Communicating with a CAN bus through an LPT printer port


… because, why not?

A few months back, I found myself in a position where I needed to develop a proof of concept for a CAN bus data logging system, but the hardware hadn’t been finalised. At the time, there were talks of using x86-based hardware, which would eventually communicate with the CAN bus via an SPI CAN controller. Unfortunately the dev board was on back-order and I wanted to keep the project moving. Continue reading

Tasks: A Time-Triggered Scheduler for Arduino


This has been a long time coming. A few years ago (back in 2012!) I wrote a basic TT scheduler which was really more of a ‘proof of concept’ and not really friendly for an everyday Arduino user. It relied on a strong knowledge of C or C++ and needed Eclipse with an Arduino plugin to get it to work. I’ve promised myself since then that I’d  write a proper Arduino library to demonstrate time-triggered scheduling to Arduino IDE users as well as learning some C++ skills for myself. Continue reading

Further task scheduling with Arduino / AVR


** EDIT 27/04/2016: I have now created a ‘user-friendly’ library compatible with the Arduino IDE **

** EDIT: I have now added the code for this to a git repo here. **

Introduction

My previous post on this topic showed a simple time triggered task scheduler for the Arduino / AVR microcontroller platforms. Although this worked very well, there were some areas which could be improved in the way the scheduler worked. To recap, the previous scheduler worked as follows:

How it worked:

  • The program sits in an empty loop when idle.
  • “Ticks” are generated by the timer driven interrupt.
  • Tasks have a period and an offset:
    • Period – How often the task is executed
    • Offset – The first tick in which the task is executed
  • The period and offset of the tasks are adjusted manually to spread the tasks out, ensuring that tasks don’t collide.
  • The ISR iterates through each task in the schedule and executes any task that is ready to run.
  • Doing this ensures that tasks are executed with precise timing (as precise as the ATMEGA timer allows). Continue reading

Reliable task scheduling with Arduino / AVR


** EDIT 27/04/2016: I have now created a ‘user-friendly’ library compatible with the Arduino IDE **

***Please note: This article is now for background information only. The code in this article has been revised, and can be found here. ***

 

There comes a point, when writing embedded software, where you need to time the execution of tasks more precisely than just having a list of functions in a loop. In order to make a system as predictable, and therefore reliable, as possible it is desirable to use a time-triggered scheduler. If you are unfamiliar with this concept, I recommend you have a scan of Dr Michael J Pont’s Patterns for Time Triggered Embedded Systems, and watch some of his lectures.

The idea is to use a small-scale operating system that runs on a microcontroller, using timer interrupts to execute tasks with microsecond precision. This isn’t a new idea; the aerospace industry have been using this approach for years to stop their aircrafts from making ‘premature landings’, but it’s rare to find it implementated on hobby-centric platforms such as Arduino. So I thought I’d have a go at writing my own. Continue reading