This application note describes crystal selection layout techniques co
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Application Note Crystal Considerations Dallas Real-Time Clocks
This application note describes crystal selection layout techniques connecting 32,768Hz crystal real-time clock (RTC). also provides information about oscillator circuit-design criteria, system design, manufacturing issues.
OSCILLATOR BASICS
oscillator used Dallas Semiconductor RTCs CMOS inverter variation Pierce-type oscillator. Figure shows general configuration. These RTCs include integrated load capacitors (CL1 CL2) bias resistors. Pierce oscillator uses crystal operating parallel mode. Crystals used parallel mode specified certain frequency with specific load capacitance. crystal trimmed factory operate specified frequency specified capacitive load. oscillator correct frequency, oscillator circuit must load crystal with correct capacitive load.
Figure Oscillator with Internal Load Capacitors Bias Resistors
COUNTDOWN CHAIN
REGISTERS
CRYSTAL
ACCURACY
frequency accuracy crystal-based oscillator circuit mainly dependent upon accuracy crystal accuracy match between crystal oscillator capacitive load. capacitive load less than crystal designed for, oscillator runs fast. capacitive load greater than what crystal designed for, oscillator runs slow. addition errors from crystal load match, crystals vary from their base frequency ambient temperature changes. Dallas RTCs "tuning fork" crystals, which exhibit error over temperature, shown Figure error 20ppm equivalent approximately minute month.
120902
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Figure Crystal Frequency Temperature
-0.04ppm/°C2 (typ)
32768 32767.5 32767 32766.5 32766 32765.5 32765 32764.5 32764 32763.5 32763 32762.5
ERROR (ppm)
-100 -120 -140 -160 -180 -200 TEMPERATURE (°C)
32762 32761.5
Note: better accuracy required, TCXO such DS32kHz used.
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CRYSTAL PARAMETERS
Crystals have several fundamental characteristics that important design oscillator circuit. Figure shows equivalent circuit crystal. Near resonate frequency circuit consists series circuit including motional inductance motional resistance motional capacitance parallel component shunt capacitance crystal. equivalent series resistance (ESR), also known motional resistance, impedance crystal when reactive components crystal cancel, i.e., resonant frequency. quality factor (Q-Factor inversely proportional. lower ESR, less energy that lost crystal. crystal with high requires more power operate takes longer start.
Figure Crystal Equivalent Circuit
load capacitance capacitive load oscillating circuit seen from pins crystal. Figure shows capacitance parallel with crystal. load capacitors used oscillator circuit, CL2, plus stray capacitance circuit, combine create overall load capacitance. Dallas RTCs have integrated capacitors. Care should taken minimize stray capacitance board layout. following formula shows relationship between load capacitor values: (CL1 CL2) (CL1 CL2) CSTRAY]
Figure Crystal Load Capacitors Equivalent Parallel Load
Most crystals allow maximum drive level 1µW. Dallas RTCs under 1µW. Drive level determined using following formula: 32,768(CO CL)VRMS]2, where VRMS value voltage across crystal.
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OSCILLATOR STARTUP TIME
Oscillator startup times highly dependent upon crystal characteristics, board leakage, layout. High excessive capacitive loads major contributors long startup times. circuit using crystal with recommended characteristics proper layout usually starts within second.
Table Crystal Specifications
PARAMETER Nominal Frequency Frequency Tolerance Load Capacitance Temperature Turnover Point Parabolic Curvature Constant Quality Factor Series Resistance Shunt Capacitance Capacitance Ratio Drive Level SYMBOL delta C0/C1 32.768 70,000 UNITS ppm/°C
40,000
0.042
Table Crystal Suppliers
MANUFACTURER PART FREQUENCY TOLERANCE (ppm) (min) typ* DRIVE LEVEL (µW) TEMP ALTERNATE CL-pF RANGE (°C) SURFACE THRUHOLE
Citizen Citizen ECS, Inc. ECS, Inc. ECS, Inc. KDS/Daiwa KDS/Daiwa Pletronics Pletronics Seiko Seiko
CFS-145 CFS-206 ECS-3X8 ECS-2X6 ECS-1X5 DT-26 DT-38 WX15 WX26 NC-38 C-001R
PART
80,000* 70,000* 90,000 90,000 90,000 40,000 50,000
8.0* 12.5 12.5 12.5 12.5 12.5 12.5 12.5 12.5 12.5
-10~+60 -10~+60 -10~+60 -10~+60 -10~+60 -10~+60 -10~+60 -10~+60 -10~+60 -20~+60 -10~+60 -10~+60
MANUFACTURER Citizen Citizen ECS, Inc. ECS, Inc. ECS, Inc. KDS/Daiwa KDS/Daiwa Pletronics Pletronics Seiko Seiko
CFS-145 CFS-206 ECS-3X8 ECS-2X6 ECS-1X5 DT-26 DT-38 WX15 WX26 NC-38 C-001R
PACKAGE DIMENSIONS (mm) x5.1
MANUFACTURER ORDERING NUMBER
1TB602G00 WX15-32.768k-6pF WX26-32.768k-6pF
*Cylinder-type dimensions barrel diameter length. They exclude leads. dimensions approximate.
