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8 Timer PWM Interrupt

Objectives

At the end of this self-learning lab, you should be able to:

  • Understand Timer
  • Use PWM
  • Use External Interrupt
  • Use Timer Interrupt

Things you need

  • Solderless breadboard
  • LED
  • A resistor, anything between a few hundred to a few thousand ohms is OK. Check the resistance with a multimeter or by reading the color code on it.
  • Momentary button
  • Hookup wire (or Dubon wires as we like to call them)

Timer

Arduino has hardware timers, each consist of a counter and several registers to control the behaviour of the counter. The counter increments from 0 to its maximum, and then back to 0 again. (In upcounting mode. There are also downcounting mode, which behave similarly)

PWM

PWM is the abbreviation of Pulse Width Modulation.

PWM

There are 2 major functions:

  • The length of the pulse width can be used to encoded data. Servo motor (which will be mentioned later) uses PWM to acquire the desire position.
  • PWM wave can be used to control the power delivered to a desired value. In the example below, the brightness of an LED can be adjusted even though only 3.3V and 0V is delivered from Blue Pill. Also, for a circuit with sufficient inductance, average analog waveform at the approximate desired voltage level can be recovered. L298N motor driver (which will be mentioned later) is an example.

In STM32, PWM waves are generated with timers. The output pin is set to HIGH only when the conuter of the timer is lower then a preset value.

To use PWM control in Arduino, we may need to use analogWrite(). Here we will use a LED to demostrate. Select one of the PWM pins (you may check the pinout). Connect the circuit as below:

Schematic for Fading LED

Click File → Examples → 03.Analog → Fading. Change the value of ledPin to the pin you've connected the LED. You should see a breathing LED light.

Section Check Box:

  • What is PWM
  • Using analogWrite()

Further Reading

Try it yourself 01

pwmWrite()

pwmWrite() is a function only available to STM32 but not genuine Arduino, which offers 16-bit PWM resolution. Make a breathing LED light the same as above using pwmWrite(). Notice that pin mode needed to be changed from OUTPUT to PWM in order to use pwmWrite(). The following sites may help you:

External Interrupt

You may have heard of interrupt service routine (ISR), which is a piece of code triggered by a interrupt signal.

On Arduino Uno, one pin 2 and pin 3 support interrupt. Connect a button to pin 2 like this.

Schematic for button

Run the following code:

const int BTN_PIN = 2;

bool g_state = false;

void handler(void);

void setup() {
  pinMode(LED_BUILTIN, OUTPUT);
  pinMode(BTN_PIN, INPUT_PULLDOWN);

  attachInterrupt(BTN_PIN, handler, RISING);
}

void loop() {
}

void handler(){
  g_state = !g_state;
  digitalWrite(LED_BUILTIN, g_state);
}

Here handler() is called once the button is pressed.

Further Reading

Try it yourself 02

Control a breathing light

Make a breathing light that turns on only when the button is pressed. Make use of external interrupt.

Timer Interrupt

It is possible to generate interrupt signals using a timer. The function can be executed at a regular time intervals. This is especially useful in real-time control.

Here we will be using the PC13 LED as an example:

const int LED_PIN = LED_BUILTIN;
const int LED_RATE = 500000;  // in microseconds; therefore the interrupt handler will be called every 0.5s

void handler_led(void);

int g_state = 0;

void setup()
{
  pinMode(LED_PIN, OUTPUT);

  // Setup LED Timer
  // In output compare mode, timer counts from 0 to its reload value repeatedly; every time the counter value reaches one of the channel compare values, the corresponding interrupt is fired.
  Timer1.setMode(TIMER_CH1, TIMER_OUTPUTCOMPARE);
  Timer1.setPeriod(LED_RATE); // in microseconds
  Timer1.attachInterrupt(TIMER_CH1, handler_led); // handler_led() is the interrupt handler
}

void loop() {
}

void handler_led() {
    g_state = !g_state;
    digitalWrite(LED_PIN, g_state);
}

Here handler_led() is called every 0.5s. The LED should be blinking at 1 Hz.

Further Readings

Assignment 01

Control a breathing light Mk-II

Make a breathing light that can be toggled by a button or a command from serial input. Use timer interrupt to adjust the brightness of the LED, and use external interrupt read the state of the button.

You may check the maple documentation and see how to use pause() and resume().

Reference

PWM

External Interrupt

Timer Interrupt

Further Reading