4502 datasheet(29/116 Pages) RENESAS

29 / 116 page

4502 Group

Rev.3.01

2005.02.02

page 29 of 112

REJ03B0105-0301

WATCHDOG TIMER

Watchdog timer provides a method to reset the system when a pro-

gram run-away occurs. Watchdog timer consists of timer

WDT(16-bit binary counter), watchdog timer enable flag (WEF),

and watchdog timer flags (WDF1, WDF2).

The timer WDT downcounts the instruction clocks as the count

source from “FFFF16” after system is released from reset.

After the count is started, when the timer WDT underflow occurs

(after the count value of timer WDT reaches “FFFF16,” the next

count pulse is input), the WDF1 flag is set to “1.”

If the WRST instruction is never executed until the timer WDT un-

derflow occurs (until timer WDT counts 65534), WDF2 flag is set to

“1,” and the RESET pin outputs “L” level to reset the microcom-

puter.

Execute the WRST instruction at each period of 65534 machine

cycle or less by software when using watchdog timer to keep the

microcomputer operating normally.

When the WEF flag is set to “1” after system is released from reset,

the watchdog timer function is valid.

When the DWDT instruction and the WRST instruction are ex-

ecuted continuously, the WEF flag is cleared to “0” and the

watchdog timer function is invalid.

The WEF flag is set to "1" at system reset or RAM back-up mode.

The WRST instruction has the skip function. When the WRST in-

struction is executed while the WDF1 flag is “1”, the WDF1 flag is

cleared to “0” and the next instruction is skipped.

When the WRST instruction is executed while the WDF1 flag is “0”,

the next instruction is not skipped.

The skip function of the WRST instruction can be used even when

the watchdog timer function is invalid.

Fig. 25 Watchdog timer function

65534 count

(Note)

Value of 16-bit timer (WDT)

WDF1 flag

➂ WRST instruction

executed

(skip executed)

RESET pin output

WDF2 flag

➄ System reset

➀ Reset

released

➀ After system is released from reset (= after program is started), timer WDT starts count down.

➁ When timer WDT underflow occurs, WDF1 flag is set to “1.”

➂ When the WRST instruction is executed, WDF1 flag is cleared to “0,” the next instruction is skipped.

➃ When timer WDT underflow occurs while WDF1 flag is “1,” WDF2 flag is set to “1” and the

watchdog reset signal is output.

➄ The output transistor of RESET pin is turned “ON” by the watchdog reset signal and system reset is

executed.

Note: The number of count is equal to the number of machine cycle because the count source of watchdog timer

is the instruction clock.

FFFF16

000016

➁

➃

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