PWM and CCP module

The CCP module (Capture / Compare / PWM) has three modes; the course focuses on PWM — motor speed, LED dimming, and servo control. The time base is Timer2 + PR2, not Timer0.

This page completes Topic 9 CCP (PWM): PDF formulas, registers, and 5 kHz at 50% example with 20 MHz crystal. Labs: Practice 4 and Practice 6 — Stepper motor.

PWM signal diagram showing period and duty cycle — Duty cycle is the high time within period T_PWM. — Topic 9 CCP Module — UNEXPO · Mechatronics · Microcontrollers

CCP module modes table: capture, compare and PWM — CCP1/CCP2: capture and compare use Timer1; PWM uses Timer2 with PR2 and CCPRxL. — Class Topic 9 — CCP Module (PWM) · UNEXPO

CCP module structure with CCPRxH, CCPRxL and CCPxCON registers — CCPRxH:CCPRxL and CCPxCON. PWM mode uses Timer2. — Class Topic 9 — CCP Module (PWM) · UNEXPO

What PWM is

PWM (Pulse Width Modulation) repeats a digital waveform: the pin toggles high and low with fixed period T_PWM. You change duty cycle — high time as a percentage of T_PWM — to simulate analog power to a motor or LED.

Step 1 — Period and duty

Parameter	Symbol	What it controls
Period	T_PWM	Frequency F_PWM = 1/T_PWM
Duty cycle	X %	High time / T_PWM
Resolution	effective bits	How many duty steps you can program

Step 2 — Three CCP modes

Mode	Timer base	Use
Capture	Timer1	Measure pulse width / frequency
Compare	Timer1	Trigger at precise instant
PWM	Timer2 + PR2	Motor, LED, servo — course focus

Capture/compare are covered in Timer 1; PWM uses Timer2 separately.

Step 3 — Key PWM registers

Register	Function
`PR2`	Period — TMR2 counts 0 → PR2 → reset
`T2CON`	Timer2 prescaler, TMR2ON
`CCPR1L` + `CCP1CON<5:4>`	Duty cycle (10 bits on PIC18)
`CCP1CON`	CCP mode — PWM = `CCP1M3:0 = 1100` → `0x0C`

Step 4 — Hardware output

With CCP1CON = 0x0C and Timer2 active, RC2 toggles automatically — no BSF LATC, 2 loop needed. Set BCF TRISC, 2 (output) once at init.

Topic 9 formulas

PWM period:

  T_PWM = 4 × (PR2 + 1) × Tosc × Timer2_Prescaler

Duty cycle (value to load):

  CCPR1L : CCP1CON<5:4> = (T_PWM × X%) / (Tosc × Prescaler)

Resolution (effective bits):

  log2( Fosc / (F_PWM × Prescaler) )

With Fosc = 20 MHz, Timer2 prescaler = 1:4, F_PWM = 5 kHz:

  PR2 = 1 / (5 kHz × 4 × Tosc × 4) − 1 = 249
  50% duty → CCPR1L = 124

PWM setup — 5 steps

BCF TRISC, 2 — RC2/CCP1 as output
Load PR2 for target frequency
Configure T2CON (prescaler, TMR2ON = 1)
Load CCPR1L and DC1B1:DC1B0 in CCP1CON if needed
CCP1CON = 0x0C — PWM mode

Full example — 5 kHz, 50% duty @ 20 MHz

config_pwm_5khz_50:
        BCF     TRISC, 2            ; RC2 output (CCP1)
        MOVLW   D'249'
        MOVWF   PR2                 ; period ~5 kHz, PS=1:4
        MOVLW   D'124'
        MOVWF   CCPR1L              ; duty ~50%
        MOVLW   B'00001101'         ; TMR2ON, prescaler 1:4
        MOVWF   T2CON
        MOVLW   0x0C                ; CCP1 PWM mode
        MOVWF   CCP1CON
        RETURN

To change motor speed: modify CCPR1L (duty) without recalculating PR2 if PWM frequency stays the same.

CCP1 vs CCP2

Module	Pin	Practice
CCP1	RC2	Practice 4 PWM, Practice 6 motor
CCP2	RC1	Second PWM output if design requires

Both in PWM mode share Timer2 and PR2 — same frequency, different duty in CCPR1L / CCPR2L.

Common mistakes

Mistake	Consequence
PWM without TMR2ON	RC2 idle
Skip TRISC	Pin does not drive
`CCP1CON` in capture mode	No PWM waveform
Wrong PR2	Unexpected frequency
Mix Timer1 with PWM	Capture uses T1; PWM uses T2

UNEXPO practices

Practica · Laboratory

Practice 4 — PWM

Calculate PR2 and CCPR1L, configure Timer2 and CCP1, connect load on RC2. Tutorial: Practice 4.

Practica · Laboratory

Practice 6 — Stepper motor

Use PWM or coil sequence to drive the motor. Requires correct CCP/Timer2 setup. Tutorial: Practice 6.

Next step

ADC — read potentiometer to adjust duty live (optional in Practice 4)
Partial III — timers + PWM review
Quick guide — PWM/CCP

Exam-style questions

Which timer does CCP PWM use?

Timer2 with period register PR2.

What value typically sets CCP1 to PWM mode?

CCP1CON = 0x0C (mode bits 1100).

Which pin outputs CCP1?

RC2.

For 50% duty with PR2 = 249, CCPR1L ≈ ?

124 (half the period in duty counts).

Ejercicio · Parcial

Calculate PR2 for F_PWM=5kHz, Fosc=20MHz, prescaler 1:4

PR2 = 1/(5kHz) / (4 × 1/20MHz × 4) − 1 = 249 per the UNEXPO Microcontrollers course example.