Intel ICH Family Real Time Clock (RTC) Accuracy
Contents
1. Notes Reference Designators Arbitrarily Assigned 3 3V Sus is Active Whenever System Plugged In Vbatt is Voltage Provided By Battery Co 32 768 KHz VCCRTC RTCX2 RTCX1 VBIAS VCCRTC RTCX1 and RTCX2 are ICHn pins VBIAS is used to bias the ICH1 2 3 4 Internal Oscillator VCCRTC powers the RTC well of the ICHn RTCX1 is the Input to the Internal Oscillator RTCX2 is the feedback for the external crystal Figure 3 Intel ICH4 RTC External Circuit 3 3V Sus Schottky Diodes pF D c3 1 0 pF 0 047 pF T 1 Xtal VCCRTC Q 1pF RTCX2 1 KO R1 Co 10MQ 32 768 KHz T 20 Ka RTCX1 C1 C2 R2 EPE 10MQ Notes The circuit changes from ICH3 are as follows 2 The use of a 0 1pF decoupling capacitor near the ball of VecRTC 1 For VccRTC and VBIAS the use of one 1KQ resistor near the coin battery instead of two 1KQ resistors 3 The original 1uF capacitor moved near the coin battery after the diode gt x RTCRST o X VBIAS AP 728 Background External RTC Circuit intel Figure 4 Intel ICH5 6 7 8 RTC External Circuit 3 3V Sus VCCRTC Schottky Diodes 1uF 0 1uF RTCX2 1KQ R1 20KQ 32 768 KHz 10MQ Vbatt Xtal RTCX1 C1 C2 1 0uF T 7 i E DX RTCRST Note The circuit change from ICH4 is the external VBIAS circuit has been integrated into ICH5 6 7 82. RTC External Circuit i n tel Figure 9 Example OP Amp Configuration AD822AR 3V Op Amp configured for unity gain Connect to Oscilloscope To RTCX1 RTCX2 or VBIAS Figure 10 shows an example of a screen capture on the RTCX1 and RTCX2 signals Figure 10 Screen Capture of RTCX1 and RTCX2 Signals on Scope File Control Setup Measure Analyze Utilities Help 9 42 AM 7 MEME mean std dev min max 250 348 my 250 138 mV 419 53 W 249 083 my 251 109 mV 409 0 mi 412 01 mV 5 60 mi 401 5 nV 431 0 mV 516 8 mV 20 69 m 4 20 m 512 0 m 532 3 mV 248 906 my 249 846 mV 4 07489 mY 246 968 my 266 328 my current AP 728 13 i n tel RTC External Circuit 3 2 2 3 2 3 14 VBIAS The external VBIAS circuit is only required for ICH1 2 3 4 All other platforms have integrated the external VBIAS circuit For ICH1 2 3 4 VBIAS is a DC voltage level that is necessary for biasing the RTC oscillator circuit This DC voltage level is filtered out from the RTC oscillation signal by the RC network of R2 and C3 see Figure 2 and Figure 3 Therefore it is a self adjusted voltage Board designers should not manually bias the voltage level on VBIAS Checking the VBIAS voltage level is used for testing purposes only to determine the right bias condition of the RTC circuit VBIAS should be at least 200 mV DC on ICH1 2 3 4 The RC network of R2 and C3 will filter out most of AC signal that exi
3. Figure 5 Intel ICH9 ICH10 Intel 5 Series Chipset and Intel 3400 Series Chipset RTC External Circuit 3 3V Sus VCCRTC Schottky Diodes 1uF 0 1uF RTCX2 1KQ i Da Q 32 768 KHz Vbatt 20 KQ Xtal EE d RTCX1 1 0 uF 1 0 uFL c1 c2 E S DX RTCRST SRTCRST Note The circuit change from ICH8 is the addition of SRTCRST implementation AP 728 m n tel Background External RTC Circuit Figure 6 Intel C200 Series Chipset and later Client Platform Controller Hubs PCHs VccDSW3 3 o X VCCRTC Schottky Diodes 1pF 0 1pF DX RTCX2 TKO 32 768 KHz Xtal R1 20KQS 20KQ 10MQ Vbatt DX Rrext 1 0 UE 1 0 uF et ez c 7 XI RTCRST X SRTCRST Notes 1 The circuit change from Intel 5 Series Chipset and Intel 3400 Series Chipset is the VccDSW3 3 input pin 2 For platforms not supporting Deep S4 S5 Deep Sx the VccDSW3 3 pins will be connected to the VccSus3 3 pins The crystal network employs R1 C1 and C2 to generate the 32 768 kHz sine wave Actual values for these components are dependent on the crystal component specification trace lengths on the motherboard and the crystal s load capacitance For ICH9 ICH10 Intel9 5 Series Chipset and Intel 3400 Series Chipset SRTCRST4 is used to reset portions of the Intel Management Engine Intel ME and sho
