
422
11.2.4
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 transfer instructions.
Figure 11-2 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
TCSR write
Address: H'FF74
H'5A
Write data
15
8 7
0
H'A5
Write data
15
8 7
0
Figure 11-2 Format of Data Written to TCNT and TCSR (Example of WDT0)
Writing to RSTCSR: RSTCSR must be written to by a word transfer to address H'FF76. It
cannot be written to with byte instructions.
Figure 11-3 shows the format of data written to RSTCSR. The method of writing 0 to the WOVF
bit differs from that for writing to the RSTE and RSTS bits.
To write 0 to the WOVF bit, the upper byte of the written word must contain H'A5 and the lower
byte must contain H'00. This clears the WOVF bit to 0, but has no effect on the RSTE and RSTS
bits. To write to the RSTE and RSTS bits, the upper byte must contain H'5A and the lower byte
must contain the write data. This writes the values in bits 6 and 5 of the lower byte into the RSTE
and RSTS bits, but has no effect on the WOVF bit.