OSA / RTOS — Quick Intro — Part 1/3

Real-Time Operating System for Microchip PIC-controllers

Published in

Jungletronics

4 min readAug 18, 2017

I’ve just stumbled across OSA.

Real-Time Operating System (RTOS) for Microchip PIC, Atmel AVR 8-bit controllers, and for STMicroelectronics.

Has anyone out there used this?

I have been using OSA/RTOS a lot!

In this series of three posts I’ll give you a hint on how to run OSA/RTOS on a PIC18 Family using MikroC Pro For Pic from Microchip.

What is OSA?

1- OSA is a cooperative multi-tasking, non-preemptive, a priority based OS (RTOS) for Microchip PIC-controllers PIC10, PIC12, PIC16, PIC18, PIC24, dsPIC, for Atmel AVR 8-bit controllers, and for STMicroelectronics STM8;

2- It works very well and is superior to the conventional superloop and state machine approaches traditionally used in multi-tasking applications;

3- Its very small footprint (being co-operative, it does not need a separate stack for each task as would a preemptive RTOS) allows it to run comfortably in devices such as a PIC12 (or AVRTiny) but also works well with PIC18’s, PIC24’s and dsPIC33's;

4- It supports many of the features of full-blown RTOS’s such as semaphores (binary and counting), events, messages and data queues, mutexes, timers, etc;

5- It does come with full source code — in both C and ASM where applicable (for each supported development platform);

6- It is a great way to get you feet wet before looking into products such as Avix-RT, Q-Kernel, uC/OS and the like;

7- RTOS allows the programmer to focus on problem-oriented tasks (algorithmic, mathematical etc.) and not have to worry about secondary tasks. All secondary tasks are performed by OSA’s kernel;

8- Secondary tasks?
— switching between parallel processes (e.g. keyboard scanning, output data to LCD, switching relays);

— checking timeouts, counting delays;

— finding the ready task with the highest priority and executing it;

— data exchange between different tasks using semaphores, messages, queues, flags, etc;

— task management;

— queue management;

— memory management;

— critical session control function;

— A resource kernel for guaranteed and enforced timing behavior;

— It enforces data coherency;

— organize driver access, critical resources and the like;

— you’ll name it…I’m into OSA for short time:-b

9- and last but not least, being FREE makes an excellent introduction to the world of RTOS’s and multi-tasking.

Main concepts

Task

A task in OSA is a C-function. This function must contain an infinite loop which has inside it at least one service that switches task context. A simple task can look like this:

void SimpleTask (void)
{
 for (;;)            // Infinite loop
   {
     OS_Yield();     // Unconditional context switching
   }
}

Scheduler

Enforces scheduling processes for serve the Tasks.

Types of schedule policies: FIFO and Round Robin

FIFO

Task that is using the scheduling policy in FIFO mode will only be interrupted if a task with a higher priority is ready for execution.

/ * Free the CPU * /
// void _sched_yield (void);/ * Application example * /
void main()
{
 task_A (uint8_t data)
 {
 
 While (1)
 {
 process();
 _sched_yield ();
 }
 }
}

Tasks of the same priority will be blocked until the running task releases the CPU.

FIFO
Advantages:
The simplest among the scheduling processes;
All processes tend to be attended to;
Low context exchange index.
 
 
Disadvantages:
Very responsive to the order of arrival;
If larger tasks arrive first, they will increase the average waiting time;
It does not guarantee a fast response time.

Round Robin

The Round Robin scheduler is similar to FIFO, but differs in the following aspect. Each task in the Round Robin scheduling policy has a certain run time, this time is called Time-Slice.

It’s a template:

Uint8_t _sched_set_interval
(
      _task_id task_id,    / * Task ID * /
      Uint8_t ms_interval
)                          / * Time-slice value in milliseconds * /Uint8_t _sched_set_interval_ticks
(
     _task_id task_id,     / * Task ID * /
     TICK_STRUCT new_interval_ptr,
     TICK_STRUCT old_interval_ptr
)

The default time-slice value may be ten times the value of the Timer interrupt period.

Round Robin
Advantages:
All tasks end up having a regular chance of being run;
No task can monopolize the CPU;
Algorithm suitable for time-shared multi-tasking systems.
 
 
Disadvantages:
More constant context switching (not suitable for low power)
Runtime (in worst case) of undetermined task (because of time-slice).

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