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Programming embedded systems

Seminar 5

EmbeddedOperating System

Dr. Tran Thanh HungDepartment of Automation Technology,

College of Engineering, Can Tho University

Email: tthung@ctu.edu.vn

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Review

In the previous seminars, we use a superloop to run a task.

In this seminar, we will use a different way torun a task.

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Outline

How to create an Embedded OS forembedded applications?

Periodic functions Implement periodic functions

Creating Embedded OS Using Embedded OS

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Seminar objectives

At the end of this seminar, by referring thelecture notes, students will be able to:

understand issue of periodic function

use timer to create a simple Embedded OS(EOS)

implement periodic functions running in EOS

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How to run a task?

We has used a super loop architecture to run a task void main (void ){

X_Init() ; //Prepare for Task X()while (1) //super loop

{X(); //Perform task X()

}}

With this architecture, it is very difficult to run the taskat a precise interval of time.

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Periodic function

There are many applications require to run tasksrepeatedly, at precise intervals

Keypad need to be scanned every 200ms The display (PC monitor, LCD) must be

refreshed 60 times every second Music must be sampled at 44.100 times per

second Clock must update one time per secondIn practice, many embedded systems must be

able to support this type of periodic function

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Periodic function

If you need to run the function X() every 50 ms, how doyou implement?

If you know the function takes 10 ms to complete

while (1) //super loop {

X(); //Call the functiondelay(40); //Delay for 40 ms

}

This approach will be fine, if:1. We know the precise duration of X()2. This duration never varies

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Periodic function

while (1) //super loop {

X(); //Call the function (10 ms duration)delay(40); //Delay for 40 ms

}

However,1. It is very difficult to know the duration of X()2. This duration often vary3. This approach is in inefficient: waste in a delay loop

Is there any way to implement periodic function, runningat precise 50 ms interval ?

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Timer delay: review

void delay_tA(unsigned int x){ TACCR0 = x; // set delay value of Timer_A

setbit(TACTL, (TASSEL1 + MC0)) ; //TASSEL = 02, MC = 01while(bit_is_clear(TACTL, TAIFG)) ; //wait Timer_A overflow (TAIFG = 1)

clearbit(TACTL ,(MC0+TAIFG)); //stop Timer}

Note: Time for 1 count: T = 1/F = Prescaler /f CLK Instead of waiting for timer overflow, you can setup

an interrupt from timer

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Example: Timer interrupt setup

Chy bi tp 4.3 Xem Timer0_A3 t mt breakpoint dng lnh delay_tA

Chy chng trnh n khi dng delay_tA Chy chng trnh n khi timer trn CPU thc hin lnh u sau khi timer trn? t bit TAIE trong TACTL v GIE trong Core

Registers\SR t bit TAIE trong TACTL ln 1 Chy chng trnh n khi dng delay_tA Chy chng trnh n khi timer trn CPU thc hin lnh u sau khi timer trn?

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Timer interrupt setup

void delay_tA(unsigned int x){ TACCR0 = x; // set delay value of Timer_A

setbit(TACTL, (TASSEL1 + MC0)) ; //TASSEL = 02, MC = 01

while(bit_is_clear(TACTL, TAIFG)) ; //wait Timer_A overflow (TAIFG = 1)clearbit(TACTL ,(MC0+TAIFG)); //stop Timer}

Instead of waiting for timer overflow, we can set up an interrupt:

void timer_interrupt_init (unsigned int x){ TACCR0 = x; // set interval value of Timer_A

_enable_interrupt();//Enable general interrupt (set GIE)setbit(TACTL, (TASSEL1 + MC0+ TAIE )) ; //Enable timer interrupt

}

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Implement periodic function

Instead of writing a program like this:

void main (void ){

X_Init() ; //Prepare for Task X()while (1) //super loop

{X(); //Call the function (10 ms duration)delay(40); //Delay for 40 ms

}

We use a better way to do:

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Implement periodic function

//file timer_interrupt.c

#include void timer_interrupt_init (unsigned int x){ TACCR0 = x; // set interval value of Timer_A

_enable_interrupt(); // Enable general interruptsetbit(TACTL, (TASSEL1 + MC0+ TAIE )) ; //Enable timer interrupt

}#pragma vector = TIMER0_A1_VECTOR

interrupt void timer_ISR(void){

X(); //Call the function }

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Implement periodic function

//file main.c

void main ( void ){ X_Init() ; //Prepare for Task X()

timer_interrupt_init (50000); //initialize timer to interrupt every 50 mswhile (1); //empty loop

}//file timer_interrupt.c#pragma vector = TIMER0_A1_VECTOR

interrupt void timer_ISR(void){

X(); //Call the function

}

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Bi tp 5.1

S dng cc hm timer_interrupt_init () vtimer_ISR (), vit chng trnh chp tt:

- LED ni vi P1.7 tn s 10Hz (50ms sng, 50ms

tt) - LED ni vi P1.6 tn s 2Hz (250ms sng,

250ms tt) - LED ni vi P1.5 tn s 1Hz (500ms sng,

500ms tt) - LED ni vi P1.4 tn s 0.5Hz (1s sng, 1s tt)

Tn s chp tt c ng nh tnh ton?

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Problem with exercise 5.1?

1. Chy bi tp 5.1 cho n khi xy ra ngt 2. Chy tng bc, xem gi tr TAIFG khi CPU

ang thc hin timer_ISR Gi tr TAIFG? Khi thc hin xong timer_ISR,

iu g xy ra?3. Lp li bc 1, xa TAIFG khi CPU ang thc

hin timer_ISR 4. Lp li bc 2 Khi thc hin xong timer_ISR, iu g xy ra?

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Implement periodic function:Problem

void main ( void ){ X_Init() ; //Prepare for Task X()

timer_interrupt_init (50000); //initialize timer to interrupt every 50 mswhile (1); //empty loop

}#pragma vector = TIMER0_A1_VECTOR

interrupt void timer_ISR(void){

X(); //Call the function }

Sometime you forget to clear the timer flag. This approach is not adequate if the system requiresvery precise timing

ClearTAIFG

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Timer_A: Compare mode

Chy tng bc bi tp 5.1 cho n khi CPUthc hin xong hm timer_interrupt_init

Trong TACTL, xa bit TAIE Trong TACCTL0, t bit CCIE t gi tr TAR nh hn TACCR0 4 n v Chy tng bc (assembly) cho n khi TAR

bng TACCR0 iu g xy ra sau ?

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Embedded OS: Initialize/******************************************************************************

//file EOS.c* @fn EOS_init (unsigned int x)* @brief Initialize for Embedded Operating System:* - timer interrupt in next x microsecond

* - no reload required* @param x : number of microsecond* @return void

*/void EOS_init(unsigned int x){ TACCR0 = x; // set interval value of Timer_A

_enable_interrupt ();// Enable general interruptsetbit(TACCTL0, CCIE); // Enable timer interrupt (compare)

setbit(TACTL, (TASSEL1 + MC0)) ; // Start timer

}

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Embedded OS: Implement

//file main.c

void main ( void ){ Your_function _Init() ; //Initialize for Your_function

EOS_init (50000); //initialize timer 2 to interrupt every 50 ms

while (1); //empty loop }

//file EOS.c #pragma vector = TIMER0_A0_VECTOR

interrupt void EOS_ISR(void){

Your_function(); //This function will be called every 50 ms}

Problem ?

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Embedded OS: Implement

void main ( void ){ Your_function _Init() ; //Initialize for Your_function

EOS_init (50000); //initialize timer 2 to interrupt every 50 mswhile (1) // {

LPM0; // Or LPM1: go to sleep}

}#pragma vector = TIMER0_A0_VECTOR

interrupt void EOS_ISR(void){

Your_function(); //This function will be called every 50 ms}

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Saving power

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Bi tp 5.2

t hm EOS_init() and EOS_ISR() vo fileEOS.c v EOS.h

S dng h iu hnh nhng (EOS), vit liBi tp 4.4 vi yu cu nh sau:

- Hin th Gi / Pht / Giy /x ra cc LED ty theocc phm SW0/SW1/SW2/SW3 c bm

(nh phm) - Tng/gim Gi / Pht / Giy /x (ang c hinth) nu phm SW12/SW15 c bm(khngnh phm)

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Lu khi s dng EOS

Gii hn ca hm EOS_int (thi gian tia gia 2 ln ngt)

Thi gian thc hin EOS_ISR phi nhhn thi gian gia 2 ln ngt (Phi thotkhi chng trnh phc v ngt cngnhanh cng tt)