CCP feature of PIC16F628

The PIC16F873 has a special feature as CCP.
CCP is the initial of Capture/Compare/PWM(Pulse Width Modulation).
PIC16F873 has one CCP module.

Because a CCP function is the same as that of PIC16F873, so some part of pages for PIC16F873 are used in the following explanation.

Capture	This is the function to capture the 16 bits value of timer1 register when an event occurs on pin RB3/CCP1. This can be used for the measurement of the period time of the signal like the frequency counter and so on.
Compare	This is the function to compare constantly the 16 bits value of timer1 register against the CCPR1 register value. This is convenient when it makes interruption occur periodically.
PWM	This is the function to make a periodic pulse generate. This function is used to control an external circuit with changing a pulse duration (Duty).

The timer resource of the capture and compare is timer1 and the timer resource of PWM is timer2.

CCP1 register is comprised of two 8 bits registers : CCPR1L for low byte and CCPR1H for high byte. The CCP1CON register controls the operation of CCP1. The special event trigger is generated by compare match and will reset Timer1.

Capture mode

In Capture mode, CCPR1H : CCPR1L captures the 16 bits value of the TMR1 register when an event occurs on pin RB3/CCP1. In Capture mode, the RB3/CCP1 pin should be configured as an input by setting the TRISC register. If the RB3/CCP1 pin is configured as an output, a write to the port can cause a capture condition.

An event is defined as follows. An event is selected by control bits CCP1M3:CCP1M0 of CCP1CON register.

CCP1M3-0	Event
0000	CCP off ( resets CCP1 module )
0100	Every falling edge
0101	Every rising edge
0110	Every 4th rising edge
0111	Every 16th rising edge

When a capture is made, the interrupt request flag bit CCP1IF of PIR1 register 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.

Timer1 must be running in timer mode or synchronized counter mode for the CCP module to use capture feature. In asynchronous counter mode, the capture operation may not work. When the capture mode is changed, a false capture interrupt may be generated, You should keep bit CCP1IE of PIE register clear to avoid false interrupts and should clear the flag bit CCP1IF of PIR1 register following any such change in operating mode.

Compare mode

In Compare mode, the 16 bits CCPR1 register value is constantly compared against the TMR1 register pair value.

When a match occurs, the following actions will be done by the definition of CCP1M3:CCP1M0 of CCP1CON register.

CCP1M3-0	Action
1000	Set output on match ( CCP1IF bit is set )
1001	Clear output on match ( CCP1IF bit is set )
1010	Remains unchanged on match ( CCP1IF bit is set )
1011	Trigger special event ( CCP1IF bit is set ) Remains unchanged on match In case of CCP1, resets TMR1

In Compare mode, the RB3/CCP1 pin should be configured as an output by setting the TRISC register.
Timer1 must be running in timer mode or synchronized counter mode for the CCP module to use compare feature. In asynchronous counter mode, the compare operation may not work.

PWM mode
In PWM(Pulse Width Modulation) mode, the CCP1 pin produces up to a 10 bits resolution PWM output.

For PWM operation, CCP1M3 and CCP1M2 bits of CCP1CON register should be set.

CCP1M3-0	Operation
11xx	PWM mode

In PWM mode, the RB3/CCP1 pin should be configured as an output by setting the TRISC register.
The PWM period is specified by the value of the PR2 register, the clock oscillation period and the prescaler value of Timer2.
The PWM duty cycle( the output stays high ) is specified by the value of CCPR1L register, the clock oscillation period and the prescaler value of Timer2. The CCPR1L contains the eight MSbs and CCP1X:CCP1Y contains the two LSbs.
The shortest duty cycle is the period time of the oscillator. In case of 10MHz oscillator, it is 0.1 microseconds. The maximum PWM resolution of the duty cycle is 1024.
If the PWM duty cycle value is longer than the PWM period, the CCP1 pin will not be cleared. Also, if the value of PWM duty cycle register( CCPR1L+CCP1X+CCP1Y ) is set to zero, the CCP1 pin will not be set.

For the details about PWM period and PWM duty cycle, refer to the page of PIC16F873.