4. RTC circuit oscillates with extremely low bias current refer to specific component datasheet for ICCRTC maximum value therefore this signal is very sensitive to the environmental conditions such as board residue solder flux dust humidity etc For example touching directly on this circuit may cause leakage that can completely attenuate the oscillation signal and make the RTC oscillation stop Measuring RTCX1 and RTCX2 is accomplished only by using the following technique to minimize any measurement equipment loading effects 1 Configure an Analog Devices AD823 AD823AN or equivalent Op Amp with very high input impedance on the order of 10E12 10E14 as a unity gain follower as shown below Note This may be different depending on the Op Amp used See Figure 9 for an illustration 2 The conductor between the signal being measured and the Op Amp input must be less than 4 mm with a direct connection preferable The VS must be connected to a voltage source that is on all the time such as an external supply or a 9 V battery 3 Collect the RTC electrical characteristics a Place an oscilloscope probe with sufficient ground reference on the Op Amp output The oscilloscope should be configured for 100 mV DIV and 20 us DIV with a trigger set to approximately 200 mV or until capture is obtained Record the RTCX1 Vpp Optional data can be captured such as DC Offset of RTCX1 and 2 Vpp of RTCX2 DC level for VBIAS and IccRTC AP 728
5. capacitance is approximately equal to Ctrace trace length 2 pF inch e Charasitic Crystal s parasitic capacitance This capacitance is created by the existence of electrode plates and the dielectric constant of the crystal blank inside the crystal part Refer to the crystal s specification to obtain this value Ideally C1 and C2 can be chosen such that C1 C2 Using the equation of Cloaq above the value of C1 C2 can be calculated to give the best accuracy closest to 32 768 kHz of the RTC circuit at room temperature However C2 can be chosen such that C2 C1 Then C1 can be trimmed to obtain 32 768 kHz In certain conditions both C1 C2 values can be shifted away from the theoretical values calculated values from the above equation to obtain the closest oscillation frequency to 32 768 kHz When C1 and C2 values are smaller then the theoretical values the RTC oscillation frequency will be higher The following example will illustrates the use of the practical values for C1 and C2 in the case that theoretical values can not ensure the accuracy of the RTC in a low temperature condition Example 1 According to a required 12 pF load capacitance of a typical crystal that is used with the ICH PCH the calculated values of C1 C2 is 18 pF at room temperature 25 C to yield a 32 768 kHz oscillation At 0 C the frequency stability of crystal gives 23 ppm assumed that the circuit has 0 ppm at 25 C This makes the RTC ci
6. intel Intel I O Controller Hub Intel ICH Platform Controller Hub PCH Family Real Time Clock RTC Electrical Mechanical and Thermal Specification EMTS AP 728 April 2012 292276 009 INFORMATION IN THIS DOCUMENT IS PROVIDED IN CONNECTION WITH INTEL PRODUCTS NO LICENSE EXPRESS OR IMPLIED BY ESTOPPEL OR OTHERWISE TO ANY INTELLECTUAL PROPERTY RIGHTS IS GRANTED BY THIS DOCUMENT EXCEPT AS PROVIDED IN INTEL S TERMS AND CONDITIONS OF SALE FOR SUCH PRODUCTS INTEL ASSUMES NO LIABILITY WHATSOEVER AND INTEL DISCLAIMS ANY EXPRESS OR IMPLIED WARRANTY RELATING TO SALE AND OR USE OF INTEL PRODUCTS INCLUDING LIABILITY OR WARRANTIES RELATING TO FITNESS FOR A PARTICULAR PURPOSE MERCHANTABILITY OR INFRINGEMENT OF ANY PATENT COPYRIGHT OR OTHER INTELLECTUAL PROPERTY RIGHT A Mission Critical Application is any application in which failure of the Intel Product could result directly or indirectly in personal injury or death SHOULD YOU PURCHASE OR USE INTEL S PRODUCTS FOR ANY SUCH MISSION CRITICAL APPLICATION YOU SHALL INDEMNIFY AND HOLD INTEL AND ITS SUBSIDIARIES SUBCONTRACTORS AND AFFILIATES AND THE DIRECTORS OFFICERS AND EMPLOYEES OF EACH HARMLESS AGAINST ALL CLAIMS COSTS DAMAGES AND EXPENSES AND REASONABLE ATTORNEYS FEES ARISING OUT OF DIRECTLY OR INDIRECTLY ANY CLAIM OF PRODUCT LIABILITY PERSONAL INJURY OR DEATH ARISING IN ANY WAY OUT OF SUCH MISSION CRITICAL APPLICATION WHETHER OR NOT INTEL OR ITS SUBCONTRACTOR WAS NEGLIGEN
