Application Note AP-728 Document Number: 292276-002 Intel® F

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Application Note AP-728 Document Number: 292276-002 Intel® F

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Intel® Family Real Time Clock (RTC) Accuracy Considerations under Test Conditions
Application Note AP-728
Document Number: 292276-002
Intel® Family Accuracy Considerations under Test Conditions
Information this document provided connection with Intel® products. license, express implied, estoppel otherwise, intellectual property rights granted this document. Except provided Intel's Terms Conditions Sale such products, Intel assumes liability whatsoever, Intel disclaims express implied warranty, relating sale and/or Intel products including liability warranties relating fitness particular purpose, merchantability, infringement patent, copyright other intellectual property right. Intel products intended medical, life saving, life sustaining applications. Intel make changes specifications product descriptions time, without notice. Designers must rely absence characteristics features instructions marked "reserved" "undefined." Intel reserves these future definition shall have responsibility whatsoever conflicts incompatibilities arising from future changes them. Intel® family contain design defects errors known errata which cause product deviate from published specifications. Current characterized errata available request. Contact your local Intel sales office your distributor obtain latest specifications before placing your product order. Copies documents which have ordering number referenced this document, other Intel literature obtained calling 1-800548-4725 visiting Intel's website http://www.intel.com.
Intel trademark registered trademark Intel Corporation subsidiaries United States other countries. *Other names brands claimed property others. Copyright Intel Corporation, 2001
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Intel® Family Accuracy Considerations under Test Conditions
Contents
Background External Circuit External Battery Connection External Circuit
External Capacitor Values Signals 3.2.1 Signals RTCX1 RTCX2 Pins 3.2.2 VBIAS. 3.2.3 SUSCLK Accuracy 3.3.1 Voltage. 3.3.2 External Capacitance Load 3.3.3 Circuit Layout Considerations 3.3.4 Environmental Conditions Accuracy Determination Frequency Measurement Technique 3.4.1 Using Timekeeping Baseline Device.10 3.4.2 Using Time Interval Analyzer
Influences under Environmental Stress Optimizations Stress Testing Conclusion
Figures
Input Output Intel® ICHn Circuit ICHn External Circuit Diode Circuit Connect External Battery External Internal Circuitry Oscillator Clock Input Connection Pins When Using Internal RTC. Example OP-Amp Configuration Screen-Capture RTCX1 RTCX2 Signals Scope Menu.11
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Intel® Family Accuracy Considerations under Test Conditions
Revision History
Rev. Initial Release General updates Draft/Changes Date March 2001
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Background External Circuit
Background External Circuit
Intel® chipsets using Intel® family component crystal circuit generate low-swing input sine wave. This input amplified driven back crystal circuit RTCX2 signal. Internal ICHn (any member family), RTCX1 signal amplified drive internal logic well generate free running full swing clock output system use. This output ICHn called SUSCLK. This illustrated Figure 1-1. Figure 1-1. Input Output Intel® ICHn Circuit
Low-Swing 32.768kHz Sine Wave Source Full-Swing 32.768kHz Output Signal
RTCX1
Internal Oscillator
SUSCLK
ICHn
low-swing 32.768 clock source generated circuit implemented motherboard (external ICHn component). schematic illustrated below. Figure 1-2. ICHn External Circuit
3.3V VCCRTC
RTCX2 RTCX1 0.047uF 18pF 18pF
Vbatt
32.768 Xtal
VBIAS
Notes Reference Designators Arbitrarily Assigned 3.3V Active Whenever System Plugged Vbatt Voltage Provided Battery
VBIAS, VCCRTC, RTCX1, RTCX2 ICHn pins VBIAS used bias ICHn Internal Oscillator VCCRTC powers well ICHn RTCX1 Input Internal Oscillator RTCX2 feedback external crystal
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Background External Circuit
crystal network employs generate 32.768 sine wave. Actual values these components dependent crystal component specification, trace lengths motherboard, crystal's load capacitance. network composed filters 32.768 sine wave produces average voltage level from sine wave power VBIAS.
