Notes On Register Access - Hitachi H8S/2646 Hardware Manual

12.2.4

Notes on Register Access

The watchdog timer's TCNT, TCSR, and RSTCSR registers differ from other registers in being
more difficult to write to. The procedures for writing to and reading these registers are given
below.
Writing to TCNT and TCSR: These registers must be written to by a word transfer instruction.
They cannot be written to with byte instructions.
Figure 12-2 shows the format of data written to TCNT and TCSR. TCNT and TCSR both have the
same write address. For a write to TCNT, the upper byte of the written word must contain H'5A
and the lower byte must contain the write data. For a write to TCSR, the upper byte of the written
word must contain H'A5 and the lower byte must contain the write data. This transfers the write
data from the lower byte to TCNT or TCSR.
TCNT write
Address: H'FF74
TCSR write
Address: H'FF74
Figure 12-2 Format of Data Written to TCNT and TCSR (WDT0)
15
H'5A
15
H'A5
8 7
Write data
8 7
Write data
0
0
423