7. trace has approximately 2 pF per inch e Trace signal coupling must be reduced Avoid routing noisy periodic signals close and parallel to RTCX1 RTCX2 and VBIAS e Ground referencing is highly recommended Environmental Conditions The crystal temperature itself will impact the RTC accuracy The deviation from room temperature will reduce the RTC accuracy unless this factor is compensated by using the practical configuration of C1 C2 value See Section 3 1 for details Condensation from humidity can also affect the RTC accuracy due to leakage on RTC signals see Section 3 2 for details Heat will damage the crystal when reworking the boards Follow the specification of the crystal to set the right temperature for operation 15 i n tel RTC External Circuit 3 4 3 4 1 3 4 2 16 RTC Accuracy Determination and Frequency Measurement Technique The accuracy of RTC clock can be determined by many different methods Two common methods for checking RTC accuracy are e Using timekeeping baseline device via BIOS and system NVRAM e Using Time Interval Analyzer on SUSCLK signal Using Timekeeping Baseline Device Accuracy of the baseline device is crucial to determining RTC accuracy The use of a watch or clock is not sufficiently accurate for 25 ppm range accuracy over the periods used during these tests It is required that a Global Positioning System device GPS be used instead A GPS contains a clock that is reset at acq
8. Intel Corporation All rights reserved 2 AP 728 Contents 1 Background External REC Circuit s science cedcteaedeaecene sees eeeeaedeasurenys RR ARE gta Nana RR AREE ARE TT AE 5 2 RTC External Battery Connection terio 0 Rea XS 4 ex creada en a ne koe Rer KRA eu dae cas ira NRT 9 3 agredau dnrbenssicem mA 10 3 1 RIC External Capacitor Valles sassis cds clesie as ch eet ca curet Coe dada a Y aa ed ER E eek 11 LEE RIC SIGNS sa icvcccusanwesaa seas danas cued a nade cae divgnsaingetanadnense qodeeae ARAA 12 3 2 1 Signals on RTCX1 and RTCX2 Pins ccc eee eee 12 3 2 2 NMBIAS siessen denanan tee daxenas DEEST RC EREPROEFRNEKEERKEORUE TRETEN RI REENUS Ded CR POOR FIRE 14 3 2 3 SUSCLK qnae 14 B S IRIC ACCURACY EE 15 3 3 1 RUC Voltage enia cater rer enne eed iind HD M UE x ni on ne Rob etna x ca og d ow i 15 3 3 2 External Capacitance Load eie pendente TAT RR RATHOR ener Fa de x Re 15 3 3 3 RTC Circuit Layout Considerations sss sss ee eee e e eee eee eee 15 3 3 4 Environmental Conditions lecce ceiesen ennt nn nn nnn nnda hann rua n n n 15 3 4 RTC Accuracy Determination and Frequency Measurement Techniq le i iiie a sa XN SX YS 9 Yar AX ZY Ka 0 a Rx RD YT YAN steded saver REFERRE 16 3 4 1 Using Timekeeping Baseline Device sess 16 3 4 2 Using the Time Interval Analyzer ss sss aves sss ce eterne ren nennen andan anu dn 16 4 Influences Under Environmental Stress sss eee eee e e eee eee eee 18 Optimiz
9. PCH via isolation diode circuit The diode circuit allows the ICH PCH s RTC well to be powered by the battery when the system power is not available but by the system power when it is available To do this the diodes are set to be reverse biased when the system power is not available Figure 2 Figure 3 and Figure 4 have the example of a diode circuitry that is used As noted a standby power supply should be used in desktop and mobile system to provide continuous power to the RTC when available which will significantly increase the RTC battery life 88 AP 728 9 i n tel RTC External Circuit Figure 7 RTC External Circuit The ICH PCH module requires an external oscillating source of 32 768 kHz connected on RTCX1 and RTCX2 pins Figure 7 represents the internal and external circuitry that comprise the oscillator of the RTC External and Internal Circuitry for the RTC Oscillator Internal Note Figure 8 C1 C2 are the required external capacitors that affect the accuracy of the RTC Choose the right capacitor value for C1 and C2 and the tolerance should be less than or equal to 596 both are important to maintain RTC accuracy Section 3 1 presents some guidelines for choosing these values Even if the ICH PCH internal RTC is not used it is still necessary to supply clock input to X1 of the ICH PCH because other signals are gated off that clock in suspend modes In this case an oscillator or a singl
10. T IN THE DESIGN MANUFACTURE OR WARNING OF THE INTEL PRODUCT OR ANY OF ITS PARTS Intel may make changes to specifications and product descriptions at any time without notice Designers must not rely on the absence or characteristics of any features or instructions marked reserved or undefined Intel reserves these for future definition and shall have no responsibility whatsoever for conflicts or incompatibilities arising from future changes to them The information here is subject to change without notice Do not finalize a design with this information The products described in this document may contain design defects or errors known as errata which may cause the product to deviate from published specifications Current