Latest revision as of 13:02, 27 July 2016

In this tutorial we are going to discuss the PWM module of PIC16F877A.
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.

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.

PWM Channel	Port Pin	Control Register	Duty Cycle Register	Period Register
PWM1	PC.2	CCP1CON	CCPR1L	PR2
PWM2	PC.1	CCP2CON	CCPR2L	PR2

PWM Working

Before going to the PIC PWM module lets understand few terminologies wrt below image:

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

Duty Cycle: The percentage of time for which the PWM signal will be ON/HIGH. $$DutyCycle = (Ton/(Ton+Toff))*100 = (Ton/Period) * 100 $$

Below diagram shows the PIC16f877a PWM block diagram and output signal generation.

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

DutyCycle: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.

PWM Signal Generation: Once the PWM is configured and Timer2 is enabled, TMR2 starts incrementing depending on the prescalar. Once the TMR2 value is equal to dutyCycle(CCPR1L+CCP1CON<5:4>) the PWM pin will be pulled LOW. The timer still continues to increment till it matches with the period PR2. After which the PWM pin will be pulled HIGH and TMR2 is reset for next cycle.

PIC16F877A PWM Registers

The below table shows the registers associated with PIC16F877A PWM.

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.

Register Configuration

The below table shows the registers associated with PIC16f877a PWM.

CCPxCON
7	6	5	4	3	2	1	0
—	—	CCPxX	CCPxY	CCPxM3	CCPxM2	CCPxM1	CCPxM0

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

CCPxM3:CCPxM0: CCPx Mode Select bits
0000 - Capture/Compare/PWM disabled (resets CCPx module)
0100 - Capture mode, every falling edge
0101 - Capture mode, every rising edge
0110 - Capture mode, every 4th rising edge
0111 - Capture mode, every 16th rising edge
1000 - Compare mode, set output on match (CCPxIF bit is set)
1001 - Compare mode, clear output on match (CCPxIF bit is set)
1010 - Compare mode, generate software interrupt on match (CCPxIF bit is set, CCPx pin is unaffected)
1011 - Compare mode, trigger special event (CCPxIF bit is set, CCPx pin is unaffected);
11xx - PWM mode

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.

Code

Program to demonstrates the LED fading using PWM signals.

Using ExploreEmbedded Libraries

Progarm to generate the PWM signals of 25% & 75% duty cycle using explore embedded libraries.

Ouptut measured using CRO:

Downloads

Download the complete project folder from the below link:
Hardware design Files and Code Library

Have a opinion, suggestion , question or feedback about the article let it out here!

