Experiment: Pulse Width Modulation

In this experiment you will learn how to generate a pulse width modulated (PWM) signal. The PWM signals will be generated purely in software. A more hardware oriented implementation with the help of timers will be investigated in later experiments.

Software

The I²C bus is explained in the I²C Experiment so for now a helper class is supplied to help you get started with the analog inputs without prior knowledge of how I²C works. The class is PCF8591:

Use the "Import Library" button to copy it into your project.

1) Hardware

In this lab you will need:

• 1x breadboard
• 1x photo resistor
• 5x 330 ohm resistor
• 1x green LED
• 1x red LED
• 2x trimming potentiometer
• cables

Mount the components on the breadboard and connect the breadboard to the LPC812 as show in the image below.

The pin(s) are specified in the mbed library with the actual pin names as well as some useful aliases:

1) Description

In this experiment you shall investigate how to generate a signal with a given duty cycle and how the LED intensity varies with duty cycle. The figure below illustrates the PWM signal structure. The high-time defines the duty cycle. It this signals directly drives a LED, the LED will be fully on when duty cycle is 0 and fully off when the duty cycle is 1 (100%).

Below is a code structure that can be used to generate a signal with a specified duty cycle. The loop should be repeated as often as possible in order to keep generating the signal. As seen the loop below will perform 100 iterations so the resolution of the duty cycle control is 1%. Higher resolution is possible by increasing the number of iterations but the trade-off is lower frequency of the duty cycle. This may, or may not, be a problem as we will investigate in later experiments.

#include "mbed.h"

DigitalOut greenLed(D1);

int main() {
    // set wanted duty cycle
    int wantedDutyCycle = ...;
        
    while(1) {
        // set output high
        ...

        for (int loopCounter = 0; loopCounter < 100; loopCounter++) {
            if (loopCounter == wantedDutyCycle) {
                // set output low
                ...
            }
        }
    }
}

Create a program that generate a fixed duty cycle and controls a LED. Define the duty cycle with a constant in the program.

About what frequency does the PWM signal have?

(measure with an oscilloscope or logic analyzer, if possible)

2) Description

In this experiment let the value from the trimming potentiometer control the duty cycle. Build on the code that was developed in the Analog Input experiment. Read the A0 value with the help of the imported PCF8591 class and set duty cycle between 0-100% accordingly.

As an extra experiment, let the light sensor control the duty cycle. If the room is dark have full intensity on the LED and vice versa.

3) Description

In the first PWM experiment, the duty cycle frequency was fixed to a relatively high value. In this experiment you shall investigate how this frequency affects the LED intensity. At some point (when lowering the frequency) you will notice a flickering on the LED.

Create a program where one trimming potentiometer controls the duty cycle and another trimming potentiometer controller the frequency.

Tip: Add a variable delay in the loop. Let the delay be 1 us, or multiples of it. At 1 us delay, a total of 100us delay will be added per cycle. This equals 10 kHz. Measure and verify that you get about this frequency. It will be slightly lower since more than just delays are performed in the loop. If the delay is 2 us the frequency will be 5 kHz, if 3 us it will be 3.3 kHz, etc.

At what frequency does the LED flickering become apparent?

4) Description

In this experiment you shall create a program that controls the intensity of two LEDs with the help of two trimming potentiometers. One loop shall now control two different PWM signals. As seen it becomes more and more complicated to control multiple signals, especially if the signals have different frequencies. The microcontroller is also fully occupied with generating the signals. If other work is performed, the PWM signals will be affected (not correct duty cycle or frequency). This is why it is typically simpler to let a timer generate the PWM signal, without software intervention (other than setup). You can experiment with this on your own.

Solution(s)