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POWER CONSUMPTION
Many RTCs designed operate from battery supply. typical application, small lithium battery used oscillator clock circuitry while main supply off. maximize battery life, oscillator must using little power possible. accomplish this, some design tradeoffs must made.
Negative Resistance
typical high-frequency oscillator circuits, normal circuit designed with margin ESR. Low-frequency crystals typically have higher ESRs. oscillator have less than margin negative resistance. oscillator circuit with margin normally consumes less current. result, oscillator often sensitive relatively small amounts stray leakage, noise, increase ESR. oscillator circuit influences power consumption. with 12.5pF internal loads consumes more power than that loads. However, oscillator with 12.5pF load capacitors usually less susceptible noise.
NOISE CRYSTAL LAYOUT GUIDELINES
Since crystal inputs Dallas RTCs have very high impedance (about 109), leads crystal like very good antenna, coupling high-frequency signals from rest system. signal coupled onto crystal pins, either cancel pulses. Since most signals board much higher frequency than 32.768kHz crystal, more likely pulses where none wanted. These noise pulses counted extra clock "ticks" make clock appear fast. following steps illustrate determine noise causing fast: Power system synchronize known accurate clock. Turn system power off. Wait period time (two hours, hours, etc.). longer time period, easier measure accuracy clock. Turn system again, read clock, compare known accurate clock. Resynchronize known accurate clock. Keep system powered wait period time equal period Step Read clock after waiting above period time compare known accurate clock. 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 board layout. very important follow basic layout guidelines concerning placement crystal board layout ensure extra clock ticks couple onto crystal pins. important place crystal close possible pins. Keeping trace lengths between crystal small possible reduces probability noise coupling
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reducing length antenna. 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 (connected ground) around crystal. This helps isolate crystal from noise coupled from adjacent signals. Figure illustration using guard ring around crystal. ensure that signals other board layers directly below crystal below traces pins. more crystal isolated from other signals board, less likely that noise 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 board layer immediately below crystal guard ring. This helps isolate crystal from noise coupling from signals other board 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. capacitance between traces/pads ground plane added internal load capacitors (CL1 CL2). Therefore, some factors must taken into account when considering adding local ground plane. example, capacitance ground plane approximated following equation: A/t, where dielectric constant board area traces/pads thickness board layer Therefore, determine ground plane appropriate given design, above parameters must taken into account ensure that capacitance from local ground plane sufficiently large enough slow down clock.
Figure Recommended Layout Crystal
Local ground plane (layer guard ring
crystal
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Fast Clocks
following most common scenarios that cause crystal-based fast. Noise coupling into crystal from adjacent signals. This problem been extensively covered above. Noise coupling usually causes grossly inaccurate. Wrong crystal. typically runs fast crystal with specified load capacitance (CL) greater than RTC-specified load capacitance used. severity inaccuracy dependent value example, using crystal with 12pF designed with causes about minutes month fast.
Slow Clocks
following most common scenarios that cause crystal-based slow. Overshoots input pins. possible cause slow periodically stopping oscillator. This inadvertently accomplished noisy input signals RTC. 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, stops oscillator until input signal voltage decreases below diode drop above VDD. This mechanism cause oscillator stop frequently input signals noisy. Therefore, care should taken ensure there overshoot input signals. Another situation that common overshoot problem having input when battery-backup mode. This problem systems that systematically shut down certain circuits keep others powered very important ensure there input signals that greater than battery voltage (unless stated otherwise device data sheet) when device battery-backup mode. Wrong crystal. typically runs slow crystal with specified less than RTC. severity inaccuracy dependent value Stray capacitance. Stray capacitance between crystal pins and/or ground slow down. Therefore, care must taken when designing board layout ensure stray capacitance kept minimum. Temperature. further operating temperature from crystal turnover temperature, slower crystal oscillates. Figures
Clock Does
following most common scenarios that cause run. single most common problem when clock does that (clock halt) EOSC (enable oscillator) been cleared, required. Many Dallas RTCs include circuit that keeps 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. Surface mount crystals have some N.C. connect) pins. Make sure that correct pins from crystal connected pins.
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CRYSTAL MANUFACTURING ISSUES
Tuning fork crystals should exposed ultrasonic cleaning. They susceptible damage from resonant vibration. Crystals should exposed temperatures above their maximum ratings. Exposure excessive temperatures damage crystal, usually increase ESR. Crystal "cans" should soldered board. This sometimes done ground case crystal. Soldering directly case crystal usually subjects unit excessive temperatures. RTCs should generally used noncondensing environments. Moisture forming around oscillator conductors cause leakage, which cause oscillator stop. Conformal coatings used protect circuit, however, conformal coating itself cause problems. Some conformal coatings, especially epoxy-based materials, have unacceptable levels ionic contamination. addition, conformal coatings can, board surface sufficiently cleaned prior conformal coating, cause contaminants concentrate around leads traces. Solder flux residue cause leakage between pins. oscillator circuits especially sensitive leakage because their low-power operation. Leakage between oscillator input output, leakage ground, often keep oscillator from running.
REFERENCES
John Vig, Quartz Crystal Resonators Oscillators Frequency Control Timing Applications, www.rakon.com/ Advanced Crystal Technology, www.act.co.uk/ Electronics, www.foxonline.com/ ECS, Inc. International, www.ecsxtal.com/
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