The Drogon Real-time Operating System (Simplified)

Firstly, DROSS is not really an operating system – the title makes it sound a lot grander than it really is…

However… I wrote it for a commercial project running on an ATmega processor to satisfy a need to get away from the existing “big loop” type of programming style that people were doing on the Arduino, and I then later ported it to a PIC project I was working on. I only maintain the ATmega version though.

DROSS is a set of library routines available under a commercial use license.

DROSS and the supporting libraries are written entirely in C. There is no C++ code, nor any C++ support for DROSS.

The DROSS library includes:

• A “wiring” library. The usual pinMode(), digitalRead(), digitalWrite(), etc. functions.
• A task scheduler. Tasks can be called at regular intervals from 1 millisecond upwards. Tasks are expected to run in under 1mS although one (just one!) event can over-run and will subsequently be preempted. The intention was that these tasks are able read/write hardware sensors, freeing up the main program (which is preemptivly interrupted by the tasks) to carry out its main task.
• Date/Time functions.
• I2C and SPI functions
• GPIO extender functions – e.g. digitalWrite() to a pin on an mcp23017.
• LCD Library (the usual character LCD devices).

The tasks can be called at regular intervals, or they can self-schedule and say when they next want to be called.

Additionally, there is a system of co-routines too – this was developed for the PIC to overcome some of the limitations of SDCC not being able to handle re-entrant floating point code.

• A replacement for the Arduino Wiring library

Written in C and some changes to make it more compatible for running in the DROSS environment. In particular one of the PWM channels is no-longer available due to DROSS itself needing one of the timers setup in a slightly different manner to the way the Arduino system does it.

• Serial handling

Simplified but fully capable serial library utilising interrupts to send and receive data.

• Interrupt driven analog port handling

The analog ports can be sampled via an interrupt driven task with the data being fed through a simple with low-pass filter.

• 2-wire (I2C) driver code

A simplified interrrupt driven 2-wire/i2c driver for the ATmega – the driver runs the bus in master mode only.