PIC18F23K20-ESLASHML datasheet(150/456 Pages) MICROCHIP

150 / 456 page

PIC18F2XK20/4XK20

DS41303G-page 150

 2010 Microchip Technology Inc.

11.4.1

PWM PERIOD

The PWM period is specified by the PR2 register of

Timer2. The PWM period can be calculated using the

formula of Equation 11-1.

EQUATION 11-1:

PWM PERIOD

When TMR2 is equal to PR2, the following three events

occur on the next increment cycle:

• TMR2 is cleared

• The CCPx pin is set. (Exception: If the PWM duty

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

• The PWM duty cycle is latched from CCPRxL into

CCPRxH.

11.4.2

PWM DUTY CYCLE

The PWM duty cycle is specified by writing a 10-bit

value to multiple registers: CCPRxL register and

DCxB<1:0> bits of the CCPxCON register. The

CCPRxL contains the eight MSbs and the DCxB<1:0>

bits of the CCPxCON register contain the two LSbs.

CCPRxL and DCxB<1:0> bits of the CCPxCON

value is not latched into CCPRxH until after the period

completes (i.e., a match between PR2 and TMR2

registers occurs). While using the PWM, the CCPRxH

Equation 11-2 is used to calculate the PWM pulse

width.

Equation 11-3 is used to calculate the PWM duty cycle

ratio.

EQUATION 11-2:

PULSE WIDTH

EQUATION 11-3:

DUTY CYCLE RATIO

The CCPRxH 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.

The 8-bit timer TMR2 register is concatenated with

either the 2-bit internal system clock (FOSC), or 2 bits of

the prescaler, to create the 10-bit time base. The system

clock is used if the Timer2 prescaler is set to 1:1.

When the 10-bit time base matches the CCPRxH and

2-bit latch, then the CCPx pin is cleared (see

Figure 11-3).

Note:

The Timer2 postscaler (see Section 14.1

“Timer2 Operation”) is not used in the

determination of the PWM frequency.

PWM Period

PR2

 1

 4TOSC 



(TMR2 Prescale Value)

Note: TOSC = 1/FOSC.

Pulse Width

CCPRxL:DCxB<1:0>





TOSC

 (TMR2 Prescale Value)

Duty Cycle Ratio

CCPRxL:DCxB<1:0>



4PR2



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