342
12.3
Operation
12.3.1
Operation in Watchdog Timer Mode
To use the WDT as a watchdog timer, set the WT/
IT
and TME bits in TCSR to 1. Software must
prevent TCNT overflow by rewriting the TCNT value (normally by writing H'00) before overflow
occurs. If TCNT fails to be rewritten and overflows due to a system crash or the like, a
WDTOVF
signal is output (figure 12.4). The
WDTOVF
signal can be used to reset external system devices.
The
WDTOVF
signal is output for 128
φ
clock cycles.
If the RSTE bit in RSTCSR is set to 1, a signal to reset the chip will be generated internally
simultaneous to the
WDTOVF
signal when TCNT overflows. Either a power-on reset or a manual
reset can be selected by the RSTS bit in RSTCSR. The internal reset signal is output for 512
φ
clock cycles.
When a watchdog reset is generated simultaneously with input at the
RES
pin, the software
distinguishes the
RES
reset from the watchdog reset by checking the WOVF bit in RSTCSR. The
RES
reset takes priority. The WOVF bit is cleared to 0.
Summary of Contents for HD6417032
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Page 525: ...490 CK RAS CAS TRp TRc TRcc tRASD1 tRASD2 tCASD3 tCASD2 TRr tCSR Figure 20 18 Self Refresh ...
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