PIC18F242 datasheet(124/332 Pages) MICROCHIP

124 / 332 page

PIC18FXX2

DS39564C-page 122

14.5

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 14-3 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 14.5.3.

FIGURE 14-3:

SIMPLIFIED PWM BLOCK

DIAGRAM

A PWM output (Figure 14-4) 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 14-4:

PWM OUTPUT

14.5.1

PWM PERIOD

The PWM period is specified by writing to the PR2

following formula:

PWM period = (PR2) + 1] • 4 • TOSC •

(TMR2 prescale value)

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

14.5.2

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 following equation is

used to calculate the PWM duty cycle in time:

PWM duty cycle

= (CCPR1L:CCP1CON<5:4>) •

TOSC • (TMR2 prescale value)

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 con-

catenated with an internal 2-bit Q clock or 2 bits of the

TMR2 prescaler, the CCP1 pin is cleared.

The maximum PWM resolution (bits) for a given PWM

frequency is given by the equation:

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: 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 12.0)

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.

Note:

If the PWM duty cycle value is longer than

the PWM period, the CCP1 pin will not be

cleared.

FOSC

FPWM

---------------

⎝⎠

⎛⎞

log

()

log

-----------------------------bits

PWM Resolution (max)

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