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Real-Time Operating System (RTOS) Wiki

Introduction

The objective of this assignment is to learn to implement code for microcontrollers in the form of a Real-time Operating System (RTOS) to accomplish set goals for two different programs. The use of RTOS tasks and MutEx Semaphores is included in the implemented solutions.

• Part A implements a Real-Time Operating System (RTOS) with an Arduino Microcontroller (MCU).
• Part B implements a RTOS with a Binary Counter.

Motivation

Requirements

"Use a RTOS to implement the following programs, the choice of MCU and RTOS is left for the student to decide upon, available are the Arduino DUE (Arm7 cortex M3), PIC16f and 18F series MCUs and Arduino UNO (AVR328)."

Part A: Simple RTOS with Arduino (Program 1)

Requirements

A simple microcontroller program must be implemented with four tasks, each of which drives a single LED output flashing at a different frequency. Four LEDs are connected to the lower half of PORTB on the microcontroller.

"The software consists of four tasks, where each task flashes an LED at a different rate:

• task_B0, flashes the LED connected to port RB0 at a rate of 250ms.
• task_B1, flashes the LED connected to port RB1 at a rate of 500ms.
• task_B2, flashes the LED connected to port RB2 once a second.
• task_B3, flashes the LED connected to port RB3 once every two seconds.
• PORTB pins RB0–RB3 are connected to the LEDs through current limiting resistors."

Methodology

Part B: RTOS with Binary Counter (Program 2)

Requirements

A microcontroller program must be implemented with three tasks.

"In this slightly more complex RTOS project a random number between 0 and 255 is generated. Eight LEDs are connected to PORTB on the microcontroller. In addition, a push-button switch is connected to bit 0 of PORTD (RD0), and an LED is connected to bit 7 of PORTD (RD7). Three tasks are used in this project: Live, Generator, and Display:

• _Task Live runs every 200ms and flashes the LED on port pin RD7 to indicate that the system is working.
• _Task Generator increments a variable from 0 to 255 continuously and checks the status of the push-button switch. When the push-button switch is pressed, the value of the current count is sent to task Display using a messaging queue.
• _Task Display reads the number from the message queue and sends the received byte to the LEDs connected to PORTB. Thus, the LEDs display a random pattern every time the push button is pressed. The microcontroller is operated from a 20MHz crystal."

While the Requirements Document specified use of a Messaging Queue permission was also given to instead make use of semaphores to check-in and check-out access to resources shared between program tasks. Use of semaphores was the option implemented.

Methodology

Features Wish List

Part B could potentially be improved through substitution of the LED array with a 3 digit Binary Coded Decimal (BCD) seven segment display. This would enable the display of a more human readable integer value for the counter being displayed instead of the binary value encoded in the 8x LED output array.

Shopping List

The following items are required to complete Part A and Part B of this project:

• PC
• Arduino UNO R3 or compatible clone
• USB cable
• Protoboard
• 9 x Red LED's
• 9 x 330 ohm current limiting resistors
• 1 x 10kohm resistor
• Momentary push button switch
• Connecting wire

Getting Started

Before starting, please read the original Project Report to access more complete documentation than this Wiki otherwise provides.

Testing

Early testing was found to have somewhat unpredictable results. However, debugging was assisted by writing status and data values to the Serial Monitor. Serial port access was required to be either restricted to a single task to enable reliable functioning. Alternatively, use of semaphores to check-in and check-out control of the serial port would provide reliable functioning.

By writing binary counter values to the Serial Monitor these were able to be checked with the status of the LED’s. When the momentary push button was held down the LED values would freeze thus enabling their status to be cross-checked with the values written to the Serial Monitor. This was helpful for checking which LED’s were assigned to the Least Significant Bit (LSB) and Most Significant Bit (MSB) of the binary counter.

Conclusion

Part A involved implementation of a very simple RTOS program for flashing four different LED outputs connected to an Arduino UNO R3 microcontroller.

Part B involved implementation of a more advanced program featuring:

• A rolling 8-bit binary counter.
• Writing of binary counter values to a global variable.
• Use of MutEx semaphores for check-in and check-out access to the global variable.
• Writing of the binary counter values to an 8x LED output array.
• Freeze / hold of displayed LED output statuses when button is held down.

Implementation of Part B in particular proved to be a very helpful exercise. This is because it became immediately clear how the use of RTOS tasks and semaphores would have been useful in past microcontroller projects as well as how helpful their usage is likely to be in future projects.

Resources

Using a RTOS - https://www.youtube.com/watch?v=bOFMXuoc5kE

What is a RTOS and how do we use it? - https://www.youtube.com/watch?v=oWmTJdYYRTg

Microchip explanation of pre-emption in RTOS - https://www.youtube.com/watch?v=rYeqygKAZoM

Concurrency in RTOS - https://www.youtube.com/watch?v=iKtvNJQoCNw

FREE RTOS ATmega328 - https://www.idreammicro.com/post/FreeRTOS-sur-ATmega328

List of Figures

All photographs, diagrams and screenshots are by the author unless otherwise indicated.

References

While this wiki provides a project summary, the complete original documentation is available in Adobe PDF format below: RTOS Project Report (~8MB PDF, 21 pages)

Appendices

Appendix A: Arduino Files

Part A: rtos.ino (Arduino)

Part B: rtos-with-binary-counter.ino (Arduino)