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External Battery Connection
External Battery Connection
module requires external battery connection maintain functionality while ICHn powered system. recommended batteries are: Duracell* 2032, 2025, 2016, which give many years operation. Batteries rated storage capacity. battery life calculated dividing capacity average current required. example, battery storage capacity (assumed usable) average current required battery life will least: 170,000 56,666 years. voltage battery affect accuracy. battery voltage must greater than time ensure accuracy clock. battery must connected ICHn isolation diode circuit. diode circuit allows ICHn's RTC-well powered battery when system power available, system power when available. this, diodes reverse biased when system power available. Figure example diode circuitry that used. noted, standby power supply should used desktop mobile system provide continuous power when available, which will significantly increase battery life. Figure 2-1. Diode Circuit Connect External Battery
3V_STBY
Vcc(RTC) 0.1uF
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External Battery Connection
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External Circuit
External Circuit
ICHn module requires external oscillating source 32.768 connected RTCX1 RTCX2 pin. Figure represents internal external circuitry that comprise oscillator RTC. Figure 3-1. External Internal Circuitry Oscillator
Internal External
required external capacitors that affect accuracy RTC. Choosing right capacitor value important maintain accuracy. Section presents some guidelines choosing these values. Note: Even ICHn internal used, still necessary supply clock input ICHn because other signals gated that clock suspend modes. However, this case, frequency accuracy clock inputs critical; cheap oscillator used single clock input driven into X1with left connect, input voltage ICHn Figure illustrates connection. However, this validated supported configuration.
Figure 3-2. Clock Input Connection Pins When Using Internal
Internal
External Connection
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External Circuit
External Capacitor Values
maintain RTC's accuracy, external capacitor values should chosen provide manufacturer's specified load capacitance (Cload) crystal when combined with parasitic capacitance circuits traces, socket used), package, ICHn input capacitances. following equation used choose external capacitance values: Cload [(C1 Cin1 Ctrace1)*(C2 Cin2 Ctrace2)]/[(C1 Cin1 Ctrace1 Cin2 Ctrace2)] Cparasitic Where:
Cload Crystal's load capacitance. This value obtained from Crystal's specification. Cin1, Cin2 input capacitances RTCX1, RTCX2 pins ICHn. These values
obtained ICHn's data sheet.
Ctrace1, Ctrace2 Trace length capacitances measured from Crystal terminals RTCX1,
RTCX2 pins. These values depend characteristics board material, width signal traces length traces. typical value this capacitance approximately equal Ctrace trace length pF/inch electrode plates dielectric constant crystal blank inside crystal part. Refer crystal's specification obtain this value.
Cparasitic Crystal's parasitic capacitance. This capacitance created existence
Ideally, chosen such that Using equation Cload above, value C1/C2 calculated give best accuracy (closest 32.768 kHz) circuit room temperature. However, chosen such that Then trimmed obtain 32.768 kHz. certain conditions, both C1/C2 values shifted away from theoretical values (calculated values from above equation) obtain closest oscillation frequency 32.768 kHz. When values smaller then theoretical values, oscillation frequency will higher. following example will illustrates practical values case that theoretical values guarantee accuracy temperature condition: Example According required load capacitance typical crystal that used with ICHn, calculated values room temperature yield 32.768 oscillation. frequency stability crystal gives (assumed that circuit °C). This makes circuit oscillate 32.767246 instead 32.768 kHz. values chosen instead this will make crystal oscillate higher frequency room temperature (+23 ppm) this configuration makes circuit oscillate closer 32.768 value practical value.
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External Circuit
Note that temperature dependency crystal frequency parabolic relationship (ppm degree squared). effect crystal's frequency when operating below room temperature) same when operating above room temperature). values calculated from equation above only theoretical values. Ideally, these values will same practical values. However, values should chosen based values that give best accuracy circuit. every specific board design, these practical values slightly different from theoretical values.
3.2.1
Signals
Signals RTCX1 RTCX2 Pins
oscillation signal sinusoidal signal that provides 32.768 frequency ICHn. This small analog signal with peak-to-peak voltage about voltage swing RTCX2 will slightly larger than voltage swing RTCX1, since RTCX2 signal amplified through internal inverter ICHn. circuit oscillates with extremely bias current (IccRTC µA-please refer specific component datasheet IccRTC value) therefore this signal very sensitive environmental conditions such board residue (solder flux, dust), humidity, etc. example, touching directly this circuit cause leakage that completely attenuate oscillation signal make oscillation stop. Measuring RTCX1 RTCX2 accomplished only using following technique minimize measurement equipment loading effects. Configure Analog Devices AD823* (AD823AN) equivalent with very high input impedance order 10E12 10E14), unity gain follower shown below. Note: This different depending Op-Amp used. Figure illustration. conductor between signal being measured input must less than with direct connection preferable. must connected voltage source that time, such external supply battery. Collect electrical characteristics: Place oscilloscope probe with sufficient ground reference Op-Amp output. oscilloscope should configured mV/DIV µs/DIV with trigger approximately until capture obtained. Record RTCX1 Vpp. Optional data captured such Offset RTCX1 RTCX2, level VBIAS, IccRTC.
