Lab Power Rail Monitor

USB-C multi-rail logger that watches 3.3 / 5 / 12 V benches for droop and glitch events down to 20 µs.

Approach & key decisions

Front end
Three differential dividers into a simultaneous-sampling ADC, with a comparator path firing a GPIO interrupt for sub-sample glitches.
Logging
RP2040 writes a circular ring to external FRAM and streams summaries over USB-CDC so a laptop can chart overnight runs.

System spec

MCU: RP2040
ADC: 12-bit simultaneous, 3 ch
Storage: FRAM, 256 Kbit
Host: USB-CDC + Python plotter

Build gallery

First functional prototype hanging off a 5 V rail during a long overnight soak.

Comparator path catching a 40 µs droop the ADC alone would have averaged out.

Debugging timeline

Firmware
USB transfers stalled when FRAM writes ran on the same core.
Moved FRAM DMA to core 1 and double-buffered the host packets.

Results & final demo

Caught a 40 µs, 380 mV droop on the shared 5 V rail that aligned with a neighboring motor driver spin-up.
Overnight logs made the supply swap an evidence-based decision instead of a guess.

The problem

Intermittent brownouts on a shared lab supply kept resetting student boards. We needed something that sat on the rail and time-stamped glitches without a full scope on every desk.

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