UNIT1 R1

UNIT1 R1 KiCad design

This directory contains the first electrically connected UNIT1 board design. It is a real KiCad hierarchy, not a wiring sketch. The RP2350A core comes from Raspberry Pi's official minimal reference; the second sheet adds the protected 12 V input, 5 V supply, DRV8461 motor stage, safe boot defaults, and the motor connector used by the firmware.

Status: R1-LAYOUT physical-design checkpoint; not released for fabrication. The PCB now has the full UNIT1 outline, mounting holes, barrel input, USB, RP2350A core, protected power stage, DRV8461, right-angle motor header, four copper layers, power/ground pours, and routed high-current motor/power paths. The unfinished logic fanout is intentionally left as ratsnest connections; no provisional crossing copper remains on the board.

External-interface contract

The finished enclosure is one self-contained controller. It has exactly three normal field connections:

The legacy chain—adapter, external buck/Big Easy Driver, Arduino, breadboard, button/switch, and the wiring between them—is not part of UNIT1. With the adapter connected, UNIT1 supplies itself and drives the motor; the computer only sends commands and receives status over USB. USB power remains protected as a service/update source and cannot back-power the 12 V motor rail.

J10, J11, and the Micro-B USB receptacle are now placed on board edges. J10 uses the current-production Würth 694103107102 land pattern and vendor STEP model as a provisional mechanical match. That jack has a 1.3 mm inside-contact class, while third-party YU1204 listings commonly claim a 5.5 mm OD / 2.5 mm ID plug. That makes the present J10 likely incompatible, but the actual plugs still have to be gauged before a replacement is selected or released.

Design files

• UNIT1_R1.kicad_sch — root RP2350A/USB/flash/reset schematic.
• UNIT1_MOTOR_POWER.kicad_sch — protected input, AP63205