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Rev. 1.00, 05/04, page 231 of 544
11.6 Usage Notes
11.6.1 Notes on Register Access
The watchdog timer's registers, TCNT and TCSR differ from other registers in being more
difficult to write to. The procedures for writing to and reading from these registers are given
below.
Writing to TCNT and TCSR (Example of WDT_0): These registers must be written to by a
word transfer instruction. They cannot be written to by a byte transfer instruction.
TCNT and TCSR both have the same write address. Therefore, satisfy the relative condition
shown in figure 11.6 to write to TCNT or TCSR. To write to TCNT, the upper bytes must contain
the value H'5A and the lower bytes must contain the write data before the transfer instruction
execution. To write to TCSR, the upper bytes must contain the value H'A5 and the lower bytes
must contain the write data.
<TCNT write>
<TCSR write>
Address : H'FFA8
Address : H'FFA8
H'5A Write data
15 8 7 0
0
H'A5 Write data
15 8 7 0
0
Figure 11.6 Writing to TCNT and TCSR (WDT_0)
Reading from TCNT and TCSR (Example of WDT_0): These registers are read in the same
way as other registers. The read address is H'FFA8 for TCSR and H'FFA9 for TCNT.