PicoDCC

PicoDCC carrier board

The carrier PCB the firmware runs on: a Raspberry Pi Pico 2 soldered down, two BTS7960 H-bridge modules (main and programming track), their fault LEDs, and headers for the display and debug. KiCad project, BOM and the Gerbers as sent to fab.

It was designed in KiCad 9.0.1 in April 2025, before this repo had any design review, and it is imported as it was built. Nothing in the KiCad files has been changed on the way in. The concerns found while importing it are listed below rather than fixed, because the boards in use are this revision.

Files

The title block carries no revision, so "1.0" is a name given on import for the only revision there has been. A board change that goes to fab gets a new fab/rev-X.Y/ folder.

KiCad plots into Gerber/ by default, which is git-ignored so that a fresh plot is never mistaken for the fab record. Opening the project in KiCad 10 and saving it upgrades the file format; do that deliberately, in its own commit.

Connections

A1 is the Pico 2. Pin numbers are the Pico's.

Not connected: 3V3, 3V3_EN, RUN, VBUS, ADC_VREF, AGND, GP22, GP28.

J2 (debug): 1 GND, 2 VSYS, 3 GP0, 4 GP1, 5 SWDIO, 6 SWCLK. J4 (power): 1 GND, 2 VSYS.

Verification

Run on import with KiCad 10.0.5 (kicad-cli), against the files in this folder:

sch erc --severity-all                4 violations, all lib_symbol_mismatch (BC108 x2, LED_Small x2)
sch erc --severity-error              0
pcb drc --schematic-parity            0 unconnected items, 0 schematic parity issues
pcb drc, all severities               28: 27 lib_footprint_mismatch, 1 starved_thermal (error)

The mismatches are library drift: the design embeds the KiCad 9 copies of stock symbols and footprints, and KiCad 10's libraries have moved on. They are not connection problems. The starved thermal is concern 4.

The schematic was last saved in August 2025, four months after the board. The zero parity result shows the board still matches it.

fab/rev-1.0/ was checked against the board by replotting the Gerbers and drill files with kicad-cli and comparing what each file draws, using the geometry comparison from bazauto/block-detection's scripts/check_fab_outputs.py. All 12 files match. The only textual difference is that KiCad 10 numbers the three custom pad-shape aperture macros in a different order from 9.0.1.

Known concerns with revision 1.0

1. The fault LEDs are drawn reverse-biased. GP16 drives D1's cathode and the anode returns to GND through R5, so driving GP16 high — what the firmware does on a trip — reverse-biases the LED. D2 on GP19 is the same. Check how the LEDs were actually fitted before relying on them: if they light, they are in the other way round from the footprint.

2. 3V3 is not broken out. J1 carries GP0–GP15 only; neither J1 nor J2 has a 3V3 pin. Anything that needs a 3.3 V supply from the board — the block detector used for programming-track ACK, for one — has to take it from the Pico's 3V3 pin (pin 36) directly.

3. Current sense resolution, and a possible overvoltage on the ADC pins. The BTS7960's IS output is a current proportional to load, at a ratio of roughly 8500:1, turned into a voltage by the 1k to ground. At that ratio 60 mA — a decoder's service-mode ACK — is about 7 µA, about 7 mV, about 9 ADC counts, which is below the sense offset the part is specified with at low current. That is why ADC-based ACK detection never worked, and why ACK detection is moving to the block detector. Separately, the BTS7960 datasheet describes the IS pin sourcing a fixed current of a few milliamps when the chip latches a fault; if that is ~4.5 mA, 1k turns it into ~4.5 V on GP26/GP27, and the RP2350's ADC-capable pins are not 5 V tolerant. Unverified — check the datasheet figure before relying on the ADC pins surviving a driver fault.

4. One starved thermal relief. J2 pin 1 (GND) connects to the B.Cu ground zone with one spoke where the zone asks for two. Harmless electrically; worth fixing in any next revision.

Licence

Copyright © 2025–2026 Paul Barrett.

The design in this folder — schematic, board, project symbol library, BOM, Gerbers and drill files — is licensed under the CERN Open Hardware Licence Version 2 – Permissive (LICENSE), the same licence as bazauto/block-detection. The firmware in the rest of the repository stays MIT.

Symbols and footprints from the stock KiCad libraries are embedded in the schematic and board, as KiCad always does. The KiCad libraries are CC-BY-SA 4.0 with an exception that a design using them is not bound by that licence, so this folder carries no obligation from them. 3D models are referenced by path, not included. See THIRD_PARTY_NOTICES.md.