HelloWorld.md

title	author	email
Lab 2: Worksheet	Dan Goldsmith	aa9863@coventry.ac.uk

Introduction

For this set of tasks we will build on the hello world LED code from the first practical.

Setup a new workspace and add the following code

#include <mbed.h>

//Setup the LED1 pin to be a digital output called ledOne
DigitalOut ledOne(LED1);
DigitalOut ledTwo(LED2);

int main() {
  // put your setup code here, to run once:
  while(1) {
      ledOne = 1; 
	  ledTwo = 1;
      wait(0.5);
      ledOne = 0;
	  ledTwo = 0;
      wait(0.5);
      }
}

Getting Digital input

As well as the on-board LED'S the development board has other sensors connected to its pins. One of these is the user button

Like the LED's the first stage of getting input from the button is to initialise the pin to accept digital input. For this we can use the digitaIn class in the MBED API.

We create an object called button, linked the the USER_BUTTON pin, and set up for digital input.

digitalIn button(BUTTON1);

Reading from the button

The button object now represents the state of the digital pin it is connected to. We can query button to get the state of the pin:

• 0 off
• 1 on

This means we can change the behaviour of our program depending on whether the button is pressed or not.

#include <mbed.h>

//Setup the LED1 pin to be a digital output called ledOne
DigitalOut ledOne(LED1);
DigitalOut ledTwo(LED2);
DigitalIn button(BUTTON1);

int main() {
  // put your setup code here, to run once:
  while(1) {

    //Read the Button State and change behaviour.
    if (button){
      ledOne = 1; 
      wait(0.5);
      ledOne = 0;
      wait(0.5);
      }
    else{
      ledTwo = 1; 
      wait(0.5);
      ledTwo = 0;
      wait(0.5);
    }
}

\clearpage

Adding some state

We have a bit of an issue here. The LED state only changes when the button is pressed. Lets add some state to keep track of when the button is pressed and change behaviour accordingly.

#include <mbed.h>

//Setup the LED1 pin to be a digital output called ledOne
DigitalOut ledOne(LED1);
DigitalOut ledTwo(LED2);
DigitalIn button(USER_BUTTON);

int main() {
  // put your setup code here, to run once:
  int buttonstate =0;
  while(1) {

    //Read the Button and change the state
    if (button){
      buttonstate = !buttonstate;
    }

    //Read the Button State and change behaviour.
    if (buttonstate){
      ledOne = 1; 
      wait(0.5);
      ledOne = 0;
      wait(0.5);
      }
    else{
      ledTwo = 1; 
      wait(0.5);
      ledTwo = 0;
      wait(0.5);
    }
  }
}

However we still have a problem here.

The state is recorded, but is only updated each cycle. Therefore

• The button has to be pressed at the time a reading is taken
• If the button is pressed over one cycle, we end up "flipping" between states

Adding an Interrupt.

Interrupts allow us to trigger an event when something happens in a system. For example if we link a button press to an interrupt routine, then each time the button is pressed the code in the function gets triggered

NOTE: While interrupts are great in the event loop style systems, we need to consider the code that runs, for example if the interrupt code is blocking, it means we the program could hang and become unresponsive. Hiving n interrupt trigger some state update, which is then handled in another part of the code is preferable.

Examining the API for interrupts https://os.mbed.com/docs/mbed-os/v5.11/apis/interruptin.html

we can see that an interrupt is triggered on one of two occasions

• Rising. The value moves from Low to High
• Falling. The value moves from High to Low

Registering a pin as an interrupt

Like before, when we set a pin up for digital input, we need to let the OS know that the pin attached to the button will be used for an Interrupt.

InterruptIn button(BUTTON1);

Creating a handler

Next we function to catch the interrupt. In this case we can move the code that flips the button state to its own function.

void handler(){
  buttonState = !buttonState;
}

Wiring the Interrupt to the handler function

Finally we need to connect the interrupt to the handler function.

  button.rise(&handler);

\clearpage

Final Interrupt based Code

#include <mbed.h>

//Setup the LED1 pin to be a digital output called ledOne
DigitalOut ledOne(LED1);
DigitalOut ledTwo(LED2);


InterruptIn button(BUTTON1);

int buttonState =0;

void handler(){
  buttonState = !buttonState;
}

int main() {
  //Wire handler function to the Interrupt/
  button.rise(&handler);

  while(1) {
  //Main Loop
  
	//Flash LED based on buttonstate.
	if (buttonState){
      ledOne = 1; 
      wait(0.5);
      ledOne = 0;
      wait(0.5);
      }
    else
	{
      ledTwo = 1; 
      wait(0.5);
      ledTwo = 0;
      wait(0.5);
    }
  }
}

\clearpage

Task: Traffic Light Program

Design Brief

You have been asked to develop an embedded driver for a pedestrian crossing traffic light system. You can develop the code either in the simulator or for the embedded board.

NOTE: Unfortunately our board only has 4 LED's available (rather than the 5 we need), In this case we can use the following traffic light sequence. IF you are creating the program in the simulator, we can add extra LEDS, to create the full sequence

State	Driver	Pedestrian
Default	Green	Red
Button Pressed (Waiting)	Green	Red
Wait for stop	Red	Red
Crossing	Red	Green
End Crossing	Flash Red	Flash Green

Design Guidence

We need:

• Something to keep track of state
• Function for each state
• Function for Button press
• Method for state to evolve.

Hardware

• 4 LEDS.
• LED1,3 (The Vertical ones) Driver
• LED2,4 Pedestrian
• BUTTON1 Crossing button.

\clearpage

Code to get you started.

#include <mbed.h>

//Setup the LED1 pin to be a digital output called ledOne
DigitalOut driverGreen(LED3);
DigitalOut pedRed(LED2);


int lightSequence = 0; //Default sequence

int main(){

  while(1){
	
	//If we are in the Default state.
	if (lightSequence == 0){
		driverGreen = 1;
		pedRed = 0;
		}
    }
	wait(1.0);
}