characterized errata are available on request Contact your local Intel sales office or your distributor to obtain the latest specifications and before placing your product order Code names featured are used internally within Intel to identify products that are in development and not yet publicly announced for release Customers licensees and other third parties are not authorized by Intel to use code names in advertising promotion or marketing of any product or services and any such use of Intel s internal code names is at the sole risk of the user Intel and the Intel logo are trademarks of Intel Corporation in the U S and other countries Other names and brands may be claimed as the property of others Copyright 2012
11. ant rate to a maximum of 85 over at least 2 hours while maintaining temperature set in step 2 4 Hold at sustained temp RH for user defined time 5 Ramp down RH at a constant rate to 25 over at least 2 hours while maintaining temperature set in step 2 6 Ramp down temperature at a constant rate to 25 C over at least 30 minutes while maintaining 25 RH 7 Jump back to Step 1 n times RTC Accuracy Determination Device Use appropriate device to check the accuracy of the RTC clock see Figure 10 88 19 20 ntel Conclusion Conclusion The ICH PCH s RTC external oscillator is an extremely sensitive circuit because it operates at a very small current Care must be taken when working with this circuit To ensure the accuracy of the ICH PCH RTC circuit for each specific board design and RTC circuit layout the external load capacitance should be optimized by choosing correct values of the tuning fork capacitors C1 C2 The occurrence of time loss under environmental stress conditions is dependent on motherboard factors cleanliness discrete component characteristics layout fork capacitor values and condensation If time loss is observed on your system check all of the sources of inaccuracy listed in this document to improve immunity of the internal ICH PCH oscillator to time loss 88 AP 728
12. ariance J So ee a Number of Samples 4096 ation The pacing number is the number of clock edges that the TIA will count before it capture for each time stamp Depending on the TIA memory size the number of samples varies 1024 4096 etc chose the largest number possible The measured frequency Fo will be calculated based upon the average period of all samples This is the frequency of the RTC clock The following formula is used to calculate the PPM PPM Fo 32 768 kHz 32 768 kHz 10E6 88 AP 728 17 18 n tel Influences Under Environmental Stress Influences Under Environmental Stress Crystal Characteristics Typical 32 768 kHz crystals have an operating temperature ceiling of 60 C thus limit the test temperature accordingly In addition the temperature coefficient of these crystals can cause time loss of approximately 3 sec day at 60 C Fork Capacitor Tuning The timekeeping of the RTC is dependent on the RTCX1 input voltage swing Oscillation that is marginal may result in failure to meet Vih of this input and thus ticks of the clock may be missed resulting in time loss Optimum Vpp of this RTCX1 signal is achieved by accurately matching the crystal s C load specification typically 12 pF Board Leakage Since this circuit operates at such low current it is very sensitive to sources of leakage on the motherboard Manufacturing residue can cause leakage as well as condensation
13. ations for Stress Testing cc ciere de Co rale tke set ea ETA RY Ee TpLa OL OE RA V A NET vents 19 6 eelsrol Ei 20 Figures 1 Input and Output of the Intel ICH PCH RTC Circuit eee 5 X Intel ICH1 2 3 RTC External Circuit sueco accord deb aa eRab ura b Ga taci e spur ax Ca der pag a RS 6 3 Intel ICH4 RTC External CIPCUlE eee 6 4 Ihtel ICH5 6 7 8 RTC External Circuit vc icsccccsasccvsvavedaviavacdstaadeansvccvavateaveved de du Eid rea E M NC Ka 7 5 Intel ICH9 ICH10 Intel amp 5 Series Chipset and Intel 3400 Series Chipset RTC External Circuit7 6 Intel C200 Series Chipset and later Client Platform Controller Hubs PCHs 8 7 External and Internal Circuitry for the RTC Oscillator esee 10 8 Clock Input Connection to X1 X2 Pins When Not Using Internal RTC sss ss s eee ee e e eee ee 10 9 Example OP Amp Configuration 2 2 Yy a aY X aS Y ern rena Yd VA 9 nu nx xe coded vaa dada a dx TE Nas 13 10 Screen Capture of RTCX1 and RTCX2 Signals on SCOPE ce ee e ee eee 13 M Tp PSI CMM 17 AP 728 3 intel Revision History Document Rev Draft Changes Date 292276 001 e Initial Release March 2001 002 e General updates August 2001 003 s ICH4 updates May 2002 004 e ICH5 updates June 2003 005 e ICH6 update Also changed RTCRST RC for ICH4 6 June 2004 006 e ICH7 update April 2005 007 e ICH8 update May 2006 008 e ICH9 update September 2007 e ICH10 Intel 5 Series C
