Oscillator tuning | NXP Semiconductors PCA2125, PCF85x3, PCF2123

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

Fig 9. Oscillator tuning

Accuracy:

In order to be able to adjust the clock with accuracy better than 1 s/day, the frequency counter used to check the output at CLKOUT should have at least an 8 digit reading with an accuracy of 1 ppm.

Given a nominal frequency of 32.768 kHz, 1 ppm = 32.728 mHz (milli Hertz). Therefore +1 ppm = 32768.0327 Hz, -1 ppm = 32767.9673 Hz. Tune the oscillator while it is at the average operating temperature of the application.

The PCF8573 can be tuned by monitoring the 128 Hz signal at the FSET output.

Tuning the PCF8583/93 is somewhat more difficult since no buffered clock signal is available. There are four different options, all with their own drawbacks:

•Measure the period of the 1 s output signal (countdown timer). This however is time consuming;

•Attach the frequency counter probe to the OSCO-pin. This adds capacity to the OSCO-pin and detunes (lowers) the oscillator frequency by ∆f. The frequency adjustment now needs to be lower by the same ∆f in order for the clock to run at the correct speed after the probe has been removed again. Obviously the difficulty here lies in determining how big ∆f is;

•In the watch industry the frequency is coupled out acoustically. A sensitive microphone is placed near the crystal. The signal is then fed as input to the tuning gear.

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

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NXP Semiconductors PCA2125, PCF85x3, PCF2123, PCA8565 user manual Oscillator tuning

Contents

Abstract Info Content KeywordsDocument information Rev Date Description Contact informationNXP Semiconductors Revision history Introduction Features Address Register name Bit Register overview PCF8563 Comparison Event counter mode Comparison of six real time clocks Features Voltage-low detector Power-on reset PORPower-on reset Oscillator-stop detection OscillatorVoltage-low detection Overview of internal and external oscillator capacitors Pierce Oscillator equivalent diagram Oscillator frequency determining components UM10301 + C Parameter Value Unit Source Typical values for crystal and surrounding capacitors Oscillation allowance Using an external oscillator Crystal 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 Daylight Saving Time DST Century and leap year, Daylight Saving TimeCentury tracking Year and leap year tracking Block Diagram PCF8563 Initialization and setting of alarm and timerInitialization of the RTC and setting the time Register Comments Address AlarmSetting the alarm Binary BCD Alarm function Register Setting the timerSetting the timer Lithium Primary cells Backup power supply Backup circuit using primary lithium cell NiCd and NiMH secondary batteries Backup circuit using secondary cell NiCd or NiMH 13.3 Capacitors Charging the backup capacitor BAS716 BAS116 BAV170 Diode selectionSome suggestions for diode D1 1N4148 PCB layout guidelines PCB layout proposal for PCF8563 using leaded components Partial circuit switch down Protection diodes Hints to keep power consumption low 0007 Rpmax as a function of bus capacitance 8473 ⋅ C b Application diagram 2, SPI interface Application diagram 1, I2C-bus interface General countdown timer behaviour First period inaccuracy when using the timerTimer delays Timer Source clock frequency Delay for n = Timer source clock Minimum timer period Maximum timer period First period delay for timer counter value n Timing requirements for I2C read and write I2C interface Block diagram I2C interface and Time counters Sequence of events example Read Oscillator startup time Checking for oscillation Troubleshooting No communication via I2C-bus Wrong time and date, wrong clock speed References Trademarks Legal informationDefinitions Disclaimers Contents