PIC16F887/877 programming in C Tutorial 8 (PWM)

PWM:

The PWM mode generates a Pulse-Width Modulated signal on the CCPx pin. The duty cycle, period and resolution are determined by the following registers:
• PR2
• T2CON
• CCPRxL
• CCPxCON
In Pulse-Width Modulation (PWM) mode, the CCP module produces up to a 10-bit resolution PWM output on the CCPx pin. Since the CCPx pin is multiplexed with the PORT data latch, the TRIS for that pin must be cleared to enable the CCPx pin output driver.

PWM Period:

The PWM period is specified by the PR2 register of Timer2. The PWM period can be calculated using the formula

                             PWM Period = [(PR2) + 1] • 4 • TOSC • (TMR2 Prescale Value)
                             Note: TOSC = 1/FOSC

When TMR2 is equal to PR2, the following three events occur on the next increment cycle:
• TMR2 is cleared
• The CCPx pin is set. (Exception: If the PWM duty cycle = 0%, the pin will not be set.)
• The PWM duty cycle is latched from CCPRxL into CCPRxH.

PWM Duty Cycle:
The PWM duty cycle is specified by writing a 10-bit value to multiple registers: CCPRxL register and DCxB<1:0> bits of the CCPxCON register. The CCPRxL contains the eight MSbs and the DCxB<1:0> bits of the CCPxCON register contain the two LSbs. CCPRxL and DCxB<1:0> bits of the CCPxCON register can be written to at any time. The duty cycle
value is not latched into CCPRxH until after the period completes (i.e., a match between PR2 and TMR2 registers occurs). While using the PWM, the CCPRxH register is read-only.

         Pulse Width = (CCPRxL:CCPxCON<5:4>) • TOSC • (TMR2 Prescale Value)

                  Duty Cycle Ratio (CCPRxL:CCPxCON<5:4>) / 4(PR2 + 1)

PWM Resolution:

The resolution determines the number of available duty cycles for a given period. For example, a 10-bit resolution will result in 1024 discrete duty cycles, whereas an 8-bit
resolution will result in 256 discrete duty cycles. The maximum PWM resolution is 10 bits when PR2 is 255.

                                     Resolution = log[4(PR2 + 1)] / log(2)  bits

Setup for PWM Operation:

The following steps should be taken when configuring the CCP module for PWM operation:
1. Disable the PWM pin (CCPx) output drivers as an input by setting the associated TRIS bit.
2. Set the PWM period by loading the PR2 register.
3. Configure the CCP module for the PWM mode by loading the CCPxCON register with the appropriate values.
4. Set the PWM duty cycle by loading the CCPRxL register and DCxB<1:0> bits of the CCPxCON register.
5. Configure and start Timer2:
• Clear the TMR2IF interrupt flag bit of the PIR1 register.
• Set the Timer2 prescale value by loading the T2CKPS bits of the T2CON register.
• Enable Timer2 by setting the TMR2ON bit of the T2CON register.
6. Enable PWM output after a new PWM cycle has started:
• Wait until Timer2 overflows (TMR2IF bit of the PIR1 register is set).
• Enable the CCPx pin output driver by clearing the associated TRIS bit.

Note: This is only for 887. For Enhanced PWM Mode please refer to pic16F887 datasheet.

Lets do all this things in easy way by using in-build mikroc library for pwm.

Code:

Lets write the code to generate PWM on pins RC1 & RC2. And switches RA0-RA3 to increase/decrease PWM duty cycle.

unsigned short current_duty, current_duty1;

void InitMain() {

//using 887 and 8MHz xtal//////////////
  ANSEL  = 0;                         // Configure AN pins as digital
  ANSELH = 0;
  C1ON_bit = 0;                       // Disable comparators
  C2ON_bit = 0;

  PORTA = 255;
  TRISA = 255;                        // configure PORTA pins as input
  PORTC = 0;                          // set PORTC to 0
  TRISC = 0;                          // designate PORTC pins as output
  PWM1_Init(1000);                    // Initialize PWM1 module at 1KHz
  PWM2_Init(2000);                    // Initialize PWM2 module at 2KHz
}

void main() {
  InitMain();
  current_duty  = 16;                 // initial value for current_duty
  current_duty1 = 16;                 // initial value for current_duty1

  PWM1_Start();                       // start PWM1
  PWM2_Start();                       // start PWM2
  PWM1_Set_Duty(current_duty);        // Set current duty for PWM1
  PWM2_Set_Duty(current_duty1);       // Set current duty for PWM2

  while (1) {                         // endless loop
    if (RA0_bit) {                    // button on RA0 pressed
      Delay_ms(40);
      current_duty++;                 // increment current_duty
      PWM1_Set_Duty(current_duty);
     }

    if (RA1_bit) {                    // button on RA1 pressed
      Delay_ms(40);
      current_duty--;                 // decrement current_duty
      PWM1_Set_Duty(current_duty);
     }

    if (RA2_bit) {                    // button on RA2 pressed
      Delay_ms(40);
      current_duty1++;                // increment current_duty1
      PWM2_Set_Duty(current_duty1);
     }

    if (RA3_bit) {                    // button on RA3 pressed
      Delay_ms(40);
      current_duty1--;                // decrement current_duty1
      PWM2_Set_Duty(current_duty1);
     }

    Delay_ms(5);                      // slow down change pace a little
  }
}

PWM1_Init();Initializes the PWM module with duty ratio 0. Parameter freq is a desired PWM frequency in Hz.

PWM1_Set_Duty(); Sets PWM duty ratio. Parameter duty takes values from 0 to 255, where 0 is 0%, 127 is 50%, and 255 is 100% duty ratio. Other specific values for duty ratio can be calculated as (Percent*255)/100.
PWM1_Start(); Starts PWM.


Schematic: