7721 datasheet(400/577 Pages) RENESAS

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WATCHDOG TIMER

7721 Group User’s Manual

15–5

Write dummy data to the watchdog timer register (address 6016) before the most significant bit of Watchdog

timer becomes “0.” When Watchdog timer is used to detect a program runaway, a watchdog timer interrupt

request occurs if writing to address 6016 is not performed owing to a program runaway and the most

significant bit of Watchdog timer becomes “0.” This means that a program runaway has occurred.

In order to reset the microcomputer when a program runaway is detected, write “1” to the software reset

bit (bit 3 at address 5E16) in the watchdog timer interrupt routine.

15.2 Operation description

RTI

Main routine

Watchdog timer interrupt routine

Watchdog timer register

(Address 6016)

8-bit dummy data

Watchdog timer

interrupt request occur

(program runaway detected)

Watchdog timer initialized

Value of watchdog timer :

“FFF16” (Note 1)

Software reset bit

(Address 5E16, b3)

“1” (Note 2)

Reset microcomputer

Notes 1: Initialize Watchdog timer before the most significant bit of Watchdog timer

becomes “0.” (Write dummy data to address 6016 before a watchdog timer

interrupt request occurs).

2: When a program runaway occurs, values of the data bank register (DT), direct

page register (DPR), etc., may be changed. When “1” is written to the software

reset bit by the addressing mode using DT, DPR, etc., set values to DT and

DPR again.

Fig. 15.2.1 Example of program runaway detection by Watchdog timer

7721 Datasheet(PDF) 400 Page - Renesas Technology Corp