A common short cut to speed up memory testing time is to implement the walking 1's and walking 0's test on a byte wide basis as opposed to the bit wide level. If this is done then the test should be proceeded with another type of test designed to ensure uniqueness of each memory location. The uniqueness test can be done by storing and checking a pattern of data = address followed by data = !address.

The byte wide walking patterns test works like this:

1) Write all memory to all zero (0x00) bytes.
2) Verify all memory as all zero (0x00) bytes.
3) Do Loop from first address to last address..
3a) Write a byte of ones (0xFF) at loop address
3b) Verify all memory is zero (0x00) bytes except
....ones (0xFF) at the loop address byte.
3c) Write the loop address byte back to zero (0x00).
3d) Verify that all memory is all zero (0x00) bytes.
4) Increment the loop address and go to 3 if
...not at the end of memory yet.
5) Write all memory to all ones (0xFF) bytes.
6) Verify all memory as all ones (0xFF) bytes.
7) Do Loop from first address to last address..
7a) Write a byte of zeros (0x00) at loop address
7b) Verify all memory is ones (0xFF) bytes except
....zeros (0x00) at the loop address byte.
7c) Write the loop address byte back to ones (0xFF).
7d) Verify that all memory is all ones (0xFF) bytes.
8) Increment the loop address and go to 7 if
...not at the end of memory yet.

Note that sometimes if this test is included in a microcontroller the order of the tests above is reveresed to do the background in memory as the all ones first and then perform the walking 1's test with its all zeros background second. This has the characteristic of leaving all RAM cleared to zeros at the conclusion of the test if the test passes. This can be useful in environments where C has been used to program the application and it is desired to have memory start out cleared to zeros.

Michael Karas