ST10F269Z2T6 datasheet(113/184 Pages) STMICROELECTRONICS

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ST10F269

17 - WATCHDOG TIMER

113/184

The PONR flag of WDTCON register is set if the output voltage of the internal 3.3V supply falls below the

threshold (typically 2V) of the power-on detection circuit. This circuit is efficient to detect major failures of

the external 5V supply but if the internal 3.3V supply does not drop under 2 volts, the PONR flag is not

set. This could be the case on fast switch-off / switch-on of the 5V supply. The time needed for such a

sequence to activate the PONR flag depends on the value of the capacitors connected to the supply and

on the exact value of the internal threshold of the detection circuit.

Notes: 1. PONR bit may not be set for short supply failure.

2. For power-on reset and reset after supply partial failure, asynchronous reset must be used.

In case of bi-directional reset is enabled, and if the RSTIN pin is latched low after the end of the internal

reset sequence, then a Short hardware reset, a software reset or a watchdog reset will trigger a Long

hardware reset. Thus, Reset Indications flags will be set to indicate a Long Hardware Reset.

The Watchdog Timer is 16-bit, clocked with the system clock divided by 2 or 128. The high Byte of the

watchdog timer register can be set to a pre-specified reload value (stored in WDTREL).

Each time it is serviced by the application software, the high byte of the watchdog timer is reloaded. For

security, rewrite WDTCON each time before the watchdog timer is serviced

Table 35 shows the watchdog time range for 40MHz CPU clock and Table 36 shows the watchdog time

range for 32MHz CPU clock.

The watchdog timer period is calculated with the following formula:

Table 34 : WDTCON Bit Value on Different Resets

Reset Source

PONR

LHWR

SHWR

SWR

WDTR

Power On Reset

Power on after partial supply failure

1) 2)

XXX

Long Hardware Reset

Short Hardware Reset

Software Reset

Watchdog Reset

Table 35 : WDTREL Reload Value (PQFP144 devices)

Reload value in WDTREL

Prescaler for fCPU = 40MHz

2 (WDTIN = ‘0’)

128 (WDTIN = ‘1’)

FFh

12.8

µs819.2ms

00h

3.276ms

209.7ms

Table 36 : WDTREL Reload Value (TQFP144 devices)

Reload value in WDTREL

Prescaler for fCPU = 32MHz

2 (WDTIN = ‘0’)

128 (WDTIN = ‘1’)

FFh

16.0

µs

1.024ms

00h

4.096ms

262.1ms

WD T

CPU

---------------

512

1W DT IN

] 63 )

256

W DT REL

])

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–

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