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Sheet目录3880

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PIC18F2X1X/4X1X

DS39636D-page 150

2009 Microchip Technology Inc.

15.4.7.1

Auto-Shutdown and

Automatic Restart

The auto-shutdown feature can be configured to allow

automatic restarts of the module following a shutdown

event. This is enabled by setting the PRSEN bit of the

PWM1CON register (PWM1CON<7>).

In Shutdown mode with PRSEN = 1 (Figure 15-10), the

ECCPASE bit will remain set for as long as the cause

of the shutdown continues. When the shutdown condi-

tion clears, the ECCP1ASE bit is cleared. If PRSEN = 0

(Figure 15-11), once a shutdown condition occurs, the

ECCPASE bit will remain set until it is cleared by firm-

ware. Once ECCPASE is cleared, the Enhanced PWM

will resume at the beginning of the next PWM period.

Independent of the PRSEN bit setting, if the auto-

shutdown source is one of the comparators, the

shutdown condition is a level. The ECCPASE bit

cannot be cleared as long as the cause of the shutdown

persists.

The Auto-Shutdown mode can be forced by writing a ‘1’

to the ECCPASE bit.

15.4.8

START-UP CONSIDERATIONS

When the ECCP module is used in the PWM mode, the

application hardware must use the proper external pull-

up and/or pull-down resistors on the PWM output pins.

When the microcontroller is released from Reset, all of

the I/O pins are in the high-impedance state. The external

circuits must keep the power switch devices in the off

state until the microcontroller drives the I/O pins with the

proper signal levels, or activates the PWM output(s).

The CCP1M1:CCP1M0 bits (CCP1CON<1:0>) allow

the user to choose whether the PWM output signals are

active-high or active-low for each pair of PWM output

pins (P1A/P1C and P1B/P1D). The PWM output

polarities must be selected before the PWM pins are

configured as outputs. Changing the polarity configura-

tion while the PWM pins are configured as outputs is

not recommended, since it may result in damage to the

application circuits.

The P1A, P1B, P1C and P1D output latches may not be

in the proper states when the PWM module is initialized.

Enabling the PWM pins for output at the same time as

the ECCP module may cause damage to the applica-

tion circuit. The ECCP module must be enabled in the

proper output mode and complete a full PWM cycle

before configuring the PWM pins as outputs. The com-

pletion of a full PWM cycle is indicated by the TMR2IF

bit being set as the second PWM period begins.

FIGURE 15-10:

PWM AUTO-SHUTDOWN (PRSEN = 1, AUTO-RESTART ENABLED)

FIGURE 15-11:

PWM AUTO-SHUTDOWN (PRSEN = 0, AUTO-RESTART DISABLED)

Note:

Writing to the ECCPASE bit is disabled

while a shutdown condition is active.

Shutdown

PWM

ECCPASE bit

Activity

Event

PWM Period

Duty Cycle

Dead Time

Duty Cycle

Dead Time

Duty Cycle

Dead Time

Shutdown

PWM

ECCPASE bit

Activity

Event

PWM Period

ECCPASE

Cleared by Firmware

Duty Cycle

Dead Time

Duty Cycle

Dead Time

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