Alioth's Journal: [retrocomputing] Memory tests

I've been spending a bit of time writing the memory tester for my Sinclair Spectrum Diagnostics board. The board itself, to recap, contains a flash ROM, a little bit of glue logic, a couple of flip flops and eight LEDs - the idea being that the code running in ROM can display the results on the LEDs, so as to use as little of the (possibly suspect) Spectrum's hardware.

It's been an interesting challenge. I have three bad RAM chips which I've been using to test the tester - one is permanetly stuck on logic 1 regardless of the address you try to read, one has failed such that it's now effectively a 1-bit memory (whatever address you set, it sets them all) - which is undetectable by a normal write to address/read back from address. And a third one whose failure where setting one bit will set another bit at an undetermined other address. My ROM now has three major tests - a simple set/reset test (which will catch the first), an inversion test (which will catch the second) and a random fill test (which catches the third).

It's the 25th anniversary of the Sinclair Spectrum on Monday, so I'll put at least my incomplete website online by then on this subject to celebrate..