8-Bit Pulse Width Modulation

Note: This project is from the Get Started with 8-bit PIC MCUs tutorial..

Pulse Width Modulation (PWM) can be useful in many ways, one being able to control the brightness of an LED. Varying the On time at a very high rate can make the LED look dimmer or brighter without the human eye detecting the flashing. The same technique can be used for speed control of motors. PWM can be a useful technique on many designs and a PWM peripheral is built into most 8-bit MCU devices.


Step by Step Instructions

Using the MPLAB® Code Configurator (MCC), this project generates the code to control the brightness of an LED using a hardware PWM peripheral to generate the variable on/off time at a very high rate. This on/off frequency averages out to appear as a variable brightness on the LED..


The project uses:

PICDEM™ Lab II Connections:

  • Connect 5 V and Gnd to the power bus of the protoboard.
  • Connect 5 V to the Vdd pin of the PIC16F1508
  • Connect Gnd to the Vss pin of the PIC16F1508
  • Connect a wire from the RA2 pin (pin 17) to the breadboard Row 23
  • Connect a wire from the ground bus to breadboard Row 27
  • Connect the LED Anode to breadboard Row 23
  • Connect the LED Cathode to breadboard Row 24
  • Connect a 330 ohm resistor between Row 24 and Row 27

To follow along with these steps, MPLAB® X should be open and the Programmer/Debugger connected to both the computer and the Development Board. The setup was described in the Setup and Installation section of this training module. You should see a screen similar to the one below to move on to step 1.



Create a new "Stand-Alone" project in MPLAB® X for a PIC16F1508. Instructions are below if this is your first project.


Open the MPLAB® Code Configurator under the Tools -> Embedded menu of MPLAB® X IDE.



Select the peripherals for your project.
In this first project these peripherals need to be selected:

  • System Module
  • Interrupt Module
  • Pin Module
  • PWM3
  • TMR2

The System Module, Interrupt Module and the Pin Module will all be automatically included when you launch the MCC. PWM3 and TMR2 need to be added from the Device Resources area. Just click on the PWM category and then double click on the PWM3 to add it to the project. Under Timers double click on TMR2 to ad that peripheral. The result will look like the picture below:



The Timer2 needs to be setup. Click on the Timer2 label in the Project Resources menu to make the Timer2 setup screen appear.


Timer2 will be driven by the instruction clock which will be setup in a future step. These selections need to be made for the TMR2 operation.

  1. Check the Enable Timer box.
  2. Set the Prescaler box to 1:16.
  3. Set the Postscaler box to 1:1.
  4. Set the Timer Period to 16.384 ms.

The window will appear as below after the setup.



The next step is to setup the PWM3 module. Click on the PWM3 in the project resources shown below.


The PWM3 setup screen will appear in the center of the MPLAB® X screen. This project requires a few setup selections.

  1. Check the Enable PWM box
  2. Check the Enable pin Output box
  3. Select Timer2 from the Select a Timer drop down menu.
  4. Set the Duty Cycle to 50%

The result will look like this.



Open the Pin Manager and then click on the PORTA pin 2 (RA2) blue lock symbol in the PWM3 row. It should turn green. This will output the PWM signal to the RA2 pin. Also make sure the Reset pin has the blue lock shown. If it's green then click on the green lock to make it blue. This will make the Reset internal. It should look like the picture here when completed:



Close the Pin Manager and then click on the Pin Module selection in the Project Resources section.


The center screen will show RA2 listed on the I/O chart. Click on the output box to make the pin an output (if not checked) and make sure Analog and WPU are not checked (click on them to uncheck them).



The System needs to be setup next. Click on the System name in the Project Resources list.


The System section will appear. In this section the oscillator settings and the configuration settings are selected.


  1. Select the INTOSC from the drop down menu for the System Clock Select.
  2. Select the 1MHz_HF selection from the Internal Clock drop down menu.

This will enable the internal 1 MHz internal oscillator as the system clock.



Each configuration setting can be changed under the Register tab of the System window. Match the selections shown here.



Click on the Generate Code button to have the MCC create the software libraries for this project.



The project will now have both generated Header Files and Source Files. It should also have a generated main.c file.
Note: MCC may offer to generate a main.c file. Click YES to allow it to complete that task.


Double click on the main.c file to open it up in the editor window.



The PWM Duty cycle is adjusted by a value of 0 to 1023 in the PWM control routine. This is a 10-bit resolution. This example will only use half of the range of 1 to 512 to change the brightness of the LED connected to the PWM output. The main loop of code adjusts the PWM value in increments of 10 and uses a For-Loop delay routine to allow the PWM value to be displayed on the RB2 LED. To perform this routine, the following code needs to be entered in the main.c file under the while (1) area where the MCC generated template says "// Add your application code".

while (1)
        // Add your application code
        for (int duty = 1; duty < 512; duty = duty + 10)
            for (int delay = 1; delay < 500; delay++)

Note: The PWM3_LoadDutyValue(duty) function was generated by the MCC and its definition is located in the pwm3.h file.

This is a simple example of how the MCC generates a library of useful functions to make creating a PWM project code much easier and quicker.


Click on the Build Project Icon (the Hammer) to compile the code and you should see a BUILD SUCCESSFUL message in the output window of MPLAB® X.

BUILD SUCCESSFUL (total time: 7s)


Make sure your project has the programming tool selected (Part of Step 1 above) and connect power to your development board.

The PICkit™ 3 has limited power capability so we recommend you power the board separately.
The ICD™ 3 can power a development board, but for simplicity we recommend you power the board separately.
The RealICE™ cannot power the development board so powering the board separately is required.

Click on the Make and Program Device icon. This will build the project again and launch the programmer. In the output window you should see a series of messages and if successful it will end with a Programming and Verify Successful message.


Output Window:

Connecting to MPLAB PICkit 3...

Currently loaded firmware on PICkit 3
Firmware Suite Version.....01.41.06
Firmware type..............Enhanced Midrange

Target voltage detected
Target device PIC16F1709 found.
Device ID Revision = 2002

The following memory area(s) will be programmed:
program memory: start address = 0x0, end address = 0x7ff
configuration memory

Device Erased...

Programming/Verify complete

The RA2 LED will light up and get brighter as time passes. It will then reset to a dim light level and repeat the change to a brighter light. This will run continuously. The picture below shows the LED is fully lit:

If it's the first time the programmer is connected to the board, the programming tool may need to download the proper operating firmware for the exact device. You make see a series of processes if this occurs. This should only happen once.

Downloading Firmware…
Downloading bootloader
Bootloader download complete
Programming download…
Downloading RS…
RS download complete
Programming download…
Downloading AP…
AP download complete
Programming download…
Firmware Suite Version…..01.34.11
Firmware type…………..Enhanced Midrange


The project can be closed in MPLAB X. The project is saved automatically when it is built but any changes to files or configuration may be ask to be saved before the project is closed.
The project can be closed under the File Menu > Close Project



If you have any problems with your project, the completed MPLAB® X project file can be downloaded from the link below:

File Download
Windows Linux Mac OSX
Project 5 Files
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