PWM – Pulse Width Modulation Tutorial | CCP Module

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	PWM Tutorial | CCP Modules
	Intermediate Level ★★☆☆☆

In this tutorial, you’ll learn what is Pulse Width Modulation? What are the applications of PWM? And How to generate PWM signals with PIC Microcontrollers. We’ll discuss the last mode of operation for the CCP Modules which is PWM and develop the necessary firmware to drive the CCP Module in PWM mode. Finally, we’ll hook an LED to our PIC Microcontroller chip and create an LED Dimmer application. So let’s get started!

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   Required Components   

The Prototyping Board Setup

   What Is A PWM Signal?   

PWM stands for Pulse Width Modulation. This feature provides microcontrollers by a mean of, seemingly, outputting analog values of voltage between (0-5)v. Instead of outputting digital values that are either Low (0v) or High (5v). The PWM signal plot looks something like as shown in the diagram below.

image source https://www.elprocus.com

The plot for this signal captures two features: (Frequency & Duty Cycle). And we’ll discuss each of them in more detail hereafter.

 PWM Frequency 

As you know from elementary physics that a frequency of a signal is a measure of how many cycles are completed each second. It’s measured in Hz (Hertz), and we can also refer to it as the inverse of the time period of each cycle (F = 1 / T). Which means by varying the time period of each cycle, we’re controlling the frequency of the output signal. In fact, that’s what we’ll be doing hereafter in this tutorial’s LAB.

 PWM Duty Cycle 

The duty cycle is a measure for how long does the output line stays ON (High) to the cycle’s full-time period as a ratio. Which means the duty cycle can be easily determined using the following formula.

PWM Signal’s Effective Voltage. The controllability of the PWM duty cycle enables microcontroller devices to extend the hard-limited ON/OFF (High/Low) digital output pattern. Thus our microcontroller can now, seemingly not exactly, output analog values between (0-5v). 

You can think of it as changing the duty cycle of a PWM signal corresponds to changing the average (effective) voltage of the output line. For example, a PWM signal with 50% DC is seemingly considered a 2.5v voltage level on the output line. Hence, 80% DC is 4v, and 100% DC is 5v, and so on.

image source http://www.pyroelectro.com

This analogy while not being 100% accurate in a theoretical sense. But practically, it’s the way we add sweet & smooth control over light instead of On-OFF behavior with the full lighting power which can be annoying for specific applications. And we can also add a soft-start soft-end motor speed control instead of running an RC robot with 100% speed and breaking to 0% instantaneously which will result in undesirable drifts.

   What Are The Applications Of PWM?   

There are numerous situations in which it’s ideal to have a PWM output signal. That’s why there are countless applications which are mainly dependent on the pulse width modulation technique. I’m going to introduce some of them down below:

• Light intensity control.
• Motor speed control.
• ِAudio signal generation.
• Servo control (valves, motors, etc).
• Voltage regulation. By switching voltage to the load with the appropriate duty cycle, the output will approximate a voltage at the desired level. The switching noise is usually filtered with an LC network.
• The solar tracking and charging systems.

And much more, you can google “PWM Applications” to find out hundreds of applications related to PWM.

   CCP Modules – PWM Mode   

In PWM Mode, the CCPx pin (CCP1=RC2, CCP2=RC1) produces up-to a 10-Bit resolution PWM output signal. Since the CCP1 pin is multiplexed with the PORTC data latch, the RC2 pin must be configured as an output pin by clearing the TRISC2 bit. And the same goes for the CCP2 pin (RC1).

 PWM Mode Diagram 

Here is the logic (Block) diagram for the PWM mode as found in the datasheet (8.3 page-67)

Or equivalently this hand-drawn version for the same diagram

 CCP Operation In PWM Mode 

As you might have noticed in the above diagram, the operation of the ccp module in PWM mode goes as follows. First, the CCP module is set to operate in PWM mode by writing to the CCP1CON register. Then, the required PWM frequency will tell us the PWM time period which we’ll be loading to the PR2 register. Timer2 should be turned ON with the pre-specified prescaler value. And the duty cycle is set by writing to the [ CCPR1L register & CCP1CON<5:4> Bits ].

