Advance Information MC68HC908MR16/MC68HC908MR32 — Rev. 4.0
156 Pulse-Width Modulator for Motor Control (PWM MC) MOTOROLA
Pulse-Width Modulator for Motor Control
To accommodate other current sensing schemes, setting
ISENS1:ISENS0 = 11 causes the polarity of the current sense p in to b e
latched half-way into the PWM cycle in center-aligned mode and at the
end of the cycle in edge-aligned mode. Therefore, even at 0 percent and
100 percent duty cycle, the current is sensed.
Distortion correction is only available in complementary mode. At the
beginning of the PWM period, the PWM uses this latched current value
or polarity bit to decide whether the top PWM value or bottom PWM
value is used. Figure 9-19 shows an example of top/bottom correction
for PWMs 1 and 2.
NOTE: The IPOLx bits and the values latched on the ISx pins are buffered so
that only one PWM register is used per PWM cycle. If the IPOLx bits or
the current sense values change during a PWM period, this new value
will not be used until the next PWM period. The ISENSx bits are NOT
buffered; therefore, changing the current sensing method could affect
the present PWM cycle.
Figure 9-19. Top/Bottom Correction for PWMs 1 and 2
When the PWM is first enabled by setting PWMEN , PWM value registers
1, 3, and 5 will be used if the ISENSx bits are configured for current
sensing correction. This is because no current will h ave previously been
sensed.
PWM1
PWM2
PWM VALUE REG. 1 = 1 PWM VALUE REG. 2 = 2
IS1 POSITIVE IS1 POSITIVE IS1 NEGATIVE IS1 NEGATIVE
PWM = 1 PWM = 1 PWM = 2 PWM = 2