Datasheet

© 2009 Microchip Technology Inc. DS39637D-page 169
PIC18F2480/2580/4480/4580
16.2 Capture Mode
In Capture mode, the CCPR1H:CCPR1L register pair
captures the 16-bit value of the TMR1 or TMR3 regis-
ters when an event occurs on the CCP1 pin (RB3 or
RC1, depending on device configuration). 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,
CCP1M<3:0> (CCP1CON<3:0>). When a capture is
made, the interrupt request flag bit, CCP1IF (PIR2<1>),
is set; it must be cleared in software. If another capture
occurs before the value in register CCPR1 is read, the
old captured value is overwritten by the new captured
value.
16.2.1 CCP1/ECCP1 PIN CONFIGURATION
In Capture mode, the appropriate CCP1/ECCP1 pin
should be configured as an input by setting the
corresponding TRIS direction bit.
16.2.2 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 may not work.
The timer to be used with each CCP module is selected
in the T3CON register (see Section 16.1.1 “CCP
Modules and Timer Resources”).
16.2.3 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.
16.2.4 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 (CCP1M<3:0>). Whenever the
CCP module is turned off or the CCP module is not in
Capture mode, 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 16-1 shows the recom-
mended method for switching between capture
prescalers. This example also clears the prescaler
counter and will not generate the “false” interrupt.
16.2.5 CAN MESSAGE TIME-STAMP
The CAN capture event occurs when a message is
received in any of the receive buffers. When config-
ured, the CAN module provides the trigger to the CCP1
module to cause a capture event. This feature is
provided to “time-stamp” the received CAN messages.
This feature is enabled by setting the CANCAP bit of
the CAN I/O Control register (CIOCON<4>). The
message receive signal from the CAN module then
takes the place of the events on RC2/CCP1.
If this feature is selected, then four different capture
options for CCP1M<3:0> are available:
• 0100 – every time a CAN message is received
• 0101 – every time a CAN message is received
• 0110 – every 4th time a CAN message is
received
• 0111 – capture mode, every 16th time a CAN
message is received
EXAMPLE 16-1: CHANGING BETWEEN
CAPTURE PRESCALERS
Note: If RC2/CCP1 or RD4/PSP4/ECCP1/P1A
is configured as an output, a write to the
port can cause a capture condition.
CLRF CCP1CON ; Turn CCP module off
MOVLW NEW_CAPT_PS ; Load WREG with the
; new prescaler mode
; value and CCP ON
MOVWF CCP1CON ; Load CCP1CON with
; this value