UPD784938 Datasheet, PDF (391/733 Pages) NEC – 16-Bit Single-Chip Microcontrollers

CHAPTER 13 WATCHDOG TIMER
13.4 Cautions
13.4.1 General cautions on use of watchdog timer
(1) The watchdog timer is one means of detecting inadvertent program loops, but it cannot detect all inadvertent program
loops. Therefore, in equipment that requires a high level of reliability, you should not rely on the on-chip watchdog
timer alone, but should use external circuitry for early detection of inadvertent program loops, to enable processing
to be performed that will restore the normal state or establish a stable state and then stop the operation.
(2) The watchdog timer cannot detect inadvertent program loops in the following cases.
<1> If watchdog timer clearance is performed in the timer interrupt service program
<2> If cases where an interrupt request or macro service is held pending (see 23.9) occur consecutively
<3> If the watchdog timer is cleared periodically when inadvertent program looping is due to an error in the program
logic (if each module of the program functions normally but the overall program does not)
<4> If the watchdog timer is periodically cleared by a group of instructions executed when an inadvertent program
loop occurs
<5> If the STOP mode or IDLE mode is entered as the result of an inadvertent program loop
<6> If watchdog timer inadvertent program loop also occurs in the event of CPU inadvertent program loop due to
external noise
In cases <1>, <2>, and <3> the program can be amended to allow detection to be performed.
In case <4>, the watchdog timer can only be cleared by a 4-byte dedicated instruction. Similarly, in case <5>, the STOP
mode or IDLE mode cannot be set unless a 4-byte dedicated instruction is used. For state <2> to be entered as the
result of an inadvertent program loop, 3 or more consecutive bytes of data must comprise a specific pattern (e.g. BT
PSWL.bit, $$, etc.). Therefore, the establishment of state <2> as the result of <4>, <5> or an inadvertent program
loop is likely to be extremely rare.
Preliminary User’s Manual U13987EJ1V0UM00
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