Fujitsu F2MC-8FX Hardware Manual page 528

CHAPTER 27 REAL TIME CLOCK
Auto-calibration
Recommended Procedure of transition to and from auto-calibration:
1) Writing "1" to RTCCRH:PAU.
2) Transition to calibration mode.
3) Writing "0" to RTCCRH:PAU after transition from auto-calibration.
Limitation:
If user want to transit to power save mode, auto-calibration must be completed. Writing "0" to
CALCR:CALEN can stop auto-calibration process.
Operation of second counter and prescaler
Prescaler value: 7FFF + offset value(FFTR)
Prescaler value: 7FFF - offset value(FFTR)
Prescaler value
8001
7FFF
7FFD
FFTR value:0H
0000
SEC
FFTR
In manual calibration or auto-calibration function, writing data to frequency fine-tuning register (FFTR)
can correct the clock error. It can be expained by the operation of second counter and prescaler. For
example, writing 8'H02 to FFTR, prescaler value change to 7FFD by using the equation 7FFF-offset
value(FFTR). When the counter value matches the prescaler value, the second counter increase one.
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Figure 27.5-1 operation of second counter and prescaler
FFTR value:2
000000H
trigger & increase 1 in sec counter
8'H00
write 02 to FFTR value
(positive FFTR value)
(negative FFTR value)
FFTR value:FEH (-2d)
000001H
8'H02
write FE to FFTR value
time
000002H
8'HFE