| 摘要 |
PURPOSE:To detect the abnormality of a fail-safe circuit itself in a simple constitution without deteriorating the functions of the fail-safe circuit and a control circuit by providing a diagnostic period signal generating circuit to produce a diagnostic period signal having the prescribed time width. CONSTITUTION:A CPU 1, a watchdog timer 2, and a mode timer 4 are reset by a power-on reset signal, the inverse of POC. The timer 4 outputs the diagnostic period signal STBY of the prescribed time width to the CPU 1 and the timer 2 respectively. Thus the timer 2 and the CPU 1 are set in a diagnostic mode, and the timer 2 stops the fail-safe diagnostic action to the CPU 1. Then the CPU 1 outputs a test pattern signal DIAGN and inputs it to the timer 2 via an AND gate 5 while the signal STBY is kept at a high level. Then the presence or absence of the abnormality of the timer 2 is detected by a diagnostic result signal (answer signal) DIAGI outputted from the time-out output Q2 of the timer 2. As a result, the abnormality of a fail-safe circuit is detected without deteriorating the functions of the fail-safe circuit itself and the CPU 1. |
