DSPIC33FJ12MC02ESS datasheet(151/284 Pages) MICROCHIP

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Preliminary

DS70265B-page 149

dsPIC33FJ12MC201/202

14.7.2

DUTY CYCLE IMMEDIATE UPDATES

When the Immediate Update Enable bit is set (IUE =

1), any write to the Duty Cycle registers updates the

new duty cycle value immediately. This feature gives

programmers the option to allow immediate updates of

the active PWM Duty Cycle registers instead of waiting

for the end of the current time base period. Duty cycle

update effects are as follows:

• If the PWM output is active at the time the new

duty cycle is written and the new duty cycle is less

than the current time base value, the PWM pulse

width will be shortened.

• If the PWM output is active at the time the new

duty cycle is written and the new duty cycle is

greater than the current time base value, the

PWM pulse width will be lengthened.

• If the PWM output is inactive at the time the new

duty cycle is written and the new duty cycle is

greater than the current time base value, the

PWM output will become active immediately and

will remain active for the new written duty cycle

value.

System stability is improved in closed-loop servo

applications by reducing the delay between system

observation and the issuance of system corrective

commands when immediate updates are enabled

(IUE = 1).

14.8

Complementary PWM Operation

In the Complementary mode of operation, each pair of

PWM outputs is obtained by a complementary PWM

signal. A dead time can be inserted during device

switching, when both outputs are inactive for a short

period

(refer

Section 14.9

“Dead-Time

Generators”).

In Complementary mode, the duty cycle comparison

units are assigned to the PWM outputs as follows:

• PxDC1 register controls PWM1H/PWM1L outputs

• PxDC2 register controls PWM2H/PWM2L outputs

• PxDC3 register controls PWM3H/PWM3L outputs

Complementary mode is selected for each PWM

I/O

pin pair by clearing the appropriate PMODx bit in the

PWMxCON1 SFR. The PWM I/O pins are set to

Complementary mode by default upon a device Reset.

14.9

Dead-Time Generators

Dead-time generation can be provided when any of

the

PWM

I/O

pairs

are

operating

Complementary Output mode. The PWM outputs use

push-pull drive circuits. Power output devices cannot

switch instantaneously, so some amount of time must

be provided between the turn-off event of one PWM

output in a complementary pair and the turn-on event

of the other transistor.

The PWM module allows two different dead times to be

programmed. These two dead times can be used in

one of two methods to increase user flexibility:

• The PWM output signals can be optimized for

different turn-off times in the high side and low

side transistors in a complementary pair of tran-

sistors. The first dead time is inserted between

the turn-off event of the lower transistor of the

complementary pair and the turn-on event of the

upper transistor. The second dead time is inserted

between the turn-off event of the upper transistor

and the turn-on event of the lower transistor.

• The two dead times can be assigned to individual

PWM I/O pin pairs. This operating mode allows

the PWM module to drive different transistor/load

combinations with each complementary PWM I/O

pin pair.

14.9.1

DEAD-TIME GENERATORS

Each complementary output pair for the PWM module

has a 6-bit down counter that is used to produce the

dead-time insertion. As shown in Figure 14-5, each

dead-time unit has a rising and falling edge detector

connected to the duty cycle comparison output.

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