PXS20RM Datasheet, PDF (664/1368 Pages) Freescale Semiconductor, Inc – PXS20 Microcontroller

Flexible Motor Control Pulse Width Modulator Module (FlexPWM)
The FORCE_OUT signal is provided mainly for commutated applications. When PWM signals are
commutated on an inverter controlling a brushless DC motor, it is necessary to restart the PWM cycle at
the beginning of the commutation interval. This action effectively resynchronizes the PWM waveform to
the commutation timing. Otherwise, the average voltage applied to a motor winding integrated over the
entire commutation interval will be a function of the timing between the asynchronous commutation event
with respect to the PWM cycle. The effect is more critical at higher motor speeds where each commutation
interval may consist of only a few PWM cycles. If the counter is not initialized at the start of each
commutation interval, the result will be an oscillation caused by the beating between the PWM frequency
and the commutation frequency.
25.3.3.4 PWM generation
Figure 25-16 illustrates how PWM generation is accomplished in each submodule. In each case, two
comparators and associated VALx registers are utilized for each PWM output signal. One comparator and
VALx register is used to control the turn on edge while a second comparator and VALx register control the
turn off edge.
VAL0
16 bit counter
FORCE_OUT
FORCE_EN
Force Init
VAL1
16 bit
comparator
PWMX_INIT
16 bit
comparator
PWM on
D
S
Q
R
PWM off
Half Comp
PWMX
(inverted
Local Sync)
Mod Comp
VAL2
16 bit
comparator
PWM23_INIT
PWM on
S
DQ
PWM23
VAL3
VAL4
16 bit
comparator
16 bit
comparator
PWM45_INIT
R
PWM off
PWM on
S
DQ
to Force Out
logic
PWM45
VAL5
16 bit
comparator
R
PWM off
Output Triggers
Compare Interrupts
Figure 25-16. PWM Generation Hardware
25-18
PXS20 Microcontroller Reference Manual, Rev. 1
Freescale Semiconductor