M38203M4 Datasheet, PDF (259/344 Pages) Mitsubishi Electric Semiconductor – 8-BIT SINGLE-CHIP MICROCOMPUTER

APPLICATION
2.8 Watchdog timer
2.8 Watchdog timer
2.8.1 Explanation of operations
The watchdog timer is a down-count timer consisting of 14 bits (6 high-order bits and 8 low-order bits).
Each time a count source is input, a count value is decremented by 1.
The watchdog timer is also available as a 6-bit timer.
(1) Basic operations
ŒBy executing a write instruction to the watchdog timer control register (address 003716), “3F16” and
“FF16” are automatically set in the watchdog timer H and watchdog timer L respectively, with the
result that a count operation starts.
A count source can be selected by the watchdog timer H count source selection bit (bit 7 at address
003716) (Refer to “(2) and (3)” on the next page).
When a specified count value is counted and the watchdog timer H underflows, an internal reset
signal is generated, so that a microcomputer is put into the reset status. Table 2.8.1 shows the
program runaway detection time ^V1 (maximum) and Figure 2.8.1 shows an internal reset signal
output timing diagram.
ŽThe program starts from the contents of the vector address at reset.
V1: The time from start of count operation until
output of internal reset signal.
Table 2.8.1 Program runaway detection time (maximum)
Detection time (maximum)
f(XIN) = 8 MHz
f(XCIN) = 32 kHz
32.768 ms
8.19 s
When the watchdog timer H count source selec-
tion bit is “0.”
XIN input signal
XCIN input signal
Internal reset signal
Approximately 1 ms at f(XIN) = 8 MHz
Detection of program runaway
(Underflow of watchdog timer H)
Fig. 2.8.1 Internal reset signal output timing
When using the watchdog timer, set by software so that data is written into the watchdog timer control
register before the watchdog timer H underflows (within the program runaway detection time shown
in Table 2.8.1). If a write instruction has not been executed to the watchdog timer control register
because of a program runaway, internal reset occurs, so that the program restores to a normal routine.
3820 GROUP USER’S MANUAL
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