4570 Datasheet, PDF(26/69 Page) Renesas Technology Corp – SINGLE-CHIP 4-BIT CMOS MICROCOMPUTER

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4570 Datasheet, PDF (26/69 Pages) Renesas Technology Corp – SINGLE-CHIP 4-BIT CMOS MICROCOMPUTER

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MITSUBISHI MICROCOMPUTERS

4570 Group

SINGLE-CHIP 4-BIT CMOS MICROCOMPUTER

WATCHDOG TIMER

Watchdog timer provides a method to reset the system when a

program runs wild. Watchdog timer consists of 16-bit timer (WDT),

watchdog timer enable flag (WEF), and watchdog timer flags

(WDF1, WDF2).

Timer WDT starts downcounting the instruction clocks as the

count source immediately after system is released from reset.

The underflow signal is generated when the count value reaches

â000016.â This underflow signal can be used as the timer 2 count

source.

When the WRST instruction is executed after system is released

from reset, the WEF flag is set to â1.â At this time, the watchdog

timer starts operating.

When the count value of timer WDT reaches âBFFF16â or

â3FFF16,â WDF1 flag is set to â1.â Then, if the WRST instruction

is not executed while the timer WDT counts 32767, the WDF2

______

flag is set to â1â and the RESET pin outputs âLâ level to reset the

microcomputer. In software using the watchdog timer, make sure

that the WRST instruction is executed in 32766 machine cycles

or less in order to keep the microcomputer operating normally.

To prevent the watchdog timer from stopping in the event of

misoperation, the WEF flag is designed not to be initialized once

the WRST instruction has been executed. Note also that, if the

WRST instruction is never executed, the watchdog timer does

not start.

FFFF16

Value of timer WDT

0000 16

Flag WEF

Flag WDF1

Flag WDF2

BFFF16

3FFF16

RESET pin output

WRST instruction WRST instruction

execution

System reset

Fig. 20 Watchdog timer function

The contents of the WEF flag, the WDF1 and WDF2 flags and

the timer WDT are initialized at the RAM back-up mode.

However, if the WDF2 flag is set to â1â at the same time that the

microcomputer enters the RAM back-up mode, system reset may

be performed.

When using the watchdog timer and the RAM back-up mode,

initialize the WDF1 flag with the WRST instruction just before

the microcomputer enters the RAM back-up mode (refer to Figure

21).

â¢â¢â¢

WRST

EPOF

POF

; Clear WDF1 flag

; POF instruction execution enabled

Oscillation stop (RAM back-up mode)

Fig. 21 Program example to enter the RAM back-up mode

when using the watchdog timer

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