33812 Datasheet, PDF (18/25 Pages) Freescale Semiconductor, Inc – SMALL ENGINE CONTROL IC

FUNCTIONAL DEVICE OPERATION
WATCHDOG TIMER OPERATION
WATCHDOG TIMER OPERATION
The purpose of the watchdog timer is to provide a RESET
to the MCU whenever the MCU is locked up in a loop or
otherwise hung up, perhaps by executing erroneous code,
such as a HALT instruction. The watchdog timer is initialized
by a power on RESET or a RESET that occurs after a fault
such as an under-voltage event on VCC.
Whenever the watchdog timer is refreshed, it is always
reloaded with the value WD_RELOAD which initially has a value
of WD_MAX seconds. Whenever a RESET occurs, the
WD_RELOAD value is set to WD_MAX seconds and the watchdog
timer is re-loaded with this value. When the RESET pulse
returns high, and, if the WDRFSH line is low, the watchdog
timer starts counting. If the watchdog timer reaches the
WD_MAX value before the next rising edge on the WDRFSH
line, the watchdog circuit will generate a RESET pulse to the
MCU and reload itself with the maximum time value of
WD_RELOAD, which will have been set back to WD_MAX
seconds.
In normal operation, the MCU will issue a WDRFSH pulse,
periodically, which re-loads the watchdog timer with the
WD_RELOAD value and starts the counting again, thus avoiding
a watchdog timer generated RESET pulse. When the
watchdog timer is refreshed by a WDRFSH pulse, before the
watchdog timer reaches the programmed value, the refresh
will prevent a RESET pulse from being issued to the MCU.
Loading the Watchdog Timer and WD_RELOAD
Aside from the RESET case, which always loads the
WD_RELOAD value and the watchdog timer with the maximum
time value, WD_MAX, there is an additional way that the
watchdog timer and the value WD_RELOAD can be re-loaded.
During initialization, if the WDRFSH pulse width is greater
than WD_LOAD, both the watchdog timer and the value
WD_RELOAD will be loaded with a timer count value,
corresponding to the width of the pulse present on the
WDRFSH input. Once this value is set, no further setting of
the WD_LOAD value is possible until a RESET is performed.
Once the WDRFSH input goes low, the watchdog timer will
begin incrementing again, counting up to the new value that
has been loaded into the reload register. The watchdog must
be refreshed by another pulse on the WDRFSH line, before
the watchdog timer counts up to the reload value, or else a
RESET pulse will be generated and sent to the MCU.
If the WDRFSH line is ever kept high for longer than
WD_RELOAD seconds, the watchdog will issue an immediate
RESET to the MCU. Upon receiving a RESET input from the
33812, the MCU should always be programmed to bring the
WDRFSH line low to avoid being locked in a “deadly
embrace” condition where the MCU and 33812 alternate
back and forth between the RESET and Normal states.
Disabling the Watchdog Timer
If the WD_INH line is pulled high through a pull-up resistor
of 10 K or less, (i.e. not tied to ground), the watchdog timer
will be inhibited from issuing a RESET to the MCU, while the
line is held in this state. This “watchdog Inhibited” state
should only be used during software testing and development
to avoid being concerned about an inadvertent watchdog
RESET.
Watchdog Timing Diagrams
Watchdog Loaded with WD_MAX
5
4
3
WDRFSH
2
1
0
Time
Holding WDRFSH high will trigger RESET every WD_MAX time
Voltage
5
RESET
4
3
2
1
Watchdog timer
& WD_RELOAD= WD_MAX
0
RESET loads the watchdog timer and WD_RELOAD with WD_MAX^Time
Voltage
5
4
3
2
1
0
PW_A<WD_LOAD
WDRFSH
Time
Refresh pulses, PW_A, on WDRFSH load the Watchdog timer with the
WD_RELOAD
Voltage
5
4
3
PW_B >WD_LOAD
2
1
0
WDRFSH
Time
During initialization, for the first WDRFSH pulse only, PW_B, that is greater
than WD_LOAD but less than WD_MAX, the Watchdog timer and WD_RELOAD
value will be loaded with a time value corresponding to the width of that
pulse, PW_B. All pulses on the WDRFSH line width less than WD_RELOAD, will
result in the Watchdog timer being reloaded with the time value correspond-
ing to PW_B. This programmability is only allowed once per RESET.
33812
18
Analog Integrated Circuit Device Data
Freescale Semiconductor