Introduction

This is a course about embedded systems programming. Embedded systems are everywhere today, including, for example, the thermostats that control a building's temperature, the power-steering controller in modern automobiles, and the control systems in charge of jet engines. (When a jet engine controller is designed to have absolute control, it is known as a Full Authority Digital Engine Controller, or FADEC for short. If a FADEC fails, very bad things can happen.)

Clearly, much can depend on these systems operating correctly. Using Ada can help you get it right, and for less cost than other languages, if you use it well. Many industrial organizations developing critical embedded software use Ada for that reason. Our goal is to get you started in using it well.

The course is based on the assumption that you know some of the Ada language already, preferably even some of the more advanced concepts. You don't need to know how to use Ada constructs for embedded systems, of course, but you do need to know at least the language basics. If you need that introduction, see the course Introduction to Ada.

We also assume that you already have some programming experience so we won't cover CS-101.

Ideally, you also have some experience with low-level programming, because we will focus on "how to do it in Ada." If you do, feel free to gloss over the introductory material. If not, don't worry. We will cover enough for the course to be of value in any case.

So, what will we actually cover?

We will introduce you to using Ada to do low level programming, such as how to specify the layout of types, how to map variables of those types to specific addresses, when and how to do unchecked programming (and how not to), and how to determine the validity of incoming data, e.g., data from sensors that are occasionally faulty.

We will discuss development using more than Ada alone, nowadays a quite common approach. Specifically, how to interface with code and data written in other languages, and how (and why) to work with assembly language.

Embedded systems interact with the outside world via embedded devices, such as A/D converters, timers, actuators, sensors, and so forth. Frequently these devices are mapped into the target memory address space. We will cover how to define and interact with these memory-mapped devices.

Finally, we will show how to handle interrupts in Ada, using portable constructs.

Definitions

Before we go any further, what do we mean by "embedded system" anyway? It's time to be specific. We're talking about a computer that is part of a larger system, in which the capability to compute is not the larger system's primary function. These computers are said to be "embedded" in the larger system: the enclosing thermostat controlling the temperature, the power steering controller in the enclosing automobile, and the FADEC embedded in the enclosing aircraft. So these are not stand-alone computers for general purpose application execution.

As such, embedded systems typically have reduced resources available, especially power, which means reduced processor speed and reduced memory on-board. For an example at the small end of the spectrum, consider the computer embedded in a wearable device: it must run for a long time on a very little battery, with comparatively little memory available. But that's often true of bigger systems too, such as systems on aircraft where power (and heat) are directly limiting factors.

As a result, developing embedded systems software can be more difficult than general application development, not to mention that this software is potentially safety-critical.

Ada is known for use in very large, very long-lived projects (e.g., deployed for decades), but it can also be used for very small systems with tight resource constraints. We'll show you how.

We used the term "computer" above. You already know what that means, but you may be thinking of your laptop or something like that, where the processor, memory, and devices are all distinct, separate components. That can be the case for embedded systems too, albeit in a different form-factor such as rack-mounted boards. However, be sure to expand your definition to include the notion of a system-on-chip (SoC), in which the processor, memory, and various useful devices are all on a single chip. Embedded systems don't necessarily involve SoC computers but they frequently do. The techniques and information in this course work on any of these kinds of computer.

Down To The Bare Metal

Ada has always had facilities designed specifically for embedded systems. The language includes constructs for directly manipulating hardware, for example, and direct interaction with assembly language. These constructs are as effective as those of any high-level programming language (yes, including C). These constructs are expressively powerful, well-specified (so there are few surprises), efficient, and portable (within reason).

We say "within reason" because portability is a difficult goal for embedded systems. That's because the hardware is so much a part of the application itself, rather than being abstracted away as in a general-purpose application. That said, the hardware details can be managed in Ada so that portability is maximized to the extent possible for the application.

But strictly speaking, not all software can or should be absolutely portable! If a specific device is required, well, the program won't work with some other device. But to the extent possible portability is obviously a good thing.

The Ada Drivers Library

Speaking of SoC computers, there is a library of freely-available device drivers in Ada. Known as the Ada Driver Library (ADL), it supports many devices on a number of vendors' products. Device drivers for timers, I2C, SPI, A/D and D/A converters, DMA, General Purpose I/O, LCD displays, sensors, and other devices are included. The ADL is available on GitHub for both non-proprietary and commercial use here: https://github.com/AdaCore/Ada_Drivers_Library.

An extensive description of a project using the ADL is available here: https://blog.adacore.com/making-an-rc-car-with-ada-and-spark