AN2159 Datasheet, PDF (4/8 Pages) STMicroelectronics – SPI protocol for STPM01/STPM10 metering devices

Application Note
Freescale Semiconductor, Inc.
Advance Timing
Control
Throttle Control
4
FROM
SIGNAL
SHAPING
D4
TO IGNITION
CIRCUIT
D3
DISABLE
ANALOG IGNITION
(FROM MCU)
Q4
DIGITAL ADVANCE TIMING
(FROM MCU)
Figure 4. Analog Ignition Control
At the heart of the advance timing control circuit is the MC68HC908JK3 MCU.
The main function of the MCU is to monitor the speed of the engine and throttle
setting, and calculating the optimum ignition time. The engine speed is
determined by using the MC68HC908JK3’s 16-bit timer to calculate the time
interval for the recurring pulses (from the signal shaping circuit) to the IRQ pin.
Together with the throttle setting input, the optimum time for the ignition pulse
is calculated. Before the ignition spark can be fired, time is required for the HT
primary coil to charge up and collapsed at the ignition instance. Therefore,
before the ignition instance, the MCU has to switch on the IGBT to charge up
the primary coil. At the ignition instance, the MCU switches off the IGBT. This
causes the current flowing in the primary coil to collapse, and inducing a high
voltage on the secondary coil to the spark plug.
It should be noted that the HT primary coil should not be charged for excessive
periods. Long period of continuous charging causes the primary coil to
overheat. Therefore, the coil should only be charged just before the ignition
pulse is required. This charging period is controlled by the MCU.
In the demo, the engine speed is indicated using an external tachometer. This
tachometer is driven by the TCH1 pin, configured as a PWM output, from the
MC68HC908JK3’s timer. The PWM pulse width is proportional to the pulses
from the pulser.
The throttle input can be simulated using two methods. An analog throttle can
be simulated using an ADC input of the MC68HC908JK3 to monitor the
variable voltage from a potentiometer, as in Figure 5a. With the 8-bit ADC, the
input level can be divided into 256 steps. The alternative method is to use a
standard MC68HC908JK3 I/O port to monitor throttle ON and throttle OFF, as
in Figure 5b.
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AN2159
MOTOROLA