AN2848 Freescale Semiconductor / Motorola, AN2848 Datasheet - Page 5

• Manufacturer Part Number: AN2848
• Description: Programming the eTPU
• Manufacturer: Freescale Semiconductor / Motorola

Architecture
Let’s look at how this architecture works by considering a classic example; see
Figure
3. If the user wanted
a Pulse Width Modulated (PWM) output signal on one of the channel pins, he would need a TCR and
comparator to cause the output to transition high at some point in time. When that transition occurs, he
would want another transition to be scheduled at a time equal to the time of the first transition, plus a high
time determined by his software.
The first transition will cause a request for the engine to service the channel. The engine executes an
instruction thread, which has been loaded by the host into the program memory, to calculate the high time
for the pulse. The parameters for this calculation would also have been provided by the host in the data
store. The engine would set up channel hardware to make a low transition when the high time expires, at
which time another service request can be issued to the engine to service the low time. If the low transition
executes a similar instruction thread, the process can continue forever without real time intervention by
the host. If the host changes the operating parameters in the data store, the PWM can be modulated.
Figure 3. Controlling a PWM
If PWM were the most complex function required of the eTPU, this note would not be needed. In fact,
systems have been implemented in automotive engine control where the eTPU detects the engine position
and speed, controls fuel, spark, transmission shifting, and emission control valves, and drives several
additional sensors and actuators. Fuel pulses have been designed to modulate the distribution of fuel in
the cylinder, and multiple spark pulses are generated to ensure complete, clean burn of the fuel. In other
eTPU applications, communication pulses have been produced, and complete closed loop motor control
Programming the eTPU, Rev. 0
Freescale Semiconductor
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