@@ Line 10: / Line 10: @@
 *16 bit Capture Register
 *16 bit Compare Register
-*10-bit PWM Regsietrs .
+*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.
@@ Line 18: / Line 18: @@
 {| class="table table-striped table-hover table-condensed table-bordered"
 |-class="info"
-!PWM Channel || Port Pin ||  Control Register || Duty Cycle Register
+!PWM Channel || Port Pin ||  Control Register || Duty Cycle Register || Period Register
 |-
-|PWM1|| PC.2  || CCP1CON || CCPR1L
+|PWM1|| PC.2  || CCP1CON || CCPR1L || PR2
 |-
-|PWM2|| PC.1  || CCP2CON || CCPR2L
+|PWM2|| PC.1  || CCP2CON || CCPR2L || PR2
-|}<br><br><br><br><br><br>
+|}<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.
+$$DutyCycle = (Ton/(Ton+Toff))*100 = (Ton/Period) * 100 $$
+<br><br>
+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.
+<br><br><br><br>
 =PIC16F877A PWM Registers=
@@ Line 39: / Line 69: @@
 =Register Configuration=
-The below table shows the registers associated with LPC1768 PWM.<br>
+The below table shows the registers associated with PIC16f877a PWM.<br>
 {| class="table table-striped table-hover table-condensed table-bordered"
 |-class="info"
@@ Line 61: / Line 91: @@
 - Compare mode, clear output on match (CCPxIF bit is set)<br>
 - Compare mode, generate software interrupt on match (CCPxIF bit is set, CCPx pin is unaffected)<br>
-- Compare mode, trigger special event (CCPxIF bit is set, CCPx pin is unaffected); CCP1 resets TMR1; CCP2 resets TMR1 and starts an A/D conversion (if A/D module is enabled)<br>
+- Compare mode, trigger special event (CCPxIF bit is set, CCPx pin is unaffected);<br>
 <b>11xx - PWM mode</b>
 <br><br>
-=PWM Working=
+=Steps to Configure PWM=
-After looking into the PWM registers, its time to see how the LPC1768 PWM module works.
+#Configure the CCP1 module for PWM operation.
+#Set the PWM period by writing to the PR2 register.
-The TC is continuously incremented and once it matches the MR1(Duty Cycle) the PWM pin is pulled Low. TC still continues to increment and once it reaches the Cycle time(Ton+Toff) the PWM module does the following things:
+#Set the PWM duty cycle by writing to the CCPR1L register and CCP1CON<5:4> bits.
-*Reset the TC value.
+#Make the CCP1 pin an output by clearing the TRISC<2> bit.
-*Pull the PWM pin High.
+#Set the TMR2 prescale value and enable Timer2 by writing to T2CON.
-*Loads the new Match register values.
-[[File:LPC1768_PWM.gif]]
 <br><br>
-Summary of PWM operations for the above image:
+=Code=
-*Slide1: The TC is being incremented as per the Pre-scalar configuration. The PWM output is high as the TC is still less that duty cycle.
+Program to demonstrates the LED fading using PWM signals.
-*Slide2: TC is incremented to 40 and still the PWM pin as HIGH.
+<html>
-*Slide3: TC is incremented to 60 and it matches the Duty Cycle(MR1=60).
+<script src="https://gist.github.com/SaheblalBagwan/2d9ab8cbf840407b7248fef40fc56e21.js"></script>
-*Slide4: Now the Comparator1(Green) will trigger the R of SR latch and Pulls the PWM output to Zero(Q=0). TC still continues to increment.
+</html>
-*Slide5: TC is incremented to 80 and PWM pin is low as TC>Duty Cycle.
+<br>
-*Slide6: Now TC is 100 and it matches the Cycle time(MR0==100).
-*Slide7: Now the Comparator2(Red) will trigger the S of SR latch and pulls the PWM output to ONE(Q==1). It also resets the TC to zero. It updates Shadow buffers with new Match values from MRO,MR1.
-<br><br>
-=Steps to Configure PWM=
-#Configure the GPIO pins for PWM operation in respective PINSEL register.
-#Configure TCR to enable the Counter for incrementing the TC, and Enable the PWM block.
-#Set the required pre-scalar value in PR. In our case it will be zero.
-#Configure MCR to reset the TC whenever it matches MR0.
-#Update the Cycle time in MR0. In our case it will be 100.
-#Load the Duty cycles for required PWMx channels in respective match registers MRx(x: 1-6).
-#Enable the bits in LER register to load and latch the new match values.
-#Enable the required pwm channels in PCR register.
-<br><br>
-=Example=
+=Using ExploreEmbedded Libraries=
-Program to demonstrates the variable PWM signal generation on PWM_1-PWM_4(P2_0 - P2_3).<br>
+Progarm to generate the PWM signals of 25% & 75% duty cycle using explore embedded libraries.
-Connect the Leds to the pins P2_0 to P2_3 and observe the led brigthness change depending on the dutycycle.
-[[File:0_LPC1768_PWM.gif]]
 <html>
-<script src="https://gist.github.com/SaheblalBagwan/c721588560b77ad30396.js"></script>
+<script src="https://gist.github.com/SaheblalBagwan/5a49f953cd43f136fe954777c4373128.js"></script>
 </html>
-<br><br><br>
+Ouptut measured using CRO:
+[[FILE:PwmOutput.png]]
+<br><br>
+=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]
+Have a opinion, suggestion , question or feedback about the article let it out here!
+{{DISQUS}}

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