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

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NXP Semiconductors

UM10301

 

User Manual PCF85x3, PCA8565 and PCF2123, PCA2125

5.Tuning CT will increase the initial accuracy, indicated by reducing the range covered to the spread of CIN. Depending on how well CT is tuned, the initial spread can be well compensated for.

6.Here the influence on accuracy due to aging of the crystal is given. This shift occurs mainly during the first year of the crystal’s life and in the graph a range of about 420 ms/day to 850 ms/day is indicated (±10 ppm). So if the RTC were running correctly initially, after a year it could be fast or slow by about 0.4 s/day.

7.As already mentioned and illustrated in Fig 7, the largest impact on the accuracy is due to the temperature dependence of the crystal. The parabolic nature of this dependence is indicated here as well and covers about 40 ppm over the temperature range -10 °C to +60 °C.

8.The impact of a change in VDD is small; a ∆VDD of 1.5 V will change the clock speed by about 17 ms/day.

From this an important recommendation follows: If possible place the crystal and IC- circuit at the spot with the least temperature variations.

10. Oscillator tuning

The PCF8563, PCF2123, PCA8565 and PCA2125 all have a CLKOUT pin which is an open drain output. It provides the option to output the buffered crystal frequency (or a lower frequency derived from the crystal frequency using division by a power of 2) which can be achieved by enabling CLKOUT in the appropriate control register and choosing the desired frequency. Refer to the datasheet of the actual device used for details on how to enable CLKOUT and set the frequency at this pin. Possible output frequencies for the PCF8563 and PCA8565 are 1 Hz, 32 Hz, 1024 Hz and 32768 Hz. The PCF2123 and PCA2125 offer some additional choices and the possible frequencies at the CLKOUT pin are 1 Hz, 1024 Hz, 2048 Hz, 4096 Hz, 8192 Hz, 16384 Hz and 32768 Hz.

PCF8573, PCF8583 and PCF8593 do not include a CLKOUT pin.

Having the CLKOUT pin enables easy tuning of the crystal frequency. A designers’ initial impulse may be to connect an oscilloscope probe to the OSCO pin, but this is not a good idea. It may cause the oscillator to stop, but even if the oscillator keeps running the added capacitance of the probe will cause a drift in oscillator frequency. By connecting a pull-up resistor to the CLKOUT pin and measuring the frequency there, a much more accurate result can be achieved. The frequency can now be tuned by adjusting the variable capacitor CT.

Remark: Touching the adjustment screw often causes the capacitance to shift. The setup is shown in Fig 9.

UM10301_1

 

© NXP B.V. 2008. All rights reserved.

User manual

Rev. 01 — 23 December 2008

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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
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