Datasheet
ST10F269 17 - WATCHDOG TIMER
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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 X X X X
Power on after partial supply failure
1) 2)
XXX
Long Hardware Reset X X X
Short Hardware Reset X X
Software Reset X
Watchdog Reset XX
Table 35 : WDTREL Reload Value (PQFP144 devices)
Reload value in WDTREL
Prescaler for f
CPU
= 40MHz
2 (WDTIN = ‘0’) 128 (WDTIN = ‘1’)
FFh 12.8µs819.2ms
00h 3.276ms 209.7ms
Table 36 : WDTREL Reload Value (TQFP144 devices)
Reload value in WDTREL
Prescaler for f
CPU
= 32MHz
2 (WDTIN = ‘0’) 128 (WDTIN = ‘1’)
FFh 16.0µs 1.024ms
00h 4.096ms 262.1ms
P
WDT
1
f
CPU
---------------
512× 1WDTIN] 63) 256 WDTREL ])[(××[+(×=