Page 114 - The Art of Designing Embedded Systems

Firmware Musings 101

repeats at a rate that is not related to the rowkolumn configuration of the
chips.
For 64k of RAM, a string 257 bytes long is perfect: 257 is prime, and
its square is greater than the size of the RAM array. Each instance of the
string will start on a different low-order address. Also, 257 has another
special magic: you can include every byte value (00 to Oxff) in the string
without effort. Instead of manually creating a string in your code, build it
in real time by incrementing a counter that overflows at 8 bits.
Critical to this, and every other RAM test algorithm, is that you write
the pattern to all of RAM before doing the read test. Some people like to
do nondestructive RAM tests by testing one location at a time, then restor-
ing that location’s value, before moving on to the next one. Do this and
you’ll be unable to detect even the most trivial addressing problem.
This algorithm writes and reads every RAM location once, so it’s
quite fast. Improve the speed even more by skipping bytes, perhaps writ-
ing and reading every 3rd or 5th entry. The test will be a bit less robust, yet
will still find most PCB and many RAM failures.
Some folks like to run a test that exercises each and every bit in their
RAM array. Though I remain skeptical of the need, since most semicon-
ductor RAM problems are rather catastrophic, if you do feel compelled to
run such a test, consider adding another iteration of the algorithm just de-
scribed, with all of the data bits inverted.

Noise Issues

Large RAM arrays are a constant source of reliability problems. It’s
indeed quite difficult to design the perfect RAM system, especially with
the minimal margins and high speeds of today’s 16- and 32-bit systems. If
your system uses more than a couple of RAM parts, count on spending
some time qualifying its reliability via the normal hardware diagnostic
procedures. Create software RAM tests that hammer the array mercilessly.
Probably one of the most common forms of reliability problems with
RAM arrays is pattern sensitivity. Now, this is not the famous pattern
problems of yore, where the chips (particularly DRAMS) were sensitive to
the groupings of ones and zeroes. Today the chips are just about perfect in
this regard. No, today pattern problems come from poor electrical charac-
teristics of the PC board, decoupling problems, electrical noise, and inad-
equate drive electronics.
PC boards were once nothing more than wiring platforms, slabs of
tracks that propagated signals with near-perfect fidelity. With very high-
speed signals, and edge rates (the time it takes a signal to go from a zero to