NXP Semiconductors PCA8565, PCF85x3, PCF2123 Block diagram I2C interface and Time counters

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

SDA

SCL

I2C interface

reset

This signal stops the time counters from counting. It is generated when an I2C READ or WRITE is initiated.

BLOCK

64Hz

Pre-scaler

1Hz clocks

clock

Watchdog

I2C watchdog (active low)

Time counters

This signal resets the I2C interface if BLOCK remains active for too long.

Fig 21. Block diagram I2C interface and Time counters

When there is no I2C activity the RTC is counting normally. Once an I2C read or write operation is initiated, the I2C interface asserts the signal BLOCK. This signal stops the time counters from counting. Additionally this results in the watchdog no longer being reset. At the next rising edge of the 1 Hz clock, time does not increment because the registers have been frozen. However, the watchdog counter increments now. Thus the increase in time is recorded and after the read operation has completed BLOCK goes low again. Now the stored clock in the watchdog is used to give an extra pulse to the time counters to make sure that correct time is kept. Also the watchdog will be reset.

The maximum watchdog value is 2. If at the second rising edge of the 1 Hz clock after a read operation was initiated, the reading operation has not been completed yet, BLOCK will still be high. The time counters don’t increase and the watchdog counter increases and reaches its maximum value. Its output is set active which resets the I2C interface which in turn resets the BLOCK signal. Again one pulse (not two) is sent to the time counters and the watchdog is reset. But now two rising edges of the 1 Hz clock didn’t reach the time counters and only one was compensated for. The RTC looses one second. The exact sequence of events is depicted in Fig 22.

UM10301_1		© NXP B.V. 2008. All rights reserved.
User manual	Rev. 01 — 23 December 2008	46 of 52

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Contents Document information Info Content KeywordsAbstract Revision history Contact informationNXP Semiconductors Rev Date DescriptionIntroduction Features Register overview PCF8563 Address Register name BitEvent counter mode ComparisonFeatures Comparison of six real time clocksPower-on reset Power-on reset PORVoltage-low detector Voltage-low detection OscillatorOscillator-stop detection Pierce Oscillator equivalent diagram Overview of internal and external oscillator capacitorsOscillator frequency determining components UM10301 + C Typical values for crystal and surrounding capacitors Parameter Value Unit SourceUsing an external oscillator Oscillation allowanceCrystal and crystal selection Effect of temperature Modes which don’t work− f nom Capacitors and capacitor selection Accuracy Influences on time accuracy Oscillator tuning Oscillator tuning 10.1 PCF2123 Offset register Year and leap year tracking Century and leap year, Daylight Saving TimeCentury tracking Daylight Saving Time DSTInitialization of the RTC and setting the time Initialization and setting of alarm and timerBlock Diagram PCF8563 Binary BCD AlarmSetting the alarm Register Comments AddressAlarm function Setting the timer Setting the timerRegister Backup power supply Lithium Primary cellsBackup circuit using primary lithium cell Backup circuit using secondary cell NiCd or NiMH NiCd and NiMH secondary batteries13.3 Capacitors Charging the backup capacitor 1N4148 Diode selectionSome suggestions for diode D1 BAS716 BAS116 BAV170PCB layout guidelines PCB layout proposal for PCF8563 using leaded components Partial circuit switch down Hints to keep power consumption low Protection diodes0007 8473 ⋅ C b Rpmax as a function of bus capacitanceApplication diagram 1, I2C-bus interface Application diagram 2, SPI interfaceTimer Source clock frequency Delay for n = First period inaccuracy when using the timerTimer delays General countdown timer behaviourFirst period delay for timer counter value n Timer source clock Minimum timer period Maximum timer periodTiming requirements for I2C read and write Block diagram I2C interface and Time counters I2C interfaceSequence of events example Read Troubleshooting Oscillator startup time Checking for oscillationNo communication via I2C-bus References Wrong time and date, wrong clock speedDisclaimers Legal informationDefinitions TrademarksContents