14. e clock input can be used to drive into X1 with X2 left as no connect please refer to specific component datasheet for RTC X1 input voltage max value Figure 8 illustrates the connection However this is not a validated or supported configuration Clock Input Connection to X1 X2 Pins When Not Using Internal RTC Internal External X1 10M x2 32 KHz uj No Connection 10 AP 728 RTC External Circuit i n tel j 3 1 AP 728 RTC External Capacitor Values To maintain the RTC s accuracy the external capacitor values should be chosen to provide the manufacturer s specified load capacitance Coad for the crystal when combined with the parasitic capacitance of the circuits traces socket if used package and ICH PCH input capacitances The following equation can be used to choose the external capacitance values Cload E C1 Cini Ctrace1 C2 Cin2 T Ctrace2 l C1 Cini Ctracet C2 Cin2 Ctrace2 Cparasitic Where e Cioaq Crystal s load capacitance This value can be obtained from Crystal s specification e Cini Cin2 input capacitances at RTCX1 RTCX2 pins of the ICH PCH These values can be obtained in the ICH PCH s data sheet e Carcel Ctrace2 Trace length capacitances measured from the Crystal terminals to RTCX1 RTCX2 pins These values depend on the characteristics of board material the width of signal traces and the length of the traces The typical value of this
15. hipset and Intel 3400 Series 009 Chipset Intel C200 Series Chipset and later Client PCHs April 2012 update 88 4 AP 728 m Background External RTC Circuit n tel 1 Background External RTC Circuit Chipsets using an Intel I O Controller Hub Intel ICH family component an Intel 5 Series Chipset an Intel 3400 Series Chipset an Intel C200 Series Chipset or later client Platform Controller Hub PCH referred to as ICH PCH for the remainder of this document use a crystal circuit to generate a low swing 32 kHz input sine wave This input is amplified and driven back to the crystal circuit via the RTCX2 signal Internal to the ICH PCH the RTCX1 signal is amplified to drive internal logic as well as generate a free running full swing clock output for system use This output pin of the ICH PCH is called SUSCLK This is illustrated in Figure 1 Figure 1 Input and Output of the Intel ICH PCH RTC Circuit Low Swing 32 768kHz i Sine Wave Source ate RIGA Internal i ICH PCH Oscillator Full Swing 32 768kHz b SUSCLK Output Signal T The low swing 32 768 kHz clock source is generated by a circuit implemented on the motherboard external to the ICH PCH component The schematic is illustrated in the following figures AP 728 5 intel Background External RTC Circuit Figure 2 Intel ICH1 2 3 RTC External Circuit 3 3V Sus Schottky Diodes 1 KO Vbatt
16. in a capacitor choice that provides greatest Vpp and the best accuracy Board Leakage Care must be taken to ensure that there is no manufacturing residue left on the motherboard when performing environmental stress testing Consider a solder paste containing less flux which is an organic acid that becomes conductive in moisture Ensure cleaning after the solder process especially for water soluble flux Pay careful attention to underneath discrete components of the RTC circuit and the ICH PCH package Another source of leakage is condensation which may occur on the motherboard during an environmental stress test This absolutely must be prevented Choice of a non condensing chamber profile can ensure that dew points are not encountered An acceptable profile is listed below showing both temperature and relative humidity RH requirements There must be sufficient air flow in the chamber to prevent temperature spots which could also cause condensation The key to preventing condensation is not allowing temperature to ramp when the board is exposed to humidity As the air temperature rises while there is moisture in the air the board will always be cooler than the air temperature thereby causing condensation Non Condensing Temperature Humidity Profile 1 Hold at ambient 25 C 25 RH for 30 minutes 2 Ramp temperature at a constant rate to a maximum of 60 C over at least 30 minutes while maintaining 25 RH 3 Ramp up RH at a const