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External Circuit
Figure 3-3. Example OP-Amp Configuration
AD822AR-3V Op-Amp configured "unity gain"
RTCX1 RTCX2 VBIAS Connect Oscilloscope
Figure 3-4, below, shows example screen capture RTCX1 RTCX2 signals: Figure 3-4. Screen-Capture RTCX1 RTCX2 Signals Scope
3.2.2
VBIAS
VBIAS voltage level that necessary biasing oscillator circuit. This voltage level filtered from oscillation signal network (see Figure 1-2). Therefore, self-adjusted voltage. Board designers should manually bias voltage level VBIAS. Checking VBIAS voltage level used testing purposes only determine right bias condition circuit.
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External Circuit
VBIAS should least network will filter most signal that exist this pin, however, noise this should kept minimal order guarantee stability oscillation. Probing VBIAS requires same technique probing RTCX1, RTCX2 signals (using OpAmp) VBIAS also very sensitive environmental conditions.
3.2.3
SUSCLK
SUSCLK square wave-form signal output from oscillation circuit. Depending quality oscillation signal RTCX1 (largest voltage swing), SUCCLK duty cycle between 30-70%. SUSCLK duty cycle beyond 30%-70% range, indicates poor oscillation signal RTCX1 RTCX2. SUSCLK probed directly using normal probe input impedance probe) appropriate signal check frequency determine accuracy ICHn's Clock (see Section 3.4.2 details).
Accuracy
This ICHn circuit current circuit designed provide accurate time keeping service, extremely current consumption (IccRTC) maximum (please refer specific component datasheet IccRTC value). result, this circuit subject adverse influences, which must addressed understood ensure best possible accuracy. accuracy affected following primary factors:
3.3.1
Voltage
accuracy affected voltage. general, when battery voltage decays, accuracy also decreases. High accuracy (less than which included Crystal's tolerant) obtained when voltage bigger than example: equivalent with 1.728 error day. ((Fo 32.7680) 32.768) 10^6 hours 86400 (86400 sec) 86400) 1000000 1.728
3.3.2
External Capacitance Load
external capacitance load values combined with external capacitor values, capacitance circuit's trace, socket, package. These values should matched actual load capacitance required crystal used accuracy. Refer Section guidelines calculate external capacitance.
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External Circuit
3.3.3
Circuit Layout Considerations
Since circuit very sensitive requires high accuracy oscillations, reasonable care must taken during layout routing circuit. Some recommendations are:
Reduce trace capacitance minimizing trace length. ICHn requires trace length
less than inch each branch (from crystal's terminal RTCXn pin). Routing circuit should kept simple reduce trace length measurement increase accuracy calculating trace capacitances. Trace capacitance depends trace width dielectric constant board's material. FR-4, 5-mil trace approximately inch.
Trace signal coupling must reduced. Avoid routing signals noisy periodic signals
close parallel RTCX1, RTCX1 VBIAS.
Ground referencing highly recommended.
3.3.4
Environmental Conditions
crystal temperature itself affect accuracy. higher lower temperature (compared room temperature), lower accuracy. Unless this factor compensated using practical configuration value Section details). Condensation from humidity also affect accuracy leakage signals (see Section details). Note: heat will damage Crystal when reworking boards. Please follow specification Crystal right temperature operation.
Accuracy Determination Frequency Measurement Technique
accuracy clock determined many different methods. common methods checking accuracy are:
Using timekeeping-baseline-device (via BIOS system NVRAM) Using Time-Interval-Analyzer SUSCLK signal
3.4.1
Using Timekeeping Baseline Device
Accuracy baseline device crucial determining accuracy. watch clock sufficiently accurate range accuracy over periods used during these tests. required that Global Positioning System device (GPS) used instead. contains clock that reset acquisition time extremely accurate time. Over time though, time will drift like other clock. this reason should reset, powered outside allowed synchronize, within minutes either setting time motherboard using standard readout.
