Nuts & Volts - September 2005

In The Trenches

by Gerard Fonte

The Business of Electronics Through Practical Design and Lessons Learned

In The Trenches

Software Development

From time to time, most engineers are required to develop software. Some have training for this and others don’t. Sometimes the training is inappropriate for the task at hand. This month, we’ll look at approaches and considerations that are important in developing good software.

Top Down, Bottom Up, or Gestalt

There are three general approaches for writing a computer program. The most general is the gestalt or all-in-one approach. This is typically used for very short programs or by inexperienced programmers. With this technique, the program is written as a block. It is then compiled and debugged as a whole. The Holy Grail for this approach is to have a perfect program the very first time. (It’s never happened to me.)

The big problem with this approach is that there are many ways that a program can fail. Worse, the same failure can have several different causes. If there is a problem with an output value, for example, it could be that the calculations are incorrect, the wrong table data is being read, or the output columns are reversed.

Trying to figure out the source of the error can be extremely difficult, frustrating, and time-consuming, which is why this approach is rarely used for serious software. (Of course, when you were graded on how many times you submitted the program for compilation, this approach had its merits. But that was way, way back then. This is now.) The top-down approach is often used for large software projects where there are a number of members working on it simultaneously. In this case, the logic core of the program is created and test-

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ed first. Then the individual sub-programs, often written by different people, are plugged into the core and tested.

Clearly, this approach is better than the gestalt method. The software is developed piece-wise, with each piece developed and debugged in isolation. The result is a series of modules that are moderately independent of one another, which allows individual modules to be modified fairly easily when needed. This approach is usually applied with “real” computers ( desktops, networks, and mainframes) that provide ample tools for examining how the code operates internally. High-level languages are typically utilized.

The bottom-up method is usually employed for embedded computer and microcomputer (uC) applications. In this approach, the subroutines are written and tested first. After these modules are completed, they are then assembled into a program with the appropriate logic.

Again, this approach is clearly superior to the gestalt approach. It is similar to the top-down method, in that separate modules are created. This simplifies debugging and changing the code. Typically, a single person writes the software. The tools for examining the code internally are usually more limited, and low-level languages are employed more often. The choice between top-down and bottom-up development can be somewhat arbitrary. If there are several people working on a software project, then top-down is probably a better approach. But if it’s just you, choose whichever method you feel most comfortable with.