NXP Semiconductors | UM10301 |
| User Manual PCF85x3, PCA8565 and PCF2123, PCA2125 |
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 I2C 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;
5.Paper “An improved low power crystal oscillator”, Werner Thommen, EESCIR’99, Duisburg, Sept. 1999, pp.
6.Epson Toyocom, Crystal Devices Catalogue “The crystal master”, 2007.
7.Micro Crystal, Product Catalog Quartz Crystals 2008.
UM10301_1 |
| © NXP B.V. 2008. All rights reserved. |
User manual | Rev. 01 — 23 December 2008 | 50 of 52 |
