4501 datasheet(142/212 Pages) RENESAS

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2.3 Timers

2-25

4501 Group

APPLICATION

Rev.2.01

Feb 07, 2005

REJ09B0192-0201

(4)

Timer operation: timer start by external input

Outline: The constant period can be measured by external input.

Specifications: System clock frequency f(XIN) = 4 MHz and timer 1 operates by INT input as a

trigger and an interrupt occurs after 1 ms.

Figure 2.3.6 shows the setting example of timer start.

(5)

Watchdog timer

Watchdog timer provides a method to reset the system when a program run-away occurs.

Accordingly, when the watchdog timer function is set to be valid, execute the WRST instruction at

a certain period which consists of timer 16-bit timers’ 65534 counts or less (execute WRST instruction

at a cycle of 65534 machine cycles or less).

Outline: Execute the WRST instruction in 16-bit timer’s 65534 counts at the normal operation. If a

program runs incorrectly, the WRST instruction is not executed and system reset occurs.

Specifications: System clock frequency f(XIN) = 4.0 MHz is used, and program run-away is detected

by executing the WRST instruction in 49 ms.

Figure 2.3.2 shows the watchdog timer function, and Figure 2.3.7 shows the example of watchdog

timer.

Fig. 2.3.2 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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