MC68HC708MP16 Datasheet, PDF (146/398 Pages) Freescale Semiconductor, Inc – Microcontrollers

Pulse Width Modulator for Motor Control (PWMMC)
Whenever an input to a dead-time generator transitions, a dead-time is
inserted (for example, both PWMs in the pair are forced to their inactive
state). The BOTTOM PWM signal is generated from the TOP PWM and
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 9-7.) 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 dead-time of two
was inserted. In this example, with the prescaler set to divide by one and
center-aligned operation selected, this distortion can be compensated
for by adding or subtracting half the dead-time value to or from the PWM
register value. This correction is further described in 9.6.3 Top/Bottom
Correction.
Further examples of dead-time insertion are shown in Figure 9-15 and
Figure 9-16. Figure 9-15 shows the effects of dead-time insertion at the
duty cycle boundaries (near 0% and 100% duty cycles). Figure 9-16
shows the effects of dead-time insertion on pulse widths smaller than the
dead-time.
Technical Data
146
MC68HC708MP16 — Rev. 3.1
Pulse Width Modulator for Motor Control (PWMMC) Freescale Semiconductor