MC68HC908MR8 Datasheet, PDF (154/372 Pages) Motorola, Inc – Microcontrollers

Pulse-Width Modulator for Motor Control
9.6.2 Dead-Time Insertion
As shown in Figure 9-12, in complementary mode, each PWM pair can
be used to drive top-side/bottom-side transistors.
NOTE: When controlling DC-to-AC inverters such as this, the top and bottom
PWMs in one pair should never be active at the same time.
In Figure 9-12, if PWM1 and PWM2 were on at the same time, large
currents would flow through the two transistors as they discharge the
bus capacitor. The IGBTs 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 and the activation of the other.
A 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. The dead-time is not affected by changes in the PWM period
caused by the prescaler.
Dead-time insertion is achieved by feeding the top PWM outputs of the
PWM generator into dead-time generators, as shown in Figure 9-13.
When output control is enabled, the odd OUT bits, rather than the
PWM generator outputs, are fed into the dead-time generators. See
9.6.4 Output Port Control Register.
Technical Data
154
MC68HC908MR8 — Rev 4.1
Pulse-Width Modulator for Motor Control (PWMMC) Freescale Semiconductor