PIC18LF4331 datasheet(149/392 Pages) MICROCHIP

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 2010 Microchip Technology Inc.

DS39616D-page 149

PIC18F2331/2431/4331/4431

16.5

PWM Mode

In Pulse-Width Modulation (PWM) mode, the CCP1 pin

produces up to a 10-bit resolution PWM output. Since

the CCP1 pin is multiplexed with the PORTC data latch,

the TRISC<2> bit must be cleared to make the CCP1

pin an output.

Figure 16-3 shows a simplified block diagram of the

CCP1 module in PWM mode.

For a step-by-step procedure on how to set up the

CCP1 module for PWM operation, see Section 16.5.3

“Setup for PWM Operation”

FIGURE 16-3:

SIMPLIFIED PWM BLOCK

DIAGRAM

A PWM output (Figure 16-4) has a time base

(period) and a time that the output is high (duty

cycle). The frequency of the PWM is the inverse of

the period (1/period).

FIGURE 16-4:

PWM OUTPUT

16.5.1

PWM PERIOD

The PWM period is specified by writing to the PR2

following equation:

EQUATION 16-1:

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

16.5.2

PWM DUTY CYCLE

The PWM duty cycle is specified by writing to the

CCPR1L register and to the CCP1CON<5:4> bits. Up

to 10-bit resolution is available. The CCPR1L contains

the eight MSbs and the CCP1CON<5:4> contains the

two LSbs. This 10-bit value is represented by

CCPR1L:CCP1CON<5:4>. The PWM duty cycle is

calculated by the following equation:

EQUATION 16-2:

CCPR1L and CCP1CON<5:4> can be written to at any

time, but the duty cycle value is not copied into

CCPR1H until a match between PR2 and TMR2 occurs

(i.e., the period is complete). In PWM mode, CCPR1H

is a read-only register.

Note:

Clearing the CCP1CON register will force

the CCP1 PWM output latch to the default

low level. This is not the PORTC I/O data

latch.

CCPR1L

CCPR1H (Slave)

Comparator

TMR2

Comparator

PR2

(Note 1)

Duty Cycle Registers

CCP1CON<5:4>

Clear Timer,

CCP1 pin and

latch D.C.

TRISC<2>

RC2/CCP1

Note 1:

8-bit timer is concatenated with 2-bit internal

Q clock or 2 bits of the prescaler to create

10-bit time base.

Period

Duty Cycle

TMR2 = PR2

TMR2 = Duty Cycle

TMR2 = PR2

Note:

The Timer2 postscaler (see Section 14.0

“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)

PWM Duty Cycle = (CCPR1L:CCP1CON<5:4>) •

TOSC • (TMR2 Prescale Value)

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