NXP Semiconductors PCF2123, PCF85x3, PCA8565, PCA2125 user manual Influences on time accuracy

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NXP Semiconductors	UM10301
	User Manual PCF85x3, PCA8565 and PCF2123, PCA2125

Fig 8. Influences on time accuracy

The various influences indicated in Fig 8 are described below:

1.The line at the top indicates the frequency tolerance of the crystal in this example. The distance between two vertical lines represents (at the top) 1 s/day or 11.57 ppm. The crystal spread covers about 8.5 lines and thus indicates a spread of about

100 ppm.

2.The production spread of the crystal can be compensated by adjusting the pulling

capacitor CT (in the graph called C6) as long as the value of CT is chosen correctly. Here the pulling range is large enough to compensate for a spread of ±175 ppm. Typically a variable capacitor has a temperature coefficient of ±300 ppm/°C to 500 ppm/°C. This capacity change has a very small influence on the accuracy of the oscillator and its influence is shown in the third line from the bottom of the graph. The

solid line shows the impact of CT (here C6) if the value is small. If CT is large the variation is larger as indicated with the dotted line.

3.Zooming in on a range of about 35 ppm…

4.The integrated load capacitance has a finite production spread and its potential influence on the accuracy of the oscillator depends on the value of the integrated

capacitor. Whether the internal capacitor is connected to the input (CIN or to the output COUT) doesn’t make a difference. The solid line (third solid line from the top) covers a range of 1.8 s/day, or around 21 ppm. A greater value of CIN could have a larger influence indicated by the dotted line.

UM10301_1		© NXP B.V. 2008. All rights reserved.
User manual	Rev. 01 — 23 December 2008	21 of 52

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Contents Info Content Keywords Document informationAbstract NXP Semiconductors Contact informationRevision history Rev Date DescriptionIntroduction Features Address Register name Bit Register overview PCF8563Comparison Event counter modeComparison of six real time clocks FeaturesPower-on reset POR Power-on resetVoltage-low detector Oscillator Voltage-low detectionOscillator-stop detection Overview of internal and external oscillator capacitors Pierce Oscillator equivalent diagramOscillator frequency determining components UM10301 + C Parameter Value Unit Source Typical values for crystal and surrounding capacitorsOscillation allowance Using an external oscillatorCrystal and crystal selection Modes which don’t work Effect of temperature− f nom Capacitors and capacitor selection Accuracy Influences on time accuracy Oscillator tuning Oscillator tuning 10.1 PCF2123 Offset register Century tracking Century and leap year, Daylight Saving TimeYear and leap year tracking Daylight Saving Time DSTInitialization and setting of alarm and timer Initialization of the RTC and setting the timeBlock Diagram PCF8563 Setting the alarm AlarmBinary BCD Register Comments AddressAlarm function Setting the timer Setting the timerRegister Lithium Primary cells Backup power supplyBackup circuit using primary lithium cell NiCd and NiMH secondary batteries Backup circuit using secondary cell NiCd or NiMH13.3 Capacitors Charging the backup capacitor Some suggestions for diode D1 Diode selection1N4148 BAS716 BAS116 BAV170PCB layout guidelines PCB layout proposal for PCF8563 using leaded components Partial circuit switch down Protection diodes Hints to keep power consumption low0007 Rpmax as a function of bus capacitance 8473 ⋅ C bApplication diagram 2, SPI interface Application diagram 1, I2C-bus interfaceTimer delays First period inaccuracy when using the timerTimer Source clock frequency Delay for n = General countdown timer behaviourTimer source clock Minimum timer period Maximum timer period First period delay for timer counter value nTiming requirements for I2C read and write I2C interface Block diagram I2C interface and Time countersSequence of events example Read Oscillator startup time Checking for oscillation TroubleshootingNo communication via I2C-bus Wrong time and date, wrong clock speed ReferencesDefinitions Legal informationDisclaimers TrademarksContents