Programming the Finite State Machine with 8-Bit PICs in Assembly and C (PDF)

This practical guide is aimed at electronics students and hobbyists. It is intended to be a valuable aid in writing programs using Finite State Machines (FSMs) in assembly language using 8-bit PIC microcontrollers. The last two chapters introduce the use of the C programming language and make a direct comparison with development in Assembly.

An FSM is a way of writing a program to make it easier to produce and modify. The machine is abstract in that it is just the structure of the program. This abstract machine can be represented by drawing a diagram on paper. The diagram is independent of the programming language used. The FSM chart gives a complete description of what the program does. It can then be implemented as source code.

The book should appeal to those with an interest in the combination of electronics and software and have an interest in how things work. The book will describe writing code for two particular microcontrollers: The 12F1822 and 16F1823. Both are mid-range and inexpensive. To read and write the programs to and from the PICs, all that is required is an FTDI TTL level USB lead (TTL-232R-5V-WE) in addition to two programs that are both available for free download as executable files and source code from Elektor. Microsoft Windows or Linux can be used.
The PIC programs are written in assembly language. This goes against the conventional wisdom of using a higher-level language such as C. One reason for this is that assembly is a good way of learning what is happening at the lowest level. This is important as microcontroller programming requires an understanding of the chip. Another reason for using the finite state machine approach is that it makes assembly programs surprisingly easy to follow. One of the main obstacles in the way of getting started with embedded programming is the installation and learning of new software tools.

The emphasis of this book is on making things straightforward with as little complication as possible. Therefore you can concentrate on understanding the code. Real projects aren't just about coding: our software has to do something real. As a consequence, a chapter deals with a method of circuit board construction.

All coding is done in a text editor of your choosing. The command line is used for running programs. If you are a Windows user, you might look at this as old fashioned. This is actually an efficient way of doing things: simple scripts for repetitive tasks save lots of mouse clicks. The last two chapters give an introduction to programming in C using the XC8 compiler. Again this is done using a text editor and the command line.

The intention has been to achieve results using an inexpensive microcontroller with simple command line tools. Much emphasis is placed on using Microchip's datasheet as this is the best place to get correct detailed information