From the PWM Block Diagram, the PR2 value is constantly compared against the TMR2 register until a match occurs which indicates the end of the PWM cycle and the starting of a new cycle. Hence, the value of PR2 determines the time period of each PWM cycle and it consequently provides a mean of controlling the PWM output Frequency!

The CCP1 pin (RC2) is set (High = 1) at the beginning of each cycle. And the DC (duty cycle) 10-Bit value is latched to the [ CCPR1H + 2-Bit internal latches ]. This DC value is constantly compared against the [ TMR2 + 2-Bit Q Clock ]. When a match occurs, the output latch for the CCP1 pin (RC2) is reset (Low = 0). And it stays low till the end of the current PWM cycle. At the beginning of a new cycle, the output latch is set again to High and so on. Hence, the DC value controls for how long the PWM output pin will be set (High). Hence, the DC 10-Bit value provides a mean for controlling the PWM output Duty Cycle!

 Notes For PWM Mode 

I- Clearing the CCP1CON register:

Clearing the CCP1CON register will force the CCP1 PWM output latch to the default low level. This is not the PORTC I/O data latch.

II- Timer2 postscaler:

The Timer2 postscaler is not used in the determination of the PWM frequency. The postscaler could be used to have a servo update rate at a different frequency than the PWM output.

III- Duty Cycle value:

If the PWM duty cycle value is longer than the PWM period, the CCP1 pin will not be cleared.

IV- Double-Buffering & Glitches:

CCPR1L and CCP1CON<5:4> can be written to at any time, but the duty cycle value is not latched into CCPR1H until after a match between PR2 and TMR2 occurs (i.e., the period is complete). In PWM mode, CCPR1H is a read-only register. The CCPR1H register and a 2-bit internal latch are used to double-buffer the PWM duty cycle. This double-buffering is essential for glitch-free PWM operation.

V- PWM Resolution:

The PWM duty cycle is specified by writing to the CCPR1L register and to the CCP1CON<5:4> bits. Up to 10-bit resolution is available. The maximum PWM resolution (bits) for a given PWM frequency is given by the following formula.

The typical resolution (in bits) for a specific PWM frequency, with a specific Prescaler ratio PS, for a system running at Fosc clock rate is given by the equation down below

   Configuring CCP Modules For PWM Mode   

Here is the step-by-step procedure that you should follow in order to operate the CCP module in the PWM mode to generate the PWM output signal with your desired duty cycle and frequency.

 Configuring The CCP1 Module For PWM Operation 

Step1 –  Configure the CCP module for PWM operation

• As mentioned in the datasheet in the CCP1CON register’s description. Writing 11xx to the CCP1M0:CCP1M3 4-Bits selects the PWM mode of operation. Note that xx means that Bit0,1 are don’t cares! that’s why we’ll neglect them while writing the code.

Step2 –  Configure the CCP1 pin (RC2) to be an output pin by clearing the corresponding TRISC2 bit

• The CCP1/RC2 pin should be configured as an output pin.

Step3 –  Determine the PWM frequency and get the PWM period value

• Let’s say we need to have a PWM signal with a frequency of 5kHz. This means that the time period should be = 1/F = 1/5000 = 200µs

Step4 –  Using The PWM period, Calculate the value which we’ll load to the PR2 Register

• This step involves choosing a value for Timer2 prescaler prior to determining the PR2 register’s value. For this step, we’ll be using the following equation

• We now have the PWM_period, the T_osc = 1/F_osc, and we’ll pick an initial value for Timer2Prescaler (say 1 for 1:1 ratio). It should be easy to solve for PR2 to get its value. The rule of thumb is that PR2 register is an 8-Bit register which means its value ranges from (0 to 255). Your result should never exceed this 255 cap. Otherwise, you should try another value for the Timer2Prescaler. Finally, after getting the PR2 value, we should move this number to the PR2 register.

