Wiring the Test Jig

Last night I finished wiring the test jig. It's not pretty, but none of the signals need to have fast rise or fall times, and there's a lot of noise immunity in the signaling levels.

The signals arrive from the Spartan-3E reference board through the connector on the right of this photo and terminate at the pin header next to it. From there, the rats nest of wires connects to the Source leads of the array of FDV301 MOSFETs. The Gate leads of this array are connected to the +3.3V power bus sourced by the reference board; if you look closely you'll see a heavy, bare copper wire running vertically through each of the four columns of FETs. The Drain leads then connect to the larger inter-board connector on the left of this picture.

I've already had one of these leads break due to flexing while the rest were being attached, so I'm planning on attaching a piece of bare perf-board to the standoffs to provide some mechanical protection while the board is handled. I'll attach identical standoffs and perf-board to the bottom of the reference board so it'll sit at the same height off the bench.

With this done, of course I couldn't wait to test it. I hooked everything up, loaded the FPGA from my laptop, and started probing the partially-populated IP board. My plan was to test the 2-bit row refresh counter, but it wasn't counting. I started debugging by checking the INH and ~INH signals, looking for any indication that they were changing. They weren't. Probing the inputs that generated this signal I saw that none of them were changing either. Then I realized that all the inputs to this circuit are statically driven from the Verilog code, and that this is exactly what I'd intended. Doh!

The counter was getting the proper CLK1 input, so why wasn't the counter counting? Could it have something to do with the unpopulated area of the board I'd clearly marked "Rfsh" (as in "Refresh")? Well, yes...

Back to my soldering bench (which is separate from my test bench -- 500 MHz scope probes are damned expensive and far too easy to melt). I put dabs of solder paste on each of the pads in the refresh enable logic, carefully placed the components, then soldered each of them with my soldering iron. Back to the test bench. Still nothing. I probed the circuit and found what appeared to be an open solder joint. Back to the soldering bench, but this time I fired up the hot air station. It takes a little longer, because the board has to come up to temp unless you want to fry things, but the results seem to be a lot more reliable. Especially when you're dealing with leads that are barely 0.5mm (0.020") wide.

With a lesson on soldering SMT components learned, I went back to the test bench. Hot! Hot! Hot! I keep forgetting that hot air work leaves the board hot, and having two full interior copper planes holds a lot of heat. When the board cooled down I hooked it up and sat there with a silly smile on my face, watching as a 42 year old design counted 0, 1, 2, 3, 0, 1, 2, 3...

It's ALIVE...