NXP Semiconductors UM10301 PCF2123 - Manual (Page 52)

NXP Semiconductors
UM10301
User Manual PCF85x3, PCA8565 and PCF2123, PCA2125
Please be aware that important notices concerning this document and the product(s)
described herein, have been included in the section 'Legal information'.
© NXP B.V. 2008. All rights reserved.
For more information, please visit: http://www.nxp.com
For sales office addresses, email to: [email protected]
Date of release: 23 December 2008
Document identifier: UM10301_1
22. Contents
1. Introduction .........................................................3
2. Features ...............................................................4
3. Comparison .........................................................6
3.1 Event counter mode ...........................................6
4. Power-on reset (POR) .........................................8
5. Voltage-low detector ...........................................8
6. Oscillator..............................................................9
6.1 Oscillation allowance........................................15
6.2 Using an external oscillator .............................. 15
7. Crystal and crystal selection............................16
7.1 Modes which don’t work ...................................17
7.2 Effect of temperature........................................17
8. Capacitors and capacitor selection ................. 19
9. Accuracy ............................................................20
10. Oscillator tuning................................................22
10.1 PCF2123 Offset register...................................24
11. Century and leap year, Daylight Saving Time. 25
11.1 Century tracking ...............................................25
11.2 Year and leap year tracking .............................25
11.3 Daylight Saving Time (DST).............................25
12. Initialization and setting of alarm and timer....26
12.1 Initialization of the RTC and setting the time .... 26
12.2 Alarm................................................................27
12.3 Setting the timer ...............................................29
13. Backup power supply ....................................... 30
13.1 Lithium Primary cells ........................................30
13.2 NiCd and NiMH secondary batteries ................ 32
13.3 Capacitors ........................................................33
13.3.1 Charging the backup capacitor......................... 34
13.3.2 Estimation of backup time with capacitor ......... 34
13.4 Diode selection.................................................35
14. PCB layout guidelines ......................................36
15. Partial circuit switch down ...............................38
16. Hints to keep power consumption low ............ 39
17. First period inaccuracy when using the timer 43
18. Timing requirements for I
2
C read and write ....45
19. Troubleshooting ................................................48
19.1 Oscillator startup time ......................................48
19.2 Checking for oscillation .................................... 48
19.3 No communication via I
2
C-bus ......................... 49
19.4 Wrong time and date, wrong clock speed ........ 50
20. References .........................................................50
21. Legal information .............................................. 51
21.1 Definitions ........................................................ 51
21.2 Disclaimers ...................................................... 51
21.3 Trademarks...................................................... 51
22. Contents............................................................. 52