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Appendix A: Hardware Reference Manual
10.4.2.8
10.4.2.8.1
Power On Reset Generator and Watchdog
Design Criteria
During conditions such as power on or transients in battery voltage due to high load or low battery, it is
required that the system behave in a deterministic and controlled manor to prevent damage to the engine
or ECU itself. This results in the demand for an external monitor of the system voltage. The circuit must
have the ability to shutdown or prevent operation of the MCU when the power gets near the limits of
normal operation. The most common implementation for this is for an external device to hold the RESET
pin of the MCU low unless the 5.0 V system power is within the limits of normal operation. This aids in
preventing situations leading to code runaway. Additional safety is sometimes required to ensure that the
application running on the MCU is not lost or executing in an unexpected way. An external watchdog
circuit requires the MCU to periodically communicate to an external chip to indicate that software has not
slowed execution or runaway. If the MCU fails to provide the proper feedback to the external watchdog,
it will toggle the RESET pin of the MCU and bring the software to a known state.
10.4.2.8.2
Implementation Recommendations
If the MCU needs an external voltage monitor to provide a reset signal, when the supply voltage is first
applied, or it goes out of range, the MC33812 provides this function. Most MCU's must not be allowed to
begin processing until their 5 Volt supply is stable and within the required range. The POR (Power On
Reset) circuit provides this function. When power is first applied to the MC33812, the RESETB pin is held
low by an internal pull down resistor. When the POR circuit sees the VCCSENSE pin exceed ~4.75 Volts,
it starts a 128 μ s timer. When this timer, times out, the POR circuit raises the RESETB pin which allows
the MCU to come out of reset and start processing. This is one of the functions of the POR circuit. Another
function of this circuit is to continuously monitor the VCCSENSE pin for any voltage transients that could
cause the VCCSENSE voltage to go out of range. These voltage interruptions could cause the MCU to
lock-up or begin executing erroneous program instructions. When any out-of-range voltage conditions are
detected, the reset generator will bring the RESETB pin low for 128 μ s and, once the condition has
cleared, will bring the RESETB pin high again to allow the MCU to restart.
The last function of the reset generator circuit on the MC33812 is the watchdog. The watchdog is a timer
circuit that can be programmed with a specific time value, between 1.0 ms and 10 seconds. Once the
watchdog timer is programmed, the MCU must send it a pulse on the WDRFSH pin, at least once during
the programmed time period, to avoid the watchdog from issuing a reset to the MCU. The purpose of the
watchdog is to monitor the MCU to ensure that the program code is running and that the MCU has not
“locked-up”. If the MCU enters an infinite program loop or it executes an erroneous halt instruction, the
watchdog of the MC3812 can toggle the RESETB pin. The MCU must be programmed to “pet” the
watchdog, (i.e. send it a pulse on the WDRFSH pin), periodically, to keep the watchdog from issuing the
reset to the MCU. Care must be taken so that the “pet” of the watchdog is done as a natural part of the
main loop execution and not down by means of a hardware timer.
To program the watchdog timer initially, the MCU must send it a pulse greater than 1 ms but, less than 10
seconds on the WDRFSH line. The default value for the watchdog timer is 10 seconds. If the MCU does
not initialize the watchdog timer, the default value will remain in effect and the watchdog will issue a reset
to the MCU on the RESETB pin when it times out, in 10 seconds. Whenever the watchdog issues a reset,
it reloads the default time value, 10 seconds, into its timer. If the watchdog is not needed, it can be
disabled by pulling the WD_INH pin high through a pull-up resistor to 5.0 Volts. For more details on setting
and using the watchdog timer please see the MC33812 data sheet.
On the reference design, the RESETB pin is connected to the RESET pin of the MC9S12P128 MCU
through a 2.2 kohm resistor. This provision is made so that an external tool can be connected to the BDM
programming header and overdrives the RESETB pin of the MC33812. By implementing this, an external
tool can maintain control of the RESET pin of the MCU during the programming sequence and prevent
an erroneous reset during programming.
50
Freescale Semiconductor
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