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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. 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 12.2 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
TCSR write
Address
Note:
*
Lower 20 bits of the address in advanced mode.
Figure 12.2 Format of Data Written to TCNT and TCSR
15
H'FFF8C *
15
H'FFF8C *
8 7
H'5A
8 7
H'A5
0
Write data
0
Write data
417
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