17. on the board encountered during temperature and or humidity testing Timekeeping Baseline Device Time of motherboards is typically compared to a baseline device like a watch or other baseline clock device believed to be accurate The case is that most timekeeping devices like this are not sufficiently accurate This can cause an additional source of error 88 AP 728 m Optimizations for Stress Testing n tel 5 AP 728 Optimizations for Stress Testing To obtain the best RTC accuracy in environmental stress conditions the above listed factors can be optimized While there is little that can be done to change crystal characteristics there are opportunities to maximize the oscillator voltage swing and to minimize board leakage described below Fork Capacitor Tuning The fork capacitors C1 C2 must be chosen to provide the greatest Vpp of RTCX1 yet still providing the best accuracy This is typically accomplished by laboratory analysis and is specific to each motherboard see Section 3 1 for details Analysis of several motherboards has shown that 18 pF is optimum for many designs This analysis is accomplished by monitoring SUSCLK accuracy with extremely sensitive measurement equipment that can measure frequency to a PPM range of less than 2 ppm The SUSCLK output is monitored and IccRTC may be monitored for various configurations of fork caps All this can be accomplished at room ambient conditions The goal is to obta
18. rcuit oscillate at 32 767246 kHz instead of 32 768 kHz If the values of C1 and C2 are chosen to be 6 8 pF instead of 18 pF this will make the crystal oscillate at a higher frequency at room temperature 23 ppm but this configuration of C1 and C2 makes the circuit oscillate closer to 32 768 kHz at 0 C The 6 8 pF value of C1 and C2 is the practical value 11 3 2 3 2 1 12 n tel j RTC External Circuit Note that the temperature dependency of crystal frequency is a parabolic relationship ppm degree squared The effect of the crystal s frequency when operating at 0 C 25 C below room temperature is the same when operating at 50 C 25 C above room temperature The values of C1 and C2 calculated from the equation above are only the theoretical values Ideally these values will be the same as practical values However the values of C1 and C2 should be chosen based on the values that give the best accuracy of the RTC circuit In every specific board design these practical values may be slightly different from theoretical values RTC Signals Signals on RTCX1 and RTCX2 Pins The RTC oscillation signal is a sinusoidal signal that provides a 32 768 kHz frequency to the ICH PCH This is a small analog signal with peak to peak voltage about 200 mV 500 mV The voltage swing on RTCX2 will be slightly larger than the voltage swing on RTCX1 since RTCX2 signal is amplified through the internal inverter of the ICH PCH The
19. st on this pin however the noise on this pin should be kept minimal in order to ensure the stability of the RTC oscillation Probing VBIAS requires the same technique as probing the RTCX1 RTCX2 signals using Op Amp VBIAS is also very sensitive to environmental conditions SUSCLK SUSCLK is a square wave form signal output from the RTC oscillation circuit Depending on the quality of the oscillation signal on RTCX1 largest voltage swing SUCCLK duty cycle can be between 30 70 If the SUSCLK duty cycle is beyond 30 70 range it indicates a poor oscillation signal on RTCX1 and RTCX2 SUSCLK can be probed directly using a normal probe 50 Q input impedance probe and is the appropriate signal to check the RTC frequency to determine the accuracy of the ICH PCH s RTC Clock see Section 3 4 2 for details AP 728 RTC External Circuit i n tel j 3 3 3 3 1 3 3 2 3 3 3 3 3 4 Note AP 728 RTC Accuracy The ICH PCH RTC circuit is a low current circuit designed to provide accurate time keeping service at an extremely low current consumption IccRTC a maximum of 5 or 6 uA please refer to specific component datasheet for IccRTC max value As a result this circuit is subject to adverse influences which must be addressed or understood to ensure the best possible RTC accuracy The RTC accuracy can be affected by the following primary factors RTC Voltage The RTC accuracy can be affected by the voltage of batter
