PIC18F258-ESLASHLSQTP datasheet(130/402 Pages) MICROCHIP

130 / 402 page

PIC18FXX8

DS41159E-page 128

15.4

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 15-3 shows a simplified block diagram of the

CCP module in PWM mode.

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

module for PWM operation, see Section 15.4.3

“Setup for PWM Operation”.

FIGURE 15-3:

SIMPLIFIED PWM BLOCK

DIAGRAM

A PWM output (Figure 15-4) has a time base (period)

and a time that the output stays high (duty cycle). The

frequency of the PWM is the inverse of the period

(1/period).

FIGURE 15-4:

PWM OUTPUT

15.4.1

PWM PERIOD

The PWM period is specified by writing to the PR2

following formula.

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 (exception: if PWM duty

cycle = 0%, the CCP1 pin will not be set)

• The PWM duty cycle is latched from CCPR1L into

CCPR1H

15.4.2

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 following equation is

used to calculate the PWM duty cycle in time.

EQUATION 15-2:

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

time, but the duty cycle value is not latched into

CCPR1H until after a match between PR2 and TMR2

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

CCPR1H is a read-only register.

The CCPR1H register and a 2-bit internal latch are

used to double-buffer the PWM duty cycle. This

double-buffering is essential for glitchless PWM

operation.

When the CCPR1H and 2-bit latch match TMR2,

concatenated with an internal 2-bit Q clock or 2 bits of

the TMR2 prescaler, the CCP1 pin is cleared.

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 (Master)

CCPR1H (Slave)

Comparator

TMR2

PR2

(Note 1)

Duty Cycle Registers

CCP1CON<5:4>

Clear Timer,

set 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.

Comparator

Period

Duty Cycle

TMR2 = PR2

TMR2 = Duty Cycle

TMR2 = PR2

Note:

The Timer2 postscaler (see Section 13.0

“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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