PIC16F87X
8.1.2
TIMER1 MODE SELECTION
8.1
Capture Mode
Timer1 must be running in timer mode or synchronized
counter mode for the CCP module to use the capture
feature. In asynchronous counter mode, the capture
operation may not work.
In Capture mode, CCPR1H:CCPR1L captures the
16-bit value of the TMR1 register when an event occurs
on pin RC2/CCP1. An event is defined as:
• Every falling edge
• Every rising edge
8.1.3
SOFTWARE INTERRUPT
• Every 4th rising edge
• Every 16th rising edge
When the capture mode is changed, a false capture
interrupt may be generated. The user should keep bit
CCP1IE (PIE1<2>) clear to avoid false interrupts and
should clear the flag bit CCP1IF following any such
change in operating mode.
An event is selected by control bits CCP1M3:CCP1M0
(CCP1CON<3:0>). When a capture is made, the inter-
rupt request flag bit CCP1IF (PIR1<2>) is set. The
interrupt flag must be cleared in software. If another
capture occurs before the value in register CCPR1 is
read, the old captured value will be lost.
8.1.4
CCP PRESCALER
There are four prescaler settings, specified by bits
CCP1M3:CCP1M0. Whenever the CCP module is
turned off, or the CCP module is not in capture mode,
the prescaler counter is cleared. Any reset will clear the
prescaler counter.
8.1.1
CCP PIN CONFIGURATION
In Capture mode, the RC2/CCP1 pin should be config-
ured as an input by setting the TRISC<2> bit.
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 8-1 shows the recom-
mended method for switching between capture pres-
calers. This example also clears the prescaler counter
and will not generate the “false” interrupt.
Note: If the RC2/CCP1 pin is configured as an
output, a write to the port can cause a cap-
ture condition.
FIGURE 8-1: CAPTURE MODE OPERATION
BLOCK DIAGRAM
Set flag bit CCP1IF
(PIR1<2>)
Prescaler
÷ 1, 4, 16
EXAMPLE 8-1: CHANGING BETWEEN
CAPTURE PRESCALERS
RC2/CCP1
Pin
CCPR1H
CCPR1L
TMR1L
CLRF
CCP1CON
;Turn CCP module off
MOVLW
NEW_CAPT_PS ;Load the W reg with
; the new precscaler
Capture
Enable
and
edge detect
; move value and CCP ON
TMR1H
MOVWF
CCP1CON
;Load CCP1CON with this
; value
CCP1CON<3:0>
Q’s
1999 Microchip Technology Inc.
DS30292B-page 59
ETC [ ETC ]
