Nxp Semiconductors PCA2125 User Manual

 

 

UM10301_1 

© NXP B.V. 2008. All rights reserved.

The internal timer is an 8-bit countdown timer which is controlled by the timer control 
register. The timer counts down from a software-loaded 8-bit binary value. It can be 
clocked by four different source clock frequencies: 4096 Hz, 64 Hz, 1 Hz or 1/60 Hz. 

This example for the PCF8563 generates an interrupt after 10 ms: 

•  Clock to be used 4096 Hz; 
•  Number of clock pulses needed = 0.01 x 4096 = 40; 
•  Error = 40 / 4096 Hz – 0.01 = -234 μs; 
•  Length of I

2

C-bus initialization: 3 start conditions, 3 pulses each + 9 bytes, 9 pulses 

each = 90 clocks @ 400 kHz = 225 μs. Creating the clock asynchronously also gives 
an error of up to 1 clock pulse (see Chapter 17 “First period inaccuracy when using 
the timer”.). The interrupt will generate an output pulse after 9.991 ms or if the 
counter is set to 41 the interrupt will start after 10.236 ms.  

•  The timer is started by the acknowledge of the start timer instruction. 

 

  

 

generate 

I

2

C start condition 

1 0 1 0   0 0 1 0 

A2 

 

I

2

C slave address, write 

0 0 0 0   0 0 0 1 

01 

 

word address 1 (control/status register 2) 

0 0 0 0   0 0 0 1 

01 

01 

clear all flags, enable timer interrupt 

  

 

generate 

I

2

C start condition (repeated start) 

1 0 1 0   0 0 1 0 

A2 

 

I

2

C slave address, write 

0 0 0 0   1 1 1 1 

0F 

 

Word address 0F

HEX

 for timer value 

0 0 1 0   1 0 0 0 

28 

0F 

Timer value set to 40, 28

HEX

 

  

 

generate 

I

2

C start condition (repeated start) 

1 0 1 0   0 0 1 0 

A2 

 

I

2

C slave address, write 

0 0 0 0   1 1 1 0 

0E 

 

word address OE for timer control byte 

1 0 0 0   0 0 0 0 

80 

0E 

Select clock frequency 4096 Hz and start timer 

  

 

generate 

I

2

C stop condition 

 

Refer to chapter 17 “First period inaccuracy when using the timer” for further details on 
timer operation.