Found my lost electrons

To confirm my analysis, I told iMPACT to verify the contents of the Flash ROM with the image I'd previously written. This loads the JTAG-to-SPI core, which sets all the non-SPI I/O pins to PULLUP. While this was running I noted that the supply current spiked to about 200 mA. I measured the voltage on the LTC3494 and SN6505B enable pins and found them just under 3V; they need to be below 0.3V for these chips to be disabled.

Having created valid test conditions, I changed out the 100K ohm pull-down resistors on the VFD +30V boost regulator and filament transformer driver, replacing them with 750 ohm resistors. Now when I access the Flash ROM from iMPACT the ammeter now barely twitches and the enable pins sit at about 0.25V. Satisfied, I changed the pull-downs on the printer drivers circuits.

I also chased down the problems with my walking-one test of the debug interface. When I reattached the FPGA to the board after reorienting it, I didn't remove all the solder from the pads. My thinking was that since I couldn't put down new solder paste, I'd just re-heat the solder already there. The problem is that now the FPGA isn't sitting flat, and some of the pins (fingers?) didn't get soldered. The debug interface pins that were misbehaving were floating. By careful inspection I found several such pins and fixed them. Or at least I think I fixed them.

With this in mind, my next test will be to do the walking-one test on the VFD interface. This will show whether any of those pins are disconnected. I wrote the code this evening, and I'll run the test on the board in the morning.