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Appendix A: Hardware Reference Manual
Additionally, a small capacitor is placed near the RESET pin of the MCU. This is used as a precaution to
filter noise that can lead to a glitch on the RESET pin. Typically this is not populated in production but is
available as a placeholder that can be populated as electro-magnetic conformance or radiated immunity
testing demands. Sizing of this capacitor should be balanced with the RESET pin functionality of the
MCU. Specifically, any internal reset of the S12P will drive the pin low for 512 PLL Clock cycles and
release. The S12P will then sample the RESET pin 256 cycles after the release to determine if the reset
event was internal or external. Sizing this external capacitor must take this into consideration along with
the any external pull up current source. For the reference design a 100pF capacitor was selected with
the 10K pull up resistor. This is based on a worst case maximum PLL Clock speed of 32MHz and
obtaining a valid high signal level after two time constants before the 256 cycles after the RESET pin is
released. Further details are provided in the datasheet for the S12P device.
An alternative solution for connecting the RESETB pin of the MC33812 to the MC9S12P128 MCU is the
XIRQ pin. By connecting the RESETB pin to an unmaskable interrupt source, the MC9S12P128 can use
its own reset detection circuitry and watchdog. This type of configuration would allow the MC9S12P128
to operate down to its minimum voltage of 3.13 V. Refer to datasheet of MC9S12P128 for further details.
The advantage of running down to this voltage level is increased time from a loss of power event to loss
of operation. Providing the maximum time during a loss of power event allows the size of the bulk storage
capacitor on the 5.0 V system voltage to be reduced in size when compared to 5.0 V only operation. The
total time required between a loss of power and loss of operation is heavily driven by the amount of data
necessary to be stored in non-volatile memory during system shutdown. If the system can perform to a
lower voltage, it has a longer time available for operation on a given capacitance. The design can then
optimize the bulk charge storage on the system supply to incorporate a small capacitor, which can provide
lower cost.
10.4.3 Power Supply
10.4.3.1
Design Criteria
System power is derived from a 12 V battery. The battery has a nominal 13.8 V output which must be
reduced to a system voltage of 5.0 V and provide at least 150mA to the system including external
modules. Performance of the 5.0 V supply must have greater than 5% accuracy to prevent the need of a
separate voltage reference for analog measurements.
10.4.3.2
Implementation Recommendations
The main control for the power supply is integrated into the MC33812 device through a pre-regulator. This
is discussed in detail in the 5.0 Volt regulator pre-driver section for the Small Engine IC block.
While the reference design implements a hard power on strategy through the ignition switch, it is possible
to architect a power control circuit that enables a more controlled power down process. This would enable
the MCU to control when power down occurs and reduce the constraints associated with power off
detection. For such a system, the module would incorporate an ignition switch signal, and a direct
connection to the battery. Power coming from the battery would go to control logic that performs an OR
logic function between the ignition switch signal and a signal from the MCU. This allows the battery power
to turn on when the ignition switch is keyed on and when the MCU has determined it needs power. When
the key is turned off, the MCU can hold power on through the OR logic and conclude any necessary
activity, such as storage of data to non-volatile memory, and then shutdown power. This functionality is
very attractive but has its trade-offs. The main trade off is cost. Additional components are required. The
second is reduced operating range. Since additional components are placed in the battery power path,
additional tolerance and error is presented to the battery voltage powering circuits, such as the 5.0 V
regulator. Low battery conditions suffer the most as the additional drop across the control logic reduces
internal voltages, creating a smaller operating range.
Freescale Semiconductor
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