Programming Embedded Systems, 2nd Edition

Memory Testing

One of the first pieces of serious embedded software you are likely to write is a memory test. Once the prototype hardware is ready, the designer would like some reassurance that he has wired the address and data lines correctly and that the memory chips are working properly. At first this might seem like a fairly simple assignment, but as you look at the problem more closely, you will realize that it can be difficult to detect subtle memory problems with a simple test. In fact, as a result of programmer naiveté, many embedded systems include memory tests that would detect only the most catastrophic memory failures. Some of these might not even notice that the memory chips have been removed from the board!

The purpose of a memory test is to confirm that each storage location in a memory device is working. In other words, if you store the number 50 at a particular address, you expect to find that number stored there until another number is written. The basic idea behind any memory test, then, is to write some set of data values to each address in the memory device and verify the data by reading it back. If all of the values read back are the same as those that were written, then the memory device is said to pass the test. As you will see, it is only through careful selection of the set of data values that you can be sure that a passing result is meaningful.

Of course, a memory test such as the one just described is unavoidably destructive. In the process of testing ...