AN974 Datasheet, PDF (2/6 Pages) STMicroelectronics – Real time clock with ST7 Timer Output Compare

Real time clock with ST7 Timer Output Compare
Figure 1. Output Compare Register Update Flowchart
STORE OCxHR AND OCxLR REGISTER
VALUES IN TEMPORARY REGISTERS
UPDATE OCxHR AND OCxLR VALUES
THROUGH TEMPORARY REGISTERS
STORE NEW OCxHR REGISTER VALUE
STORE NEW OCxLR REGISTER VALUE
Based on the assumption that f_CPU is 4-MHz, Table 1 gives some basic possible time bases
with the Output compare register offset value (OCxHR, OCxLR).
f_timercnt = ^fCPU
(cc1,cc0)
4 MHz
=
8
= 500 KHz
TimeBase TimeBase
(OCxHR OCxLR )_offset = _Ttimercnt = _{2 us}
T_timercnt = 2 us
Table 1. Output Compare Register and Timebase Values with f_timercnt at 500KHz
Time Base [ms]
2
10
20
50
100
131.07
OCxHR OCxLR offset [hex]
03E8 1388
2710
61A8
C350
FFFF
3 REAL TIME CLOCK APPLICATION
The real time clock application is based on four software counters (100ms, seconds, minutes
and hours) updated by a 100ms ST7 timer output compare 1 interrupt.
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