AN1857 Freescale Semiconductor / Motorola, AN1857 Datasheet - Page 6

• Manufacturer Part Number: AN1857
• Description: A 3-Phase ac Induction Motor Control System Based on the MC68HC908MR32
• Manufacturer: Freescale Semiconductor / Motorola

Application Note
Fault Protection
PWM Output
Alignment
6
Conditions can arise in the external drive circuitry, requiring that the
PWM signals become inactive immediately. These conditions include
overcurrent, overvoltage, overtemperature, or other error conditions.
The four fault input pins on the MR32’s PWMMC module can be
configured to react in a number of different ways, upon the detection of
a fault. Each fault input has its own interrupt vector. In all fault conditions,
the output of the PWM generator is forced to a known inactive state.
A number of fault control and recovery options is available to the
systems architect. In some cases, it may be desirable to selectively
disable PWM(s) solely with software. Manual and automatic recovery
mechanisms are available that allow certain acceptable fault situations
to occur, such as starting a motor and using a fault input to limit the
maximum startup current. The fault inputs can be partitioned if the MR32
is used to control multiple motors.
Depending on the system design, there is a choice between edge- or
center-aligned PWM signals output from the MR32’s PWM generator.
The PWM counter uses the value in the timer modulus register to
determine its maximum count.
In center-aligned mode, a 12-bit up/down counter is used to create the
PWM period. The PWM resolution in center-aligned mode is two clock
periods (highest resolution is 250 ns @ a processor speed of 8 MHz).
The PWM period will be equal to:
In edge-aligned mode, a 12-bit up-only counter is used to create the
PWM period. Therefore, the PWM resolution in edge-aligned mode is
one clock (highest resolution is125 ns @ a processor speed of 8 MHz).
Again, the timer modulus register is used to determine the maximum
count. The PWM period will be equal to:
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[(timer modulus) x (PWM clock period) x 2].
[(timer modulus) x (PWM clock period)].
MOTOROLA
AN1857