ST10F280_12 datasheet(170/239 Pages) STMICROELECTRONICS

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Watchdog timer

ST10F280

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Doc ID 8673 Rev. 3

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.

Note:

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

Note:

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

The Table 34 shows the watchdog time range for 40MHz CPU clock.

The watchdog timer period is calculated with the following formula:

Table 33.

WDTCON bits value on different resets

Reset source

PONR

LHWR

SHWR

SWR

WDTR

Power On Reset

XXX

Power on after partial supply

failure

Long Hardware Reset

Short Hardware Reset

Software Reset

Watchdog Reset

Table 34.

WDTREL reload value

Reload value in WDTREL

Prescaler for fCPU = 40MHz

2 (WDTIN = ‘0’)

128 (WDTIN = ‘1’)

FFh

12.8

μs819.2ms

00h

3.276ms

209.7ms

WDT

CPU

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

512

1WDTIN

] 63 )

256

WDTREL

])

[

–

(

[

(

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