Nxp Semiconductors PCA2125 User Manual

 

 

UM10301_1 

© NXP B.V. 2008. All rights reserved.

•  In the datasheets of PCF8583 and PCF8593 the following method is described: 

Using the alarm function (via the I

2

C-bus) a signal faster than 1 Hz can be generated 

at the interrupt output for fast setting of a trimmer. Procedure: 
ƒ

 Power-on; 

ƒ

  Initialization (alarm functions). 

Routine: 
ƒ

  Set clock to time T and set alarm to time T + ∆T 

ƒ

  At time (T + ∆T) (Interrupt) repeat the routine.  

However, this only works well when ∆T is an integer number of seconds. The 1/10 s 
and the 1/100 s are derived from a combination of 1 Hz, 2 Hz, 4 Hz, 8 Hz, 16 Hz, 
32 Hz etc. signals. The accuracy is therefore only < ± 5 ms. Generating an alarm 
after T + ∆T with ∆T = 20 ms will show a jitter of plus or minus 5 ms which makes 
automatic tuning very complicated. 

 

The PCF2123 incorporates an offset register which can be used to implement several 
functions, e.g.: 

• Ageing adjustment 
• Temperature compensation 
• Accuracy tuning 

 

The offset is made once every two hours in the normal mode, or once every hour in the 
course mode. Each LSB will introduce an offset of 2.17 ppm for normal mode and 4.34 
ppm for course mode. These values are based on a nominal 32.768 kHz clock. The 
offset value is coded in two’s complement giving a range of +63 LSB’s to -64 LSB’s. The 
correction is made by adding or subtracting 64 Hz clock correction pulses, thereby 
changing the period of a single second. 

In normal mode, the correction is triggered once every two hours and then correction 
pulses are applied once per minuted until the programmed correction value has been 
implemented. 

In course mode, the correction is triggered once per hour and then correction pulses are 
applied once per minute up to a maximum of 60. When absolute correction values of 
greater than 60 are used, additional correction pulses are made in the 59

th

 minute. 

It is possible to monitor when correction pulses are applied. The correction interrupt 
enable (CIE) mode will generate a 1/128 second pulse on  INT for every correction 
applied. In the case where multiple correction pulses are applied, a 1/128 second 
interrupt pulse will be generated for each correction pulse applied. Correction is applied 
to the 1 Hz clock. Any timer or clock output using a frequency of 1 Hz or below will also 
be affected by the correction pulses. For more details, refer to the PCF2123 datasheet.