystal below traces pins. more crystal isolated from other signals board, less likely that noise will coupled into crystal. There should minimum 0.200 inches between digital signal trace connected should isolated from component that generates electromagnetic radiation (EMR). This true discrete module type RTCs. also helpful place local ground plane layer immediately below crystal guard ring. This helps isolate crystal from noise coupling from signals other layers. Note that ground plane needs vicinity crystal only entire board. Figure illustration local ground plane. Note that perimeter ground plane does need larger than outer perimeter guard ring. Note that care must taken concerning local ground plane because stray capacitance that introduces. This capacitance will added crystal pins large enough could slow clock down. Therefore, some factors must taken into account when considering adding local ground plane. example, capacitance ground plane approximated where dielectric constant area ground plane thickness layer.
Therefore, determine ground plane appropriate given design, above parameters must taken into account insure that capacitance from local ground plane sufficiently large enough slow down clock.
APPLICATION NOTE
EXAMPLE CRYSTAL PLACEMENT Figure
CLOCK ACCURACY OVER TEMPERATURE
accuracy real time clock directly dependent upon frequency crystal. Therefore, since resonant frequency crystal dependent upon temperature, real time clock will also dependent upon temperature. resonant frequency crystal expressed following basic formula: T0)2 where nominal frequency parabolic curvature constant turnover temperature temperature
values above parameters found data sheet crystal being used. temperature characteristic nominal Daiwa crystal illustrated Figure where 32.768 kHz, -0.042 ppm/°C, 23°C. seen this figure, frequency parabolic relationship temperature; temperature deviates from ideal 23°C, crystal frequency becomes increasingly slower. Figure shows same basic curve; however, axis been changed show frequency deviation ppm) from crystal's nominal frequency 23°C. This curve illustrates more clearly frequency crystal will affect accuracy clock. frequency deviation translates into accuracy approximately minute month. With this mind, quick glance Figure will give approximate expected accuracy given temperature. above information should help provide basic understanding temperature will affect Dallas Semiconductor real time clocks.
APPLICATION NOTE
CRYSTAL FREQUENCY TEMPERATURE Figure
FREQUENCY DEVIATION TEMPERATURE Figure
APPLICATION NOTE
TROUBLESHOOTING
This section provided summary most frequent causes real time clock inaccuracies. Most these problems have been mentioned earlier, repeated here quick reference. This section been divided into three parts. first part will consider factors that cause real time clock fast second part will consider factors that cause real time clock slow. third part deals with clocks that run.
FAST CLOCKS
following most common scenarios that cause crystal-based real time clock fast. Noise coupling into crystal from adjacent signals: This problem been extensively covered above. Noise coupling will usually cause real time clock grossly inaccurate. Wrong crystal: real time clock will typically fast crystal with specified load capacitance greater than used. severity inaccuracy dependent value example using crystal with will cause real time clock about minutes month fast.
SLOW CLOCKS
following most common scenarios that cause crystal-based real time clock slow. Overshoot real time clock input pins: possible cause real time clock slow periodically stopping oscillator. This inadvertently accomplished noisy input signals real time clock. input signal rises voltage that greater than diode drop (~0.3V) above VDD, protection diode input will forward bias, allowing substrate flooded with current. This, turn, will stop oscillator until input signal voltage decreases below diode drop above VDD. This mechanism cause oscillator stop frequently input signals noisy. Therefore, care should taken insure that there overshoot input signals. Another situation that common overshoot problem having input real time clock volt when real time clock battery back-up mode. This problem systems that systematically shut down certain circuits, keep others powered very important insure that there input signals real time clock that greater than battery voltage when device battery back-up mode. Wrong crystal: real time clock will typically slow crystal with specified load capacitance less than used. severity inaccuracy dependent value Stray capacitance: Stray capacitance between crystal pins slow real time clock down. Therefore care must taken when designing layout insure that stray capacitance kept minimum. Temperature: further operating temperature from crystal turnover temperature, slower crystal will oscillate. Figures
APPLICATION NOTE
Clock Does
following most common scenarios that cause crystal-based real time clock run. single most common problem when clock does that (clock halt) EOSC (enable oscillator) been cleared, required. Many Dallas Semiconductor RTCs include circuit that will keep oscillator from running when power first applied. This allows system wait shipment customer, without drawing power from backup battery. When system powered first time, software/firmware must enable oscillator prompt user correct time date. Parasitic loads caused condensation, incomplete solder flux removal other loads keep oscillator from running. Floating supply pins. Many Dallas RTCs have multiple supply inputs. unused input, such Vbat, must grounded. supply left floating, communication with work. Surface mount crystals have some connect) pins. Make sure that correct pins from crystal connected pins. Note that oscillator circuit Dallas RTCs power; signal oscillator input pins only hundred millivolts peak peak
APPLICATION NOTE
REFERENCES
Eaton, Timekeeping Advances Through COS/MOS Technology, Application Note ICAN-6086. Eaton, Micropower Crystal-Controlled Oscillator Design Using COS/MOS Inverters, Application Note ICAN-6539. Meyer, "MOS Crystal Oscillator Design," IEEE Journal Solid-State Circuits, Vol. SC-15, 222-227, April 1980. Williamson, Oscillators Microcontrollers, Intel Application Note AP-155.
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