Step5 –  Set the prescaler of Timer2

• Adjust the Timer2 prescaler value to match the value which you’ve already used in the previous step. This is done by writing to the T2CON register T2CKPSx bits.

Step6 –  Step The PWM Duty Cycle, by writing to the CCPR1L register & CCP1CON<5:4> Bits

• To set or change the Duty Cycle of the PWM output signal, you should calculate and write the DC value to the 10-Bit buffer register which consists of CCPR1L:CCP1CON<5:4>. The CCPR1L contains the eight MSBs and the CCP1CON<5:4> contains the two LSbs. This 10-bit value is represented by CCPR1L:CCP1CON<5:4>.
• The following equation is used to calculate the PWM duty cycle in time:

• Let’s say you’re willing to get 40% DC for your PWM output. Then you should multiply 0.40 by PWM_period To get the PWM_DutyCycle in time. Now, substitute for PWM_DutyCycle , T_osc , TMR2_prescaler in the previous equation and solve for the 10-Bit value between the ( )
• After getting the 10-Bit duty cycle from your calculations, just write this value to the buffer CCPR1L:CCP1CON<5:4>

Step7 –  Turn ON Timer2

• Don’t forget to turn ON the Timer2 module! it should be obvious that it’s the working-horse of our system. However, it happens to find out that you’ve missed doing so and that’s why nothing is working at all.

   Implementing PWM Firmware Driver   

 Implementing CCP1 PWM Output Driver 

Step1 – Configure the CCP module for PWM operation

Step2 – Configure the CCP1 pin (RC2) to be an output pin

Step3 – Determine the PWM frequency

Let’s say our objective is to generate a 2kHz PWM signal, and that is our desired PWM_Frequency. And let’s also assume our microcontroller is running at 4MHz. Well, it’s time to choose the TMR2_prescaler value to be capable of using the following equation in order to get the PR2 register’s value. let’s pick 1:1 PS and substitute in the frequency equation

After substitution, our equation will be as follows 

Solving for PR2 will yield the following result. (PR2 = 499), which exceeds the 8-Bit range (0-255). Then it’s not OK to use this prescaler value and we’ll pick another value!

Let’s check a prescaler of 1:4 and solve for PR2 once again.

Solving for PR2 will now yield the following result. (PR2 = 124). which falls in the 8-Bit range (0-255). So, it’s OK to use this prescaler value and let’s proceed to the next step.

Step4 – Write to the PR2 Register

Step5 – Set the prescaler of Timer2

Step6 – Set The PWM Duty Cycle

This step involves both calculating and writing the 10-Bit DC value as discussed earlier. Let’s say you’re willing to get a 70% Duty Cycle for your PWM output. Then the PWM_DutyCycle Time is calculated by multiplying the PWM_period by 0.70 as shown down below

Now, we’re able to use the duty cycle equation which we’ve seen earlier. And solve for the 10-Bit value between the ( )

This will result in (CCPR1L:CCP1CON<5:4>) = 350

This actually means that 70% DC maps to 350

Therefore, 100% DC maps to (100*350)/70 = 500, and 50% DC maps to 250, and so on!

Now, we’ve got to write the 350 value to the 10-Bit buffer register. This is done as shown below

Step7 – Turn ON Timer2

   LED Dimmer – LAB   

       1. Coding       

Open the MPLAB IDE and create a new project name it “PWM_LED_Dimmer”. If you have some issues doing so, you can always refer to the previous tutorial using the link below.

Set the configuration bits to match the generic setting which we’ve stated earlier. And if you also find troubles creating this file, you can always refer to the previous tutorial using the link below.

Now, open the main.c file and let’s start developing the firmware for our project.

Our first task is to configure the CCP module to operate in PWM mode. And also configure the PWM output frequency to be 2kHz. Then in the main routine loop, we’ll be gradually increasing the Duty Cycle of the PWM output from 0 to 100% and gradually drop it back to 0% once again and so on.