20. uisition time to an extremely accurate time Over time though the GPS time will drift like any other clock For this reason the GPS should be reset powered on outside and allowed to synchronize within 15 minutes of either setting time on motherboard or using it for a standard readout Using the Time Interval Analyzer Using an Agilent TIA Time Interval Analyzer or the equivalent is a very accurate way of measuring the RTC frequency This tool can measure the 32 768 kHz frequency of the RTC with a tolerance typically less than 20 mHz mili Hertz It is ideal to use this equipment to determine the configurations of the fork tuning capacitors C1 and C2 which are described in Section 3 1 This is to get the most accurate frequency on the RTC circuit The following is a description of how to use an Agilent TIA to calculate the frequency of the RTC circuit 1 The frequency of RTC oscillator can be monitored on the SUSCLK signal of the ICH PCH Locate SUSCLK signal on the board and connect this signal to the probe of Agilent TIA with a properly connected ground pin 2 Run the Agilent E1743A program Target frequency should be set to 32 768 kHz Refer to Agilent TIA user manual for more information on how to setup the equipment 3 On TIA menu click on Measure View Setup the setup dialog box will appear as shown in Section 11 AP 728 RTC External Circuit i n tel Figure 11 TIA Menu Measure View Setup a Allan V
21. uld not be connected to a jumper or button on the platform The only time this signal gets asserted driven low in combination with RTCRST should be when the coin cell battery is removed or not installed and the platform is in the G3 state Pulling this signal low independently without RTCRST also being driven low may cause the platform to enter an indeterminate state Similar to RTCRST it is imperative that SRTCRSTZ not be pulled low in the SO to S5 states 88 AP 728 m RTC External Battery Connection n tel 2 RTC External Battery Connection The RTC module requires an external battery connection to maintain its functionality and its RAM while the ICH PCH is not powered by the system The recommended batteries are Duracell 2032 2025 or 2016 which can give many years of operation Batteries are rated by storage capacity The battery life can be calculated by dividing the capacity by the average current required For example if the battery storage capacity is 170 mAh assumed usable and the average current required is 5 uA the battery life will be at least 170 000 Ah 5 pA 34 000 h 3 88 years The voltage of the battery can affect the RTC accuracy VccRTC must be greater than the min voltage specification at all times to ensure the accuracy of the RTC clock and correctly storing RTC SRAM data Please refer to specific component datasheet for VccRTC min voltage value The battery must be connected to the ICH
22. y In general when the battery voltage decays the RTC accuracy also decreases High accuracy less than 10 ppm which does not include Crystal s tolerance can be obtained when VccRTC is larger than the min voltage specification For example 20 ppm is equivalent with 1 728 sec error in one day PPM lt Fo 32 7680 32 768 10 6 1 day 24 hours day 60 min hour 60 sec min 86400 sec 20 ppm 1 day 86400 sec 20 86400 1000000 1 728 sec External Capacitance Load The external capacitance load values are combined with the external capacitor values the capacitance of the circuit s trace socket and package These values should be matched to the actual load capacitance required of the crystal used for the RTC accuracy Refer to Section 3 1 for guidelines to calculate the external capacitance RTC Circuit Layout Considerations Since the RTC circuit is very sensitive and requires high accuracy oscillations reasonable care must be taken during the layout and routing of the RTC circuit Some recommendations are e Reduce trace capacitance by minimizing the RTC trace length The ICH PCH requires a trace length less than 1 inch on each branch from crystal s terminal to RTCXn pin Routing the RTC circuit should be kept simple to reduce the trace length measurement and increase accuracy on calculating trace capacitances Trace capacitance depends on the trace width and dielectric constant of board s material On FR 4 a 5 mil
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