Advance Information MC68HC908MR16/MC68HC908MR32 — Rev. 4.0
150 Pulse-Width Modulator for Motor Control (PWM MC) MOTOROLA
Pulse-Width Modulator for Motor Control
Figure 9-13. Dead-Time GeneratorsWhenever an input to a dead-time generator transitions, a dead-time is inserted (for example, both PWMs in the pair are forced to their ina ctive state). The bottom PWM signal is generated from the t op PWM an d the dead-time. In the case of output control enabled, the odd OUTx bits control the top PWMs, the even OUTx bits control the bottom PWMs with respect to the odd OUTx bits (see Table 9-6). Figure 9-14 shows the effects of the dead-time insertion.As seen in Figure 9-14, some pulse width distortion occurs when the dead-time is inserted. The active pulse widths are reduced. For example, in Figure 9-14, when the PWM value register is equal to two, the ideal waveform (with no dead-time) has pulse widths equal to four. However, the actual pulse widths shrink to two after a de ad-time of two was inserted. In this example, with the prescaler set to divide by one a nd center-aligned operation selected, this distortion can be compensated for by adding or subtracting half the dead-time value to or from the PWM
FAULT
POLARITY/OUTPUT DRIVE
PWMGEN<1:6>
PWMPAIR12
PWMPAIR34
PWMPAIR56
DEAD-TIME
TOP/BOTTOM
GENERATION
POSTDT (TOP)
TOP/BOTTOM
GENERATION
TOP/BOTTOM
GENERATION
TOP
BOTTOM
TOP
BOTTOM
TOP
BOTTOM
PWM1
PWM2
PWM3
PWM4
PWM5
PWM6
6
MUX
PWM (TOP)
OUTX
SELECT
PREDT (TOP)
MUX
PWM (TOP)
OUTX
SELECT
PREDT (TOP)
MUX
PWM (TOP)
OUTX
SELECT
PREDT (TOP)
DEAD-TIME
DEAD-TIME
POSTDT (TOP)
DEAD-TIME
DEAD-TIME
POSTDT (TOP)
DEAD-TIME
(TOP)
(TOP)
(TOP)
(PWM1)
(PWM2)
(PWM3)
(PWM4)
(PWM5)
(PWM6)
OUTPUT CONTROL
OUT1
OUT3
OUT5
OUTCTL
OUT2
OUT4
OUT6
(OUTCTL)
CURRENT SENSING
PWM GENERATOR