2009 Microchip Technology Inc.
DS39636D-page 133
PIC18F2X1X/4X1X
14.2
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,
CCPxM3:CCPxM0 (CCPxCON<3:0>). When a capture
is made, the interrupt request flag bit, CCPxIF, is set; it
must be cleared in software. If another capture occurs
before the value in register CCPRx is read, the old
captured value is overwritten by the new captured value.
14.2.1
CCP PIN CONFIGURATION
In Capture mode, the appropriate CCPx pin should be
configured as an input by setting the corresponding
TRIS direction bit.
14.2.2
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 Counter
mode, the capture operation will not work. The timer to be
used with each CCP module is selected in the T3CON
14.2.3
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.
14.2.4
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 (CCPxM3:CCPxM0). Whenever
the CCP module is turned off or Capture mode is dis-
abled, 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
a non-zero prescaler. Example 14-1 shows the
recommended method for switching between capture
prescalers. This example also clears the prescaler
counter and will not generate the “false” interrupt.
EXAMPLE 14-1:
CHANGING BETWEEN
CAPTURE PRESCALERS
(CCP2 SHOWN)
FIGURE 14-1:
CAPTURE MODE OPERATION BLOCK DIAGRAM
Note:
If RB3/CCP2 or RC1/CCP2 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
CCPR1H
CCPR1L
TMR1H
TMR1L
Set CCP1IF
TMR3
Enable
Q1:Q4
CCP1CON<3:0>
CCP1 pin
Prescaler
÷ 1, 4, 16
and
Edge Detect
TMR1
Enable
T3CCP2
CCPR2H
CCPR2L
TMR1H
TMR1L
Set CCP2IF
TMR3
Enable
CCP2CON<3:0>
CCP2 pin
Prescaler
÷ 1, 4, 16
TMR3H
TMR3L
TMR1
Enable
T3CCP2
T3CCP1
T3CCP2
T3CCP1
TMR3H
TMR3L
and
Edge Detect
4
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