| 摘要 |
PURPOSE:To detect runaway of a program in a microprocessor and, at the same time, to prevent the runaway, by detecting access to an undefined address or an abnormal drop in supply voltage in addition to a watch dog timer signal. CONSTITUTION:A one-shot timer 5 is driven and an output signal of a fixed length is generated from a terminal Q when a watch dog timer (WDT) is operated through a program set in a ROM2 and pulses are generated at regular intervals from an interface 4, but a CPU1 is reset when a WDT signal becomes extinct. When runaway occurs in the program, an undefined address area is accessed and, simultaneously, a trap address decoder 10 generates a signal SAT and resets the CPU1. In the free run mode, under which all address areas are repeatedly accessed, of runaway mode, no malfunction detection can be made by means of the WDT only. In the case of drop in supply voltage, the drop is detected by a voltage detecting circuit 20 and a signal for resetting the CPU1 is outputted. |
