PIC16F818/819
9.1.2
TIMER1 MODE SELECTION
9.1
Capture Mode
Timer1 must be running in Timer mode or Synchro-
nized 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 the CCP1 pin. An event is defined as:
• Every falling edge
• Every rising edge
9.1.3
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. 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.
9.1.4
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. This means that any
Reset will clear the prescaler counter.
9.1.1
CCP PIN CONFIGURATION
In Capture mode, the CCP1 pin should be configured
as an input by setting the TRISB<x> 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
Note 1: If the CCP1 pin is configured as an
output, a write to the port can cause a
capture condition.
a
non-zero prescaler. Example 9-1 shows the
2: The TRISB bit (2 or 3) is dependent upon
the setting of configuration bit 12
(CCPMX).
recommended method for switching between capture
prescalers. This example also clears the prescaler
counter and will not generate the “false” interrupt.
FIGURE 9-1:
CAPTURE MODE
OPERATION BLOCK
DIAGRAM
EXAMPLE 9-1:
CHANGING BETWEEN
CAPTURE PRESCALERS
CLRF
CCP1CON
;Turn CCP module off
MOVLW NEW_CAPT_PS ;Load the W reg with
;the new prescaler
Set Flag bit CCP1IF
(PIR1<2>)
Prescaler
÷ 1, 4, 16
;move value and CCP ON
;Load CCP1CON with this
;value
MOVWF CCP1CON
CCPR1H
CCPR1L
TMR1L
CCP1 pin
Capture
Enable
and
Edge Detect
TMR1H
CCP1CON<3:0>
Q’s
DS39598E-page 66
2004 Microchip Technology Inc.
MICROCHIP [ MICROCHIP ]
