Renesas 4514 User Manual

4513/4514 Group User’s Manual

1-35

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

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

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

(WDF1, WDF2).

The timer WDT downcounts the instruction clocks as the count

source. The underflow signal is generated when the count value

reaches “0000

16

.” 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 “1”. At this time, the watchdog

timer starts operating.

When the count value of timer WDT reaches “BFFF

16

” or “3FFF

16

,”

the WDF1 flag is set to “1.” If the WRST instruction is never ex-

ecuted while timer WDT counts 32767, WDF2 flag is set to “1,” and

the RESET pin outputs “L” level to reset the microcomputer. Ex-

ecute the WRST instruction at each period of 32766 machine cycle

or less by software when using watchdog timer to keep the micro-

computer operating normally.

To prevent the WDT stopping in the event of misoperation, WEF

flag is designed not to initialize once the WRST instruction has

been executed. Note also that, if the WRST instruction is never ex-

ecuted, the watchdog timer does not start.

The contents of WEF, WDF1 and WDF2 flags and timer WDT are

initialized at the RAM back-up mode.

If WDF2 flag is set to “1” at the same time that the microcomputer

enters the RAM back-up state, system reset may be performed.

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

tialize the WDF1 flag with the WRST instruction just before the

microcomputer enters the RAM back-up state (refer to Figure 21)

WRST              ; WDF1 flag reset

••

•

POF                 

EPOF             ; POF instruction enabled

(RAM back-up state)     

•

•

•

Oscillation 
stop

FFFF

16

0000 

16

3FFF

16

BFFF

16

The value of timer (WDT)

WDF1 flag

WDF2 flag

WRST 
instruction 
executed

RESET pin output

System reset

WRST 
instruction 
executed

WEF flag