Software-Based Memory Testing

One piece of software that nearly every embedded developer must write.
once the prototype hardware is ready, the board's designer would like some reassurance that she has wired the address and data lines correctly, and that the various memory chips are working properly.

And, even if that's not the case, it is desirable to test any onboard RAM at least as often as the system is reset. It is up to the embedded software developer, then, to figure out what can go wrong and design a suite of tests that will uncover problems.

If the problem is with a data line, several data bits may appear to be "stuck together" (i.e., two or more bits always contain the same value, regardless of the data transmitted). Similarly, a data bit may be either "stuck high" (always 1) or "stuck low" (always 0). These problems can be detected by writing a sequence of data values designed to test that each data pin can be set to 0 and 1, independently of all the others.

If an address line has a wiring problem, the contents of two memory locations may appear to overlap. In other words, data written to one address will actually overwrite the contents of another address instead. This happens because an address bit that is shorted or open will cause the memory device to see a different address than the one selected by the processor.

Address Bus Test

The smallest set of addresses that will cover all possible combinations is the set of "power-of-two" addresses. These addresses are analogous to the set of data values used in the walking 1's test. The corresponding memory locations are 00001h, 00002h, 00004h, 00008h, 00010h, 00020h, etc. In addition, address 00000h must also be tested. The possibility of overlapping locations makes the address bus test harder to implement. After writing to one of the addresses, you must check that none of the others has been overwritten.

Data Bus Test

A good way to test each bit independently is to perform the so-called "walking 1's test." Table 1 shows the data patterns used in an 8-bit version of this test. The name of this test, walking 1's, comes from the fact that a single data bit is set to 1 and "walked" through the entire data word. The number of data values to test is the same as the width of the data bus.

00000001
00000010
00000100
00001000
00010000
00100000
01000000
10000000

Table 1. Consecutive Data Values for the Walking 1's Test