Fujitsu MB90390 Series Hardware Manual page 269

Table 15.2-1 Timer Control Register (Lower)
Bit name
bit7 to
Reserved
bit5
bit4,
Undefined
bit3
bit2 UPDT: Update bit
OE:
bit1
Output enable bit
bit0 ST: Start bit
These are reserved bits. Always write "0" to these bits.
The UPDT bit is prepared for modifying the Second/Minute/Hour counter values.
To modify the counter values, write the modified data in the Second/Minute/Hour
registers. Then set the UPDT bit to "1". The register values are loaded to the counter
at the next CO signal from the 22-bit prescaler. The UPDT bit is reset by the hardware
when the counter values are updated. However, if the set operation by software and
the reset operation by hardware occur at the same time, the UPDT bit will not be
reset.
Note:
If this bit is set during "59 second", normal up count operation is executed and this
bit is reset to "0" without reflecting the Second/Minute/Hour register values.
Writing "0" to the UPDT bit has no effect and a read-modify-write (RMW)
instruction results in reading "0".
When the OE bit is set to "1", the WOT external pin serves as the output for the
Watch Timer. Otherwise it can be used as a general purpose I/O or for another
peripheral block.
When the ST bit is set to "1", the Watch Timer loads Second/Minute/Hour values
from the registers and starts its operation. When it is reset to "0", all the counters and
the prescalers are reset to "0" and halts.
CHAPTER 15 WATCH TIMER
Function
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