AN1857 Freescale Semiconductor / Motorola, AN1857 Datasheet - Page 7

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

PWM Counter
Timebase
PWM Load
Operations
Direct Output
Control
Dead-Time
Insertion
AN1857
MOTOROLA
CAUTION:
To permit lower PWM frequencies, a prescaler is provided which will
divide the PWM clock frequency by 1, 2, 4, or 8. This prescaler is
buffered and will not be used by the PWM generator until the LDOK bit
located in a PWM control register is set and a new PWM reload cycle
begins.
When generating sine waves to a motor, an interrupt routine is typically
used to step through a sine table located in FLASH memory, scale that
sine value, and output the result to the system from the PWM generator.
The rate at which the sine table is scanned can be derived from an
interrupt from the PWM generator. The PWM module can be
programmed to provide an interrupt rate of every one, two, four, or eight
PWM reload cycles.
In some cases, the user may desire to bypass the PWM generator and
directly control the PWM outputs. A mechanism exists to disconnect the
PWM generator from its outputs and directly control the six PWM
outputs. When this mode is used, the PWM generator continues to run;
however, its normal PWM output is disabled, overridden by direct output.
When the PWM generator is used in complementary mode, automatic
dead-time insertion can be provided to prevent turning on both top and
bottom inverter transistors in the same phase leg at the same time.
When controlling dc-to-ac inverters, the top and bottom PWMs in one
pair must never be active at any given time.
If the top and bottom transistors are turned on simultaneously, large
currents will flow through the two transistors as they attempt to discharge
the bus supply voltage. The transistors could be weakened or destroyed.
Simply forcing the two PWMs to be inversions of each other is not always
sufficient. Since a time delay is associated with turning off the transistors
in the motor drive, there must be a “dead-time” between the deactivation
of one PWM power transistor and the activation of the opposite transistor
in a top and bottom pair.
Dead-time can be specified in the dead-time write-once register. This 8-
bit value specifies the number of CPU clock cycles to use for the dead-
time.
Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
MC68HC908MR32 Pulse-Width Modulator
Application Note
7