View source for PIC16F877A PWM ← PIC16F877A PWM

[[Category:PIC Tutorials]]
In this tutorial we are going to discuss the PWM module of PIC16F877A.<br>
First we will see how to configure the PWM registers to generate signals of required PWM, At the end we will see how to use the ExploreEmdedded PWM  library.
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[[File:0_LPC1768_PWM.gif]]

=PIC16F877A PWM Module=
PIC16F877A microcontroller has two independent CCP(Capture/Compare/PWM) modules, named as CCP1 and CCP2. Each CCP module has two 8-bit resistors(CCPxH,CCPxL) that can be use as:
*16 bit Capture Register
*16 bit Compare Register
*10-bit PWM Register.

In this tutorial we will be discussing only the PWM part of CCP. PIC has 2PWM module with a resolution of 10-bits.
The 8-Msb bits are stored in CCPRxL and remeining 2-bits in CCPxCON register.
Below tables shows the PWM module of PIC.

{| class="table table-striped table-hover table-condensed table-bordered"
|-class="info"
!PWM Channel || Port Pin ||  Control Register || Duty Cycle Register || Period Register
|-
|PWM1|| PC.2  || CCP1CON || CCPR1L || PR2
|-
|PWM2|| PC.1  || CCP2CON || CCPR2L || PR2
|}<br><br>


=PWM Working=

Before going to the PIC PWM module lets understand few terminologies wrt below image:<br><br>
[[FILE:Pwm.png]]

<b>Ton:</b> Time for which the signal is ON/HIGH.<br>
<b>Toff:</b> Time for which the signal is OFF/LOW.<br>
<b> Period:</b>  The cycle time of the signal which is nothing but the sum of Ton and Toff period.<br>
Period= Ton+Toff<br>
Freq of Pwm signal = 1/Period.<br>

<b>Duty Cycle:</b> The percentage of time for which the PWM signal will be ON/HIGH.
$$DutyCycle = (Ton/(Ton+Toff))*100 = (Ton/Period) * 100 $$
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Below diagram shows the PIC16f877a PWM block diagram and output signal generation.
[[FILE:Pic16f877a_Pwm.png]]<br>

PIC uses TIMER2 for generating the PWM signals. Lets relate the above concepts with the PIC registers.<br>
<b>Period:</b>PR2 register is used to specify the PWM Period. The PWM period can be calculated using the following formula.
$$PWM Period = [(PR2) + 1] * 4 * TOSC * (TMR2 Prescale Value)$$

<b>DutyCycle:</b>CCPR1L and CCP1CON<5:4> are used to specify the DutyCycle.The CCPR1L contains the eight MSbs and the CCP1CON<5:4> contains the two LSbs.

<b>PWM Signal Generation:</b> Once the PWM is configured and Timer2 is enabled, TMR2 starts incrementing depending on the prescalar.
Once the <b>TMR2</b> value is equal to dutyCycle<b>(CCPR1L+CCP1CON<5:4>)</b> the PWM pin will be pulled LOW. The timer still continues to increment till it matches with the period <b>PR2</b>. After which the PWM pin will be pulled HIGH and TMR2 is reset for next cycle.
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=PIC16F877A PWM Registers=
The below table shows the registers associated with PIC16F877A PWM.<br>
{| class="table table-striped table-hover table-condensed table-bordered"
|-class="info"
!Register || Description
|-
|CCPxCON|| This register is used to Configure the CCP module for Capture/Compare/PWM opertaion. 
|- 
|CCPRxL|| This register holds the 8-Msb bits of PWM, lower 2-bits will be part of CCPxCON register.
|-
|TMR2|| Free running counter which will be compared with CCPR1L and PR2 for generating the PWM output.
|}<br><br>

=Register Configuration=
The below table shows the registers associated with LPC1768 PWM.<br>
{| class="table table-striped table-hover table-condensed table-bordered"
|-class="info"
|CCPxCON 
|-
|7 || 6 || 5 || 4 || 3 || 2 || 1 || 0  
|-
|— || — || CCPxX || CCPxY || CCPxM3 || CCPxM2 || CCPxM1 || CCPxM0
|}

<b>CCPxX:CCPxY:</b>  PWM Least Significant bits<br>
These bits are the two LSbs of the PWM duty cycle. The eight MSbs are found in CCPRxL.

<b>CCPxM3:CCPxM0:</b> CCPx Mode Select bits<br>
0000 - Capture/Compare/PWM disabled (resets CCPx module)<br>
0100 - Capture mode, every falling edge<br>
0101 - Capture mode, every rising edge<br>
0110 - Capture mode, every 4th rising edge<br>
0111 - Capture mode, every 16th rising edge<br>
1000 - Compare mode, set output on match (CCPxIF bit is set)<br>
1001 - Compare mode, clear output on match (CCPxIF bit is set)<br>
1010 - Compare mode, generate software interrupt on match (CCPxIF bit is set, CCPx pin is unaffected)<br>
1011 - Compare mode, trigger special event (CCPxIF bit is set, CCPx pin is unaffected);<br>
<b>11xx - PWM mode</b>
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=Steps to Configure PWM=
#Configure the CCP1 module for PWM operation.
#Set the PWM period by writing to the PR2 register.
#Set the PWM duty cycle by writing to the CCPR1L register and CCP1CON<5:4> bits.
#Make the CCP1 pin an output by clearing the TRISC<2> bit.
#Set the TMR2 prescale value and enable Timer2 by writing to T2CON.
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=Code=
Program to demonstrates the LED fading using PWM signals.
<html>
<script src="https://gist.github.com/SaheblalBagwan/2d9ab8cbf840407b7248fef40fc56e21.js"></script>
</html>
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=Using ExploreEmbedded Libraries=
Progarm to generate the PWM signals of 25% & 75% duty cycle using explore embedded libraries.
<html>
<script src="https://gist.github.com/SaheblalBagwan/5a49f953cd43f136fe954777c4373128.js"></script>
</html>

Ouptut measured using CRO:
[[FILE:PwmOutput.png]]
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=Downloads=
Download the complete project folder from the below link:<br>
[https://github.com/ExploreEmbedded/Pic16f877a_ExploreUltraPicDevKit/archive/master.zip Hardware design Files and Code Library]


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