Keith and I have been plugging away for the last couple weeks turning on and testing the various bits of TeleMetrum, and I can now very happily report that we have all the hardware working! Amazingly, we got almost everything right in the hardware design. Only two things need to be changed.

We need to be able to control the chip select line on the the SPI non-volatile memory chip, which means we need one more GPIO line to it from the processor. To free up a line, we gave up the ability to put the accelerometer in to self test mode. Two trace cuts and two jumpers to the current board implement this change.

The last thing we got working just this evening was the ability to sense the presence of an ejection charge igniter. We need another resistor in each of the two sense circuits to form a voltage divider so that we don't overload the ADC input. Not sure how we missed that in our many design reviews! Oh well. To ensure robust ignition of the electric matches we prefer to use for ejection charges, we charge a 1000uF cap to a voltage higher than our ADC inputs can handle. Fixing this problem on the current boards requires changing the values of two resistors, then tacking extra resistor to one end of each of the existing ones and then wiring the other ends of the new resistors to ground. Since the resistors in question are all 0402 surface mount parts, this is a royal pain... but with my finest dental-pick soldering iron tip, and working under the inspection microscope, it is indeed possible.

Alternatively, if the ability to sense igniter continuity isn't important, just leaving off R14 and R15 at least prevents the 3.3V processor from going whacko due to having 5V applied to two ADC inputs!

I'm really impressed at how well these first two boards reflow soldered in the electric skillet in my basement came out. Getting the right amount of solder paste on the board is something I'm likely to get better at with practice, but so far we've only had a few (very obvious during inspection) shorts to clean up, and a couple poorly soldered pins. All in all, I couldn't be happier with how this has gone... and I'm really looking forward to flying one of these boards sometime soon as an auxiliarly payload to gather some initial flight data!