Datasheet

Section 13 Watchdog Timer (WDT)
Rev. 6.00 Mar. 18, 2010 Page 477 of 982
REJ09B0054-0600
13.6 Usage Notes
13.6.1 Notes on Register Access
The write method for TCNT, TCSR, and RSTCSR differs from that of normal registers so that
they cannot be easily rewritten. Use the following procedures to read and write these registers.
(1) Writing to TCNT and TCSR
Word transfer instructions must be used to write to TCNT and TCSR. These registers cannot be
written with byte transfer instructions. This is shown in figure 13.6.
For writing, TCNT and TCSR are allocated to the same address. To write to TCNT, transfer a
word in which the upper byte is H'5A and the lower byte is the write data. To write to TCSR,
transfer a word in which the upper byte is H'A5 and the lower byte is the write data. When these
transfer operations are performed, the lower byte data is written to TCNT or TCSR.
TCNT write
TCSR write
Address:
Address:
15 8 7 0
H'5A
H'FF74
Write data
15 8 7 0
H'A5
H'FF74
Write data
Figure 13.6 Writing to TCNT, TCSR
(2) Writing to RSTCSR
Use word transfer operations to write to RSTCSR. This register cannot be written using byte
transfer instructions. This is shown in figure 13.7.
The method used to write a 0 to the WOVF bit and the method used to write the RSTE and RSTS
bits are different.
To write a 0 to the WOVF bit, set the upper byte to H'A5 and the lower byte to H'00 and transfer
that data. This will clear the WOVF bit to 0. This operation does not affect the RSTE and RSTS
bits. To write the RSTE and RSTS bits, set the upper byte to H'5A and the lower byte to the data
to be written and transfer that data. This will write the data in bits 6 and 5 of the lower byte to the
RSTE and RSTS bits. This operation does not affect the WOVF bit.