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Sheet目录3880

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2009 Microchip Technology Inc.

DS39636D-page 141

PIC18F2X1X/4X1X

15.4

Enhanced PWM Mode

The Enhanced PWM mode provides additional PWM

output options for a broader range of control applica-

tions. The module is a backward compatible version of

the standard CCP module and offers up to four outputs,

designated P1A through P1D. Users are also able to

select the polarity of the signal (either active-high or

active-low). The module’s output mode and polarity are

configured

setting

the

P1M1:P1M0

and

CCP1M3:CCP1M0 bits of the CCP1CON register.

Figure 15-1 shows a simplified block diagram of PWM

operation. All control registers are double-buffered and

are loaded at the beginning of a new PWM cycle (the

period boundary when Timer2 resets) in order to

prevent glitches on any of the outputs. The exception is

the PWM Delay register, PWM1CON, which is loaded

at either the duty cycle boundary or the period bound-

ary (whichever comes first). Because of the buffering,

the module waits until the assigned timer resets instead

of starting immediately. This means that Enhanced

PWM waveforms do not exactly match the standard

PWM waveforms, but are instead offset by one full

instruction cycle (4 TOSC).

As before, the user must manually configure the

appropriate TRIS bits for output.

15.4.1

PWM PERIOD

The PWM period is specified by writing to the PR2

following equation.

EQUATION 15-1:

PWM frequency is defined as 1/[PWM period]. When

TMR2 is equal to PR2, the following three events occur

on the next increment cycle:

TMR2 is cleared

The CCP1 pin is set (if PWM duty cycle = 0%, the

CCP1 pin will not be set)

The PWM duty cycle is copied from CCPR1L into

CCPR1H

FIGURE 15-1:

SIMPLIFIED BLOCK DIAGRAM OF THE ENHANCED PWM MODULE

Note:

The Timer2 postscaler (see Section 12.0

“Timer2 Module”) is not used in the

determination of the PWM frequency. The

postscaler could be used to have a servo

update rate at a different frequency than

the PWM output.

PWM Period = [(PR2) + 1] 4 TOSC

(TMR2 Prescale Value)

CCPR1L

CCPR1H (Slave)

Comparator

TMR2

Comparator

PR2

(Note 1)

Duty Cycle Registers

CCP1CON<5:4>

Clear Timer,

set CCP1 pin and

latch D.C.

Note: The 8-bit TMR2 register is concatenated with the 2-bit internal Q clock, or 2 bits of the prescaler, to create the 10-bit time

base.

TRISx<x>

CCP1/P1A

TRISx<x>

P1B

TRISx<x>

P1D

Output

Controller

P1M1<1:0>

CCP1M<3:0>

PWM1CON

CCP1/P1A

P1B

P1C

P1D

P1C

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