CCP feature of PIC16F873

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

Capture	This is the function to capture the 16 bits value of timer1 register when an event occurs on pin RC2/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 and CCP2 can be worked at the same time. However, because they are using the same timer resources, the interaction occurs.
Interaction is shown below.

CCP1	CCP2	Interaction of two CCP modules
Capture	Capture	Same TMR1 time-base. The captured time value is different but it can be used at the same time.
Capture	Compare	Timer1 is cleared by compare operation. So, it's better not to use the capture of CCP1.
Capture	PWM	None.
Compare	Capture	Timer1 is cleared by compare operation. So, it's better not to use the capture of CCP2.
Compare	Compare	Timer1 is cleared by either compare operation. So, it isn't possible to use at the same time.
Compare	PWM	None.
PWM	Capture	None.
PWM	Compare	None.
PWM	PWM	The PWMs will have the same frequency and update rate.

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.

CCP2 register is comprised of two 8 bits registers : CCPR2L for low byte and CCPR2H for high byte. The CCP2CON register controls the operation of CCP2. The special event trigger is generated by compare match and will reset Timer1 and start an A/D conversion if the A/D modules is enabled.

Following, I will explain the operation of CCP1. CCP2 operates the same as CCP1, except where noted.

Capture mode

In Capture mode, CCPR1H : CCPR1L captures the 16 bits value of the TMR1 register when an event occurs on pin RC2/CCP1. In Capture mode, the RC2/CCP1 pin should be configured as an input by setting the TRISC register. If the RC2/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 CCPx 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 ( CCPxIF bit is set )
1001	Clear output on match ( CCPxIF bit is set )
1010	Remains unchanged on match ( CCPxIF bit is set )
1011	Trigger special event ( CCPxIF bit is set ) Remains unchanged on match In case of CCP1, resets TMR1 In case of CCP2, resets TMR1 and starts an A/D conversion

In Compare mode, the RC2/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 CCPx 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 RC2/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.

PWM period

The PWM period is the time which matches TRM2 and PR2. It can be calculated using the following formula.

PWM period

= (PR2+1) x 4 x Tosc x TMR2 prescale value

Example 1 ( Maximum period on 10MHz )

Condition

PR2 = 255

Tosc = 0.1 microseconds ( 10MHz clock )

TMR2 prescale value = 16

PWM period	= ( 255+1 ) x 4 x 0.1 x 10^-6 x 16
	= 1638.4 x 10^-6
	= 1.6384 x 10^-3
	= 1.6384 milliseconds ( = 610Hz )

Example 2 ( Minimum period on 10MHz )

Condition

PR2 = 0

Tosc = 0.1 microseconds ( 10MHz clock )

TMR2 prescale value = 1

PWM period	= 1 x 4 x 0.1 x 10^-6 x 1
	= 0.4 x 10^-6
	= 0.4 microseconds ( = 2.5MHz )

PWM duty cycle

The duty cycle is the time which the output stays high. It can be calculated using the following formula.

PWM duty cycle

= ( CCPR1L+CCP1X+CCP1Y(10bits) ) x Tosc x TMR2 prescale value

Example 3 ( Maximum duty cycle on 10MHz )

Condition

CCPR1L+CCP1X+CCP1Y = 1023

Tosc = 0.1 microseconds ( 10MHz clock )

TMR2 prescale value = 16

PWM duty cycle	= 1023 x 0.1 x 10^-6 x 16
	= 1636.8 x 10^-6
	= 1.6368 x 10^-3
	= 1.6368 milliseconds

This value is the value that 1/1024 is shorter than PWM period compared with example 1.

You find that the PWM duty cycle can be controlled in the precision of 1024 pieces of division to the PWM period.

Example 4 ( Minimum duty cycle on 10MHz )

Condition

CCPR1L+CCP1X+CCP1Y = 1

Tosc = 0.1 microseconds ( 10MHz clock )

TMR2 prescale value = 1

PWM duty cycle	= 1 x 0.1 x 10^-6 x 1
	= 0.1 x 10^-6
	= 0.1 microseconds

You find that the minimum value of the PWM duty cycle is an oscillator period.

CCPR1L+CCP1X+CCP1Y can be written to at any time, but the duty cycle value is not latched into CCPR1H until after a match between PR2 and TMR2 occurs. In PWM mode, CCPR1H is a read-only register.

The following steps should be taken when configuring the CCP module for PWM operation.

1. Set the PWM period by writing to the PR2 register.

2. Set the PWM duty cycle by writing to the CCPR1L register and CCP1X and CCP1Y bits of CCP1CON register.

3. Make the CCP1 pin an output by clearing the TRISC.

4. Set the TMR2 prescale value and enable Timer2 by writing to T2CON register.

5. Configure the CCP1 module for PWM operation.