Sometimes, an oscilloscope can come in very handy.

About a year ago I was working on a signal processing driver that, for several weeks, I could seem to get to work correctly (part of the ordeal is documented here. At the time, I was working under the assumption that I had a problem with the timing of the incoming signal (fronts were taking too long to get to my driver and the delay was both random and huge – at least, that was the hypothesis). What I was trying to do is to get the signal I received to come out of the board somewhere, so I could probe it. There being no test points on the board anywhere1 the only way to do that was to output the signal through a GPIO2 and probe that (the incoming signal was tucked away nice and snug so I couldn’t get at it).

Once I got it working, the problem was relatively easy to see: the input signal had been inverted by an opto3. Fixing the problem was a simple matter of inverting the signal I read on my GPIO before passing it to the parser.

Yesterday, I was faced with a very similar problem: I am developing a driver that interfaces with an FPGA4 that decodes a signal very similar to the one I was working with a year ago. I plugged everything in together and voila! Nothing!

I am happy to say my plea from last year has been heard, though: this time, I had a nice test point I could probe, giving me the signal exactly as it would be passed to the FPGA (so it was connected somewhere between the opto and the FPGA). What I saw was a beautiful almost-square wave pattern (these are differential signals, a little like RS-485) that was exactly the opposite of what I should be seeing (i.e. the signal was high when it should be low and vice-versa). One call to the guy who wrote the firmware for the FPGA and one E-mail later and I had a new FPGA firmware that did the right thing.

So, an ordeal was spared me, just by adding less than a gram of gold to a PCB5 in a few strategic places.

  1. Test points are points on the printed circuit board where you can easily put an oscilloscope probe to see the current passing through the circuit they’re connected to. 

  2. A GPIO is a General-Purpose Input/Output. Software can use it to directly interact with the physical world. They can be programmed as either an input or an output and can be represented by a single bit or boolean in the software. 

  3. An “opto” is an optical coupler: it de-couples an electrical signal from a circuit by using a light on one end and an optical sensor in another. Thus, if there’s a problem with the incoming signal’s current or voltage, the opto might blow out, but the rest of the circuit remains in tact. 

  4. An FPGA, or Field-Programmable Gate Array, is a chip you’ll find in many modern circuits – probably including your phone. It can be programmed with a firmware and thus coerced to do almost anything, including simulating a complete computer. FPGAs are often used as a relatively low-cost replacement for specialized high-speed circuitry, such as those required for certain types of signal processing. 

  5. A PCB is a Printed Circuit Board. If you crack open one of your gadgets you’ll see a few of them. They’re the (often red or green) boards with the little gold lines drawn on them and the little chip thingies sticking out. Don’t touch! You’ll ruin it!