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10.2.4 Notes on Register Access
The watchdog timer's TCNT, TCSR, and RSTCSR registers differ from other registers in being
more difficult to write. The procedures for writing and reading these registers are given below.
Writing to TCNT and TCSR: These registers must be written by a word transfer instruction.
They cannot be written by byte instructions. Figure 10-2 shows the format of data written to
TCNT and TCSR. TCNT and TCSR both have the same write address. The write data must be
contained in the lower byte of the written word. The upper byte must contain H'5A (password for
TCNT) or H'A5 (password for TCSR). This transfers the write data from the lower byte to TCNT
or TCSR.
TCNT write
Address
H'FFA8*
TCSR write
Address
H'FFA8*
Note:
*
Lower 16 bits of the address.
Figure 10-2 Format of Data Written to TCNT and TCSR
15
8 7
H'5A
15
8 7
H'A5
303
0
Write data
0
Write data
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