H8SSLASH2225 datasheet(563/1071 Pages) RENESAS

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Section 13 Watchdog Timer (WDT)

Rev. 5.00 Aug 08, 2006 page 477 of 982

REJ09B0054-0500

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:

H'5A

H'FF74

Write data

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.

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