It’s been always a better practice to create a dedicated function for each task of these. However, for this lab, I’ll be only creating a function for manipulating the DC value in order to fit it in the PWM DC buffer register. The rest of the code will reside normally in the main routine.

The full code listing for this lab is as follows

       2. Simulation       

To simulate the project, just hook an LED to the RC2/CCP1 pin with a 300Ω resistor. Add the hex file and run the simulator. Here is the schematic for this lab

It’s highly recommended to hook an oscilloscope to the PWM output line in order to have a clear vision of what is exactly going on. Due to many factors, LED dimming may not be running in real-time on simulators. And here is the typical simulation results!

       3. Prototyping       

Prototyping this project should also be an easy task. The tough part has been already done before writing the code. And here is the output in case you’re curious.

   Tips + Concluding Remarks   

  1  

PWM Calculations

From my own experience, I’ve seen many embedded practitioners struggling with the calculations part of operating the PWM module. That’s why I’m going to summarize it all up once again for you.

Where are actually calculating a couple of values:

• The period to be loaded in the PR2 register
• The duty cycle 10-Bit value to be loaded in the [ CCPR1L register & CCP1CON<5:4> Bits ]

If you’re still remembering the first section of this tutorial when I’ve told you that the PWM signal plot captures 2 features: (The PWM frequency & The PWM Duty Cycle). And we (the programmers) specify each of those features to meet the design requirements.

Obviously, setting the output PWM frequency is done by writing to the PR2 register. And the output PWM Duty Cycle is set by writing to the [ CCPR1L register & CCP1CON<5:4> Bits ].

That’s why we’ve to calculate these two values. We’ve to do so in order to control both the Frequency and Duty Cycle of the output PWM signal.

1- Frequency Control

To control the frequency of the PWM output, we write to the PR2 register and its value is given by the following equation

The only missing parameter other than PR2 is the TMR2PS which you should try guessing & testing. This should be done in such a way that gives you an 8-Bit value for the PR2 register. If the result has exceeded the (0-255) range for 8-Bit values, you should try another value for the prescaler until this condition is met.

2- Duty Cycle Control

To control the DutyCycle of the PWM output, we write to the DC buffer 10-Bit register which consists of (CCPR1L:CCP1CON<5:4>). Calculating the value which we should write to this register in order to get a specific DC percentage % is done using the formula down below.

  2  

PWM Timer2 Utilization

The CCPx modules (CCP1 & CCP2) in PWM Mode both are utilizing the Timer2 module as a hardware resource for controlling the timing features of the PWM output signal. This fact necessarily means that both CCP1 & CCP2 will be restricted to generate PWM signals with the exact same frequency. And also you can not use the Timer2 module for any other function within your system. However, you can disable the respective CCPx module for a specific time period during which you can use the Timer2 as a general purpose 8-Bit Timer. Then you can switch back to enable the PWM output, only if it’s feasible for your system, and it should be carefully implemented.

  3  

PWM Resolution

You can always double-check the output typical resolution for your PWM signal using the formula shown down below as I’ve stated earlier in this tutorial.

But for now, let’s use this knowledge in a practical sense. In the previous Lab, our system was being clocked at a 4Mhz rate, with a 1:4 PS ratio, and the desired output PWM frequency was 2kHz. Well, the previous equation is now telling us the following results

Well, this results means that we typically need 9-Bits to represent the Duty Cycle for the PWM output ranging from (0 to 512). What I’ve done here is a ceiling process. Which is approximating the result to the upper next integer.

However, it’s not an accurate estimation for the maximum duty cycle range. Yes, we need only 9-Bits in resolution in order to get a 0-to-100% DutyCycle variation capability. But it’s not exactly what we should be doing.

Interestingly, raising 2 to the power of this exact resolution 8.965 yields the following result

2^8.965 = 500

Which is exactly the maximum value for the duty cycle! Which will give you a 100% DC if you’ve loaded it to the PWM buffer register (CCPR1L:CCP1CON<5:4>).

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