UPD784026 Renesas Electronics Corporation., UPD784026 Datasheet - Page 403

• Manufacturer Part Number: UPD784026
• Description: 16/8-bit Single-chip Microcontrollers
• Manufacturer: Renesas Electronics Corporation.

12.4 CAUTIONS
12.4.1 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
(2) The watchdog timer cannot detect inadvertent program loops in the following cases.
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.
<1> If watchdog timer clearance is performed in the timer interrupt processing program
<2> If cases where an interrupt request or macro service is held pending (see 20.9) occur consecutively
<3> If the watchdog timer is cleared periodically when inadvertent program looping is due to an error in the program
<4> If the watchdog timer is periodically cleared by a group of instructions executed when an inadvertent program
<5> If the STOP mode, HALT mode, or IDLE mode is entered as the result of an inadvertent program loop
<6> If watchdog timer runaway also occurs in the event of CPU runaway 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, HALT 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.
logic (if each module of the program functions normally but the overall program does not)
loop occurs
CHAPTER 12 WATCHDOG TIMER FUNCTION
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