| 摘要 |
A watchdog timer includes an instruction decoder, a delay circuit and a counter. The instruction decoder decodes a watchdog timer initialization instruction regularly executed to generate an instruction pulse for initializing the count of the counter. The delay circuit delays the rising edge of the instruction pulse, and supplies the delayed instruction pulse to the counter as a signal for initializing the count. The delay circuit prevents the pulse signal from being supplied to the counter when the operation frequency of the microcomputer is high or when the supply voltage to the microcomputer is low, so that the count of the counter overflows, and the overflow signal causes the microcomputer to be reset. This makes it possible to reset the microcomputer before it runs away, thereby solving a problem of a conventional watchdog timer in that the microcomputer can produce, if it runs away, an unexpected signal from its port before it is reset, and hence can impair the security of the system.
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