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ST72324BLJ2B5 Datasheet(PDF) 45 Page - STMicroelectronics |
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ST72324BLJ2B5 Datasheet(HTML) 45 Page - STMicroelectronics |
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45 / 151 page  ST72F324L, ST72324BL 45/151 10 ON-CHIP PERIPHERALS 10.1 WATCHDOG TIMER (WDG) 10.1.1 Introduction The Watchdog timer is used to detect the occur- rence of a software fault, usually generated by ex- ternal interference or by unforeseen logical condi- tions, which causes the application program to abandon its normal sequence. The Watchdog cir- cuit generates an MCU reset on expiry of a pro- grammed time period, unless the program refresh- es the counter’s contents before the T6 bit be- comes cleared. 10.1.2 Main Features ■ Programmable free-running downcounter ■ Programmable reset ■ Reset (if watchdog activated) when the T6 bit reaches zero ■ Optional reset on HALT instruction (configurable by option byte) ■ Hardware Watchdog selectable by option byte 10.1.3 Functional Description The counter value stored in the Watchdog Control register (WDGCR bits T[6:0]), is decremented every 16384 fOSC2 cycles (approx.), and the length of the timeout period can be programmed by the user in 64 increments. If the watchdog is activated (the WDGA bit is set) and when the 7-bit timer (bits T[6:0]) rolls over from 40h to 3Fh (T6 becomes cleared), it initiates a reset cycle pulling low the reset pin for typically 500ns. The application program must write in the WDGCR register at regular intervals during normal operation to prevent an MCU reset. This down- counter is free-running: it counts down even if the watchdog is disabled. The value to be stored in the WDGCR register must be between FFh and C0h: – The WDGA bit is set (watchdog enabled) – The T6 bit is set to prevent generating an imme- diate reset – The T[5:0] bits contain the number of increments which represents the time delay before the watchdog produces a reset (see Figure 28. Ap- proximate Timeout Duration). The timing varies between a minimum and a maximum value due to the unknown status of the prescaler when writ- ing to the WDGCR register (see Figure 29). Following a reset, the watchdog is disabled. Once activated it cannot be disabled, except by a reset. The T6 bit can be used to generate a software re- set (the WDGA bit is set and the T6 bit is cleared). If the watchdog is activated, the HALT instruction will generate a Reset. Figure 27. Watchdog Block Diagram RESET WDGA 6-BIT DOWNCOUNTER (CNT) fOSC2 T6 T0 WDG PRESCALER WATCHDOG CONTROL REGISTER (WDGCR) DIV 4 T1 T2 T3 T4 T5 12-BIT MCC RTC COUNTER MSB LSB DIV 64 0 5 6 11 MCC/RTC TB[1:0] bits (MCCSR Register) 1 |
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