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External Circuit
3.4.2
Using Time Interval Analyzer
Using HP-TIA (Time Interval Analyzer) very accurate measuring frequency. This tool measure 32.768 frequency with tolerance typically less than (mili-Hertz). ideal this equipment determine configurations fork tuning capacitors which described Section 3.1. This most accurate frequency circuit. following description HP-TIA calculate frequency circuit: frequency oscillator monitored SUSCLK signal ICHn. Locate SUSCLK signal board connect this signal probe HP-TIA with properly connected ground pin. HP-E1743A program. Target frequency should 32.768 kHz.(Refer HP-TIA user manual more information setup equipment.) menu, click "Measure/View Setup," setup dialog will appear shown Section
Figure 3-5. Menu
pacing number number clock edges that will count before capture each time stamp. Depending memory size, number samples varies (1024, 4096, etc.), chose largest number possible. measured frequency (F0) will calculated based upon average period samples. This frequency clock. following formula used calculate PPM: |(F0 32.768 kHz)| (32.768 kHz)] 10E6
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External Circuit
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Influences under Environmental Stress
Influences under Environmental Stress4
Crystal Characteristics: Typical 32.768 crystals have operating temperature ceiling thus limit test temperature accordingly. addition, temperature coefficient these crystals cause time-loss approximately sec/day Fork Capacitor Tuning: timekeeping dependent RTCX1 input voltage swing. Oscillation that marginal result failure meet this input thus "ticks" clock missed resulting time-loss. Optimum this RTCX1 signal achieved accurately matching crystal's C-load specification (typically pF). Board Leakage: Since this circuit operates such current, very sensitive sources leakage motherboard. Manufacturing residue cause leakage well condensation board encountered during temperature and/or humidity testing. Timekeeping Baseline Device: Time motherboards typically compared baseline device, like watch other baseline clock device believed accurate. case that most timekeeping devices like this sufficiently accurate. This cause additional source error.
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Influences under Environmental Stress
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Optimizations Stress Testing
Optimizations Stress Testing
obtain best accuracy environmental stress conditions, above listed factors optimized. While there little that done change crystal characteristics, there opportunities maximize oscillator voltage swing minimize board leakage; described below: Fork Capacitor Tuning: fork capacitors (C1/C2) must chosen provide greatest RTCX1) still providing best accuracy. This typically accomplished laboratory analysis specific each motherboard (see Section details). Analysis several motherboards shown that optimum many designs. This analysis accomplished monitoring SUSCLK accuracy with extremely sensitive measurement equipment that measure frequency range less than ppm. SUSCLK output monitored (and IccRTC monitored) various configurations fork caps. this accomplished room/ ambient conditions. goal obtain capacitor choice that provides greatest best accuracy. Board Leakage: Care must made ensure that there manufacturing residue left motherboard when performing environmental stress testing. Consider solder paste containing less flux which organic acid that becomes conductive moisture. Ensure cleaning after solder process, especially water soluble flux. careful attention underneath discrete components circuit ICHn BGA. Another source leakage condensation which occur motherboard during environmental stress test. This absolutely must prevented. Choice non-condensing chamber profile, ensure that dew-points encountered. acceptable profile listed below, showing both temperature relative humidity (%RH) requirements. There must sufficient flow chamber prevent temperature spots, which could also cause condensation. preventing condensation allowing temperature ramp when board exposed humidity. temperature rises, while there moisture air, board will always cooler than temperature thereby causing condensation. Non-Condensing Temperature/Humidity Profile: Hold ambient °C/25%RH) minutes. Ramp temperature constant rate maximum over least minutes while maintaining 25%RH. Ramp constant rate maximum over least hours while maintaining temperature step Hold sustained temp/RH user defined time. Ramp down constant rate over least hours while maintaining temperature step Ramp down temperature constant rate over least minutes while maintaining Jump back Step times. Accuracy Determination Device: appropriate device check accuracy clock (see Section details).
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Optimizations Stress Testing
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Conclusion
Conclusion
ICHn's external oscillator extremely sensitive circuit because operates very small current. Care must taken when working with this circuit.
guarantee accuracy ICH(n) circuit each specific board design circuit layout, external load capacitance should optimized choosing correct values tuning fork capacitors C1/C2. occurrence time-loss under environmental stress conditions dependent motherboard factors (cleanliness, discrete component characteristics, layout, fork capacitor values), condensation. time-loss observed your system, check sources inaccuracy listed this document improve immunity internal ICHn oscillator time loss.
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Application Note AP-728 Document Number: 292276-002 Intel® F

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