mc33812 Freescale Semiconductor, Inc, mc33812 Datasheet - Page 18

• Manufacturer Part Number: mc33812
• Description: Multifunctional Ignition And Injector Driver
• Manufacturer: Freescale Semiconductor, Inc

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.
reloaded with the value WD
of WD
WD
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
line, the watchdog circuit will generate a RESET pulse to the
MCU and reload itself with the maximum time value of
WD
seconds.
periodically, which re-loads the watchdog timer with the
WD
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
WD
time value, WD
watchdog timer and the value WD
than WD
WD
corresponding to the width of the pulse present on the
WDRFSH input. Once this value is set, no further setting of
the WD
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.
WD
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
of 10 K or less, (i.e. not tied to ground), the watchdog timer
18
33812
FUNCTIONAL DEVICE OPERATION
WATCHDOG TIMER OPERATION
The purpose of the watchdog timer is to provide a RESET
Whenever the watchdog timer is refreshed, it is always
In normal operation, the MCU will issue a WDRFSH pulse,
Aside from the RESET case, which always loads the
During initialization, if the WDRFSH pulse width is greater
Once the WDRFSH input goes low, the watchdog timer will
If the WDRFSH line is ever kept high for longer than
If the WD_INH line is pulled high through a pull-up resistor
RELOAD
MAX
RELOAD
RELOAD
RELOAD
RELOAD
RELOAD
MAX
LOAD
value before the next rising edge on the WDRFSH
LOAD
seconds. Whenever a RESET occurs, the
, which will have been set back to WD
value is set to WD
value and starts the counting again, thus avoiding
value and the watchdog timer with the maximum
will be loaded with a timer count value,
seconds, the watchdog will issue an immediate
value is possible until a RESET is performed.
, both the watchdog timer and the value
MAX
, there is an additional way that the
RELOAD
MAX
seconds and the watchdog
RELOAD
which initially has a value
RELOAD
can be re-loaded.
WATCHDOG TIMER OPERATION
MAX
Voltage
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
Voltage
Refresh pulses, PW
WD
During initialization, for the first WDRFSH pulse only, PW
than WD
value will be loaded with a time value corresponding to the width of that
pulse, PW
result in the Watchdog timer being reloaded with the time value correspond-
ing to PW
Voltage
1
0
5
4
3
2
1
0
5
4
3
2
1
0
RESET loads the watchdog timer and WD
5
4
3
2
RELOAD
1
0
5
4
3
2
Holding WDRFSH high will trigger RESET every WD
LOAD
B
B
. This programmability is only allowed once per RESET.
. All pulses on the WDRFSH line width less than WD
but less than WD
A
, on WDRFSH load the Watchdog timer with the
Analog Integrated Circuit Device Data
MAX
WDRFSH
WDRFSH
, the Watchdog timer and WD
PW
MAX
RESET
WDRFSH
Watchdog timer
& WD
A
<
WD
PW
Freescale Semiconductor
RELOAD
RELOAD
LOAD
B
>
WD
= WD
LOAD
with WD
MAX
MAX
B
, that is greater
time
MAX
RELOAD,
RELOAD
Time
Time
Time
Time
will