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Appendix A: Hardware Reference Manual
An additional IGBT can be implemented for a two-cylinder application that has two independent ignition
coils. This second ignition driver would require the capability to be driven directly from the MCU. This
would give up the fault detection on the second coil that is provided by the MC33812. Diagnostics for the
second ignition coil could be created through additional circuitry beyond the IGBT. Such a circuit would
also require the use of an analog channel of the MCU to detect faults.
10.4.8 Input Signal Conditioning
10.4.8.1
Design Criteria
All sensor and switch inputs must have a filter to reduce noise. The filter must provide appropriate
response for each type of signal.
10.4.8.2
Implementation Recommendations
Low pass filters are implemented on each input signal based on a nominal frequency response:
Input Signal Frequency Response
Module Pin
TPS
ATEMP
ETEMP
MAP
O2
TILT
ENGSTOP
3.0 dB Frequency Cut-off
1.0 kHz
100 Hz
100 Hz
5.0 kHz
500 Hz
100 Hz
100 Hz
These filters can be adjusted to obtain the desired response. As a design recommendation for using the
analog pins of the MC9S12P128, a minimum capacitance of 10 nF should be maintained at the analog
pin. This ensures that transfer of charge during an analog measurement does not impact the
measurement result. Placement of this capacitor should be near the analog pin to reduce noise and
minimize impedance.
Digital inputs associated with switches must be designed for the desired voltage range. This is done by
altering the voltage divider between the ECU pin and the MCU pin. Switch inputs for TILT and Engine
Stop are configured for 12 V operation and may require additional validation based on actual voltages.
The battery voltage is a special case input. The battery input is subject to intensive transient conditions
and requires additional consideration beyond a simple filter. At the connector input, it must be protected
to survive conditions such as reverse polarity and load dump. These are addressed in the reference
design by a reverse blocking diode and a 1500 W TVS. This circuit should be modified to address specific
application demands. Sizing and response of these components should be optimized to very specific test
pulses that represent known transient conditions. For the actual battery voltage measurement itself, a
separate circuit should be considered to balance measurement accuracy with transient protection. The
reference design uses a simple reverse blocking diode with good tolerance of the forward drop voltage.
This is important to maintain the integrity of the battery voltage measurement. Poor tolerance on the input
diode leads to poor accuracy of the battery voltage measurement. Alternative battery measuring circuits
can provide better protection beyond reverse battery. As these circuits incorporate additional components
care must taken to ensure a battery measurement can be made to meet the performance goals of the
system.
Small Engine Reference Design User Manual , Rev. 3.0
54
Freescale Semiconductor
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