PIC18F23K20-ESLASHML datasheet(145/456 Pages) MICROCHIP

145 / 456 page

 2010 Microchip Technology Inc.

DS41303G-page 145

PIC18F2XK20/4XK20

11.2

Capture Mode

In Capture mode, the CCPRxH:CCPRxL register pair

captures the 16-bit value of the TMR1 or TMR3

registers when an event occurs on the corresponding

CCPx pin. An event is defined as one of the following:

• every falling edge

• every rising edge

• every 4th rising edge

• every 16th rising edge

The event is selected by the mode select bits,

CCPxM<3:0> of the CCPxCON register. When a cap-

ture is made, the interrupt request flag bit, CCPxIF, is

set; it must be cleared by software. If another capture

occurs before the value in register CCPRx is read, the

old captured value is overwritten by the new captured

value.

11.2.1

CCP PIN CONFIGURATION

In Capture mode, the appropriate CCPx pin should be

configured as an input by setting the corresponding

TRIS direction bit.

11.2.2

TIMER1/TIMER3 MODE SELECTION

The timers that are to be used with the capture feature

(Timer1 and/or Timer3) must be running in Timer mode

or Synchronized Counter mode. In Asynchronous Coun-

ter mode, the capture operation may not work. The timer

to be used with each CCP module is selected in the

T3CON register (see Section 11.1.1 “CCP Modules

and Timer Resources”).

11.2.3

SOFTWARE INTERRUPT

When the Capture mode is changed, a false capture

interrupt may be generated. The user should keep the

CCPxIE interrupt enable bit clear to avoid false inter-

rupts. The interrupt flag bit, CCPxIF, should also be

cleared following any such change in operating mode.

11.2.4

CCP PRESCALER

There are four prescaler settings in Capture mode; they

are specified as part of the operating mode selected by

the mode select bits (CCPxM<3:0>). Whenever the

CCP module is turned off or Capture mode is disabled,

the prescaler counter is cleared. This means that any

Reset will clear the prescaler counter.

Switching from one capture prescaler to another may

generate an interrupt. Also, the prescaler counter will

not be cleared; therefore, the first capture may be from

non-zero

prescaler.

Example 11-1

shows

the

recommended method for switching between capture

prescalers. This example also clears the prescaler

counter and will not generate the “false” interrupt.

EXAMPLE 11-1:

CHANGING BETWEEN

CAPTURE PRESCALERS

(CCP2 SHOWN)

Note:

If the CCPx pin is configured as an output,

a write to the port can cause a capture

condition.

CLRF

CCP2CON

; Turn CCP module off

MOVLW

NEW_CAPT_PS ; Load WREG with the

; new prescaler mode

; value and CCP ON

MOVWF

CCP2CON

; Load CCP2CON with

; this value

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