Overview

RIOS is a task scheduler written entirely in C that:

• Is simple and understandable for the beginning embedded programmer
• Can provide basic non-preemptive or preemptive multitasking capabilities for cooperative tasks
• Requires only a few dozen lines of C code. Reduces need for RTOS (real-time operating system).

Versions

• Non-preemptive, simple tasks

#include "RIMS.h"

/*
   Copyright (c) 2013 Frank Vahid, Tony Givargis, and
   Bailey Miller. Univ. of California, Riverside and Irvine.
   RIOS version 1.2
*/
typedef struct task {
   unsigned long period;      // Rate at which the task should tick
   unsigned long elapsedTime; // Time since task's last tick
   void (*TickFct)(void);     // Function to call for task's tick
} task;

task tasks[2];
const unsigned char tasksNum = 2;
const unsigned long tasksPeriodGCD = 200; // Timer tick rate
const unsigned long periodToggle   = 1000;
const unsigned long periodSequence = 200;

void TickFct_Toggle(void);
void TickFct_Sequence(void);

unsigned char TimerFlag = 0;
void TimerISR(void) {
   if (TimerFlag) {
      printf("Timer ticked before task processing done.\n");
   }
   else {
      TimerFlag = 1;
   }
   return;
}

int main(void) {
   // Priority assigned to lower position tasks in array
   unsigned char i = 0;
   tasks[i].period      = periodSequence;
   tasks[i].elapsedTime = tasks[i].period;
   tasks[i].TickFct     = &TickFct_Sequence;
   ++i;
   tasks[i].period      = periodToggle;
   tasks[i].elapsedTime = tasks[i].period;
   tasks[i].TickFct     = &TickFct_Toggle;

   TimerSet(tasksPeriodGCD);
   TimerOn();

   while(1) {
      // Heart of the scheduler code
      for (i=0; i < tasksNum; ++i) {
         if (tasks[i].elapsedTime >= tasks[i].period) { // Ready
            tasks[i].TickFct(); //execute task tick
            tasks[i].elapsedTime = 0;
         }
         tasks[i].elapsedTime += tasksPeriodGCD;
      }
      TimerFlag = 0;
      while (!TimerFlag) {
         Sleep();
      }
   }
}

// Task: Toggle an output
void TickFct_Toggle(void)   {
   static unsigned char init = 1;
   if (init) { // Initialization behavior
      B0 = 0;
      init = 0;
   }
   else { // Normal behavior
      B0 = !B0;
   }
}

 // Task: Sequence a 1 across 3 outputs
void TickFct_Sequence(void) {
   static unsigned char init = 1;
   unsigned char tmp = 0;
   if (init) { // Initialization behavior
      init = 0;
      B2   = 1;
      B3   = 0;
      B4   = 0;
   }
   else { // Normal behavior
      tmp = B4;
      B4  = B3;
      B3  = B2;
      B2  = tmp;
   }
}

• Non-preemptive, state machine tasks

#include "RIMS.h"
/*
   Copyright (c) 2013 Frank Vahid, Tony Givargis, and
   Bailey Miller. Univ. of California, Riverside and Irvine.
   RIOS version 1.2
*/
typedef struct task {
   unsigned long period;      // Rate at which the task should tick
   unsigned long elapsedTime; // Time since task's last tick
   int state;                 //Current state
   int (*TickFct)(int);       // Function to call for task's tick
} task;

enum TG_States { TG_None, TG_s1 };
int TickFct_Toggle(int);

enum SQ_States { SQ_None, SQ_s1, SQ_s2, SQ_s3 };
int TickFct_Sequence(int);

unsigned char TimerFlag = 0;
void TimerISR(void) {
   if (TimerFlag) {
      printf("Timer ticked before task processing done.\n");
}
   else {
      TimerFlag = 1;
   }
   return;
}

int main(void) {
   task tasks[2];
   const unsigned char tasksNum = 2;
   const unsigned long tasksPeriodGCD = 200; // Timer tick rate
   const unsigned long periodToggle   = 1000;
   const unsigned long periodSequence = 200;

   unsigned char i = 0;
   tasks[i].period      = periodToggle;
   tasks[i].elapsedTime = tasks[i].period;
   tasks[i].TickFct     = &TickFct_Toggle;
   tasks[i].state       = TG_None;
   i++ ;
   tasks[i].period      = periodSequence;
   tasks[i].elapsedTime = tasks[i].period;
   tasks[i].TickFct     = &TickFct_Sequence;
   tasks[i].state       = SQ_None;

   TimerSet(tasksPeriodGCD);
   TimerOn();

   while(1) {
      // Heart of the scheduler code
      for (i = 0; i < tasksNum; ++i) {
         if (tasks[i].elapsedTime >= tasks[i].period) { // Ready
            tasks[i].state = tasks[i].TickFct(tasks[i].state);
            tasks[i].elapsedTime = 0;
         }
         tasks[i].elapsedTime += tasksPeriodGCD;
      }
      TimerFlag = 0;
      while (!TimerFlag) {
         Sleep();
      }
   }
   return 0;
}

int TickFct_Toggle(int state) {
   switch(state) { // Transitions
      case TG_None: // Initial transition
         B0 = 0; // Initialization behavior
         state = TG_s1;
         break;
      case TG_s1:
         state = TG_s1;
         break;
      default:
         state = TG_None;
    }
    switch(state) { // State actions
      case TG_s1:
         B0 = !B0;
         break;
      default:
         break;
   }
   return state;
}
int TickFct_Sequence(int state) {
   switch(state) { // Transitions
      case SQ_None: // Initial transition
         state = SQ_s1;
         break;
      case SQ_s1:
         state = SQ_s2;
         break;
      case SQ_s2:
         state = SQ_s3;
         break;
      case SQ_s3:
         state = SQ_s1;
         break;
      default:
         state = SQ_None;
    }
    switch(state) { // State actions
      case SQ_s1:
         B2 = 1;
         B3 = 0;
         B4 = 0;
         break;
      case SQ_s2:
         B2 = 0;
         B3 = 1;
         B4 = 0;
         break;
      case SQ_s3:
         B2 = 0;
         B3 = 0;
         B4 = 1;
         break;
      default:
         break;
   }
   return state;
}

Porting to AVR microcontrollers

For more information on running RIOS on an AVR, check here.

Student Projects

• All projects (unedited)
• Highlights

About

RIOS stands for Riverside/Irvine Operating System, developed at the University of California (UC) Riverside and UC Irvine. RIOS is not really an operating system but rather a simple task scheduler, the key element of a real-time operating system in embedded systems.

For information, contact eslucr@gmail.com.

Developed by: Frank Vahid and Bailey Miller, University of California Riverside; and Tony Givargis, University of California Irvine.

This material is based upon work supported by the National Science Foundation under Grants 0614957, 1016792, 1136146, 1563652. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.