Worksheet1_RTOS.md

NOTES:

As the MBED simulator does not support the RTOS elements, we will need to do this in platformIO and test on the boards.

We also need to make a change to the platformio.ini file to ake sure the RTOS elements are incuded when compiling. If we add the line below, everything works.

build_flags = -D PIO_FRAMEWORK_MBED_RTOS_PRESENT

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Our first multithreaded program

In this example we will write code to blink 2 LEDS.

One LED will be contrlled by the Main Loop The Second LED will have its own thread.

#include "mbed.h"
 
DigitalOut led1(LED1);
DigitalOut led2(LED2);

//Create an empty thread object ready to attach a function
Thread thread;
 
//Function to deal with blinking the second LED
void led2_thread() {
	//Loop forever
    while (true) {
        led2 = !led2;
        wait(1);
    }
}
 
//Main Program
int main() {

	//Attach the function to the Thread then Start it
    thread.start(led2_thread);
    
    while (true) {
        led1 = !led1;
        wait(0.5);
    }
}

Lets break that down a bit:

The led2_thread function is a seperate block of code that deals with blinking the second LED.

The main function (remeber this is also a thread) contains code the flash the first LED.

What Happens when the code executes

• Main thread is created
  • Spawns a second thread attached to the function
  • Main thead is is Running
  • Second thread is on Ready

Scheduler then allocates a time slice to each thread switiching between the threads every N time slices. Note the code here is very simple with only a few instructions per interation. It is likey that the code will execute within one time slice.

Therefore:

• Main Thread executes code untill it hits the wait() statement
  • Thread State changed to waiting
• Scheduler selects a thread from ready and moves it to Running
• Second thread runs till it hits the wait() statement
  • Thread state is chaged to waiting.

As it is probable that both threads are now waiting. Therefore there are no threads ready, and the system goes into low power mode.

When the timer for either threads wait command is complete, the thread is moved from waiting to ready, and can be selected by the scheduler for execution

Approx Time		Thread 1	Thread 2
0	Start	Running	Ready
0	Thread 1 hits Wait	Waiting	Ready
0	Scheduler	Waiting	Running
0	Thread 2 hits wait	Waiting	Waiting
0-0.5	Time Passes	Waiting	Waiting
0.5	Thread 1 Wait finished	Ready	Waiting
0.5	Scheduler	Running	Waiting
0.5	Thread 1 hits wait	Waiting	Waiting
1.0	Thread 2 Wait finished	Waiting	Ready
1.0	Thread 1 Wait finished	Ready	Ready
1.0	Scheduler...

Task

Modify the code to blink all 4 LEDS

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Revisiting the Interrupt

Process	Arrives At	Execute Time
A	1	1
B	2	5
C	3	3

Example

Here we want to add state to the Threads. This is a basic form of message passing.

Here we add a global variable, buttonstate that can be upaded by our Interrupt routine. Therefore we just use the Interrupt routine to updat the state, avioding the problem of the routine takeing controll f the main execution.

#include "mbed.h"
 
DigitalOut led1(LED1);
DigitalOut led2(LED2);
DigitalOut led3(LED3);

//InterruptIn button(p12);
InterruptIn button(BUTTON1);

//Create an empty thread object ready to attach a function
Thread thread;
//And a Global Variable to hold state
int buttonState = 0;

//Function to deal with blinking the second LED
void led2_thread() {
	while(true){
    //for (int x=0; x<5; x++) {
        if (buttonState){
            for (int x=0;x<4;x++){
                led2 = !led2;
                wait(0.1);
            }
            buttonState = 0;
	        
        }
	    wait(0.1);
	}
}

//Deal with Button Press
void handler(){
    buttonState = !buttonState;
    led3 = buttonState; //Debug with lights
    
}
 

 
//Main Program
int main() {
    button.rise(&handler);
	//Attach the function to the Thread then Start it
    thread.start(led2_thread);
    
    while (true) {
        led1 = !led1;
        wait(0.25);
        //Debug with Light
        led3=buttonState;
    }
}

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Sending Signals to Threads

#include "mbed.h"
 
DigitalOut led1(LED1);
DigitalOut led2(LED2);
DigitalOut led3(LED3);

//InterruptIn button(p12);
InterruptIn button(BUTTON1);

//Create an empty thread object ready to attach a function
Thread thread;

//Function to deal with blinking the second LED
void led2_thread() {
	while(true){
        //Wait on the Signal to be sent: Note Automatically cleared
        Thread::signal_wait(0x01);
            for (int x=0;x<4;x++){
                led2 = !led2;
                wait(0.25);
            }
	}
}

//Deal with Button Press
void handler(){
    led3 = !led3; 
    thread.signal_set(0x01); //Send the signal (id 0x01) to the Thread
    
}
 

 
//Main Program
int main() {
    button.rise(&handler);
	//Attach the function to the Thread then Start it
    thread.start(led2_thread);
    
    while (true) {
        led1 = !led1;
        wait(0.25);
    }
}

Threads and State Tasks

• Modify the Four threads example from before:
  • Led1 Blinks Continuously
  • On Button Press: Two things happen simultaneously
    • Led2 Blinks 10 Times
    • Led3 Blinks 20 Times
  • After Led3 Finishes: LED4 Blinks 10 Times