ST10R172L datasheet(24/68 Pages) STMICROELECTRONICS

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ST10R172L - WATCHDOG TIMER

WATCHDOG TIMER

The Watchdog Timer is a fail-safe mechanism which limits the malfunction time of the

controller. The Watchdog Timer is always enabled after device reset and can only be disabled

in the time interval until the EINIT (end of initialization) instruction has been executed. In this

way, the chip’s start-up procedure is always monitored. The software must be designed to

service the Watchdog Timer before it overflows. If, due to hardware or software related

failures, the software fails to maintain the Watchdog Timer, it will overflow generating an

internal hardware reset and pulling the RSTOUT pin low to reset external hardware

components.

The Watchdog Timer is a 16-bit timer, clocked with the system clock divided either by 2 or by

128. The high byte of the Watchdog Timer register can be set to a pre-specified reload value

(stored in WDTREL) in order to allow further variation of the monitored time interval. Each

time it is serviced by the application software, the high byte of the Watchdog Timer is

reloaded. The table below shows the watchdog time range which for a 50MHz CPU clock

rounded to 3 significant figures.

011

SSP clock = CPU clock divided by 16

3.13 MBit/s

100

SSP clock = CPU clock divided by 32

1.56 MBit/s

101

SSP clock = CPU clock divided by 64

781 KBit/s

110

SSP clock = CPU clock divided by 128

391 KBit/s

111

SSP clock = CPU clock divided by 256

195 KBit/s

Reload value

in WDTREL

Prescaler for fCPU

2 (WDTIN = ‘0’)

128 (WDTIN = ‘1’)

FFH

10.24 µs

655 µs

00H

2.62 ms

168 ms

Table 9 Watchdog timer range

SSPCKS Value

Synchronous baud rate

Table 8 Synchronous baud rate and SSPCKS reload values

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