NXP Semiconductors UM10301 PCF2123 - Manual (Page 50)

NXP Semiconductors
UM10301
User Manual PCF85x3, PCA8565 and PCF2123, PCA2125
UM10301_1 © NXP B.V. 2008. All rights reserved.
User manual Rev. 01 — 23 December 2008 50 of 52
19.4 Wrong time and date, wrong clock speed
When writing into the RTC registers care has to be taken that only valid values are
written. For example, seconds only should go to 59, but it is possible to write for example
61 into the seconds register. Care needs to be taken that the software routines convert
correctly to the BCD values used in the RTC. If a wrong value is written, at some point
the register will return to a valid value, but then still the time indicated will not be correct,
since the initial write was wrong.
If the clock is running too fast, usually the cause is spurious signals from other parts of
the application that somehow couple into the oscillator signal and are interpreted as
additional clock signals. As indicated in the parabolic curve of Fig 7, the crystal frequency
will reduce if temperature decreases or increases with respect to the turn over
temperature. Assuming that the load capacitance fits the crystal selected, the oscillator
will run at 32.768 kHz and can only run slower under influence of temperature.
Refer to Chapter 14 “PCB layout guidelines” to avoid the problem of spurious signals
coupling into the oscillator.
If the RTC runs too slow (or it appears to loose time) make sure that read and write
operations are finalized within 1 second of initiating them. Refer to Chapter 18 “Timing
requirements for I
2
C read and write”.
20. References
The documents below provide further useful information.
1. Product data sheets of all real time clocks for which this manual is valid.
2. AN10716_1; Background information and theory related to Real Time Clocks and
crystals.
3. AN10652_1; Improved timekeeping accuracy with PCF8563 using external
temperature sensor. Rev 1, 2 November 2007.
4. UM10204_3; I
2
C-bus specification and user manual. Rev 3, 19 June 2007.
5. Paper “An improved low power crystal oscillator”, Werner Thommen, EESCIR’99,
Duisburg, Sept. 1999, pp. 146-149.
6. Epson Toyocom, Crystal Devices Catalogue “The crystal master”, 2007.
7. Micro Crystal, Product Catalog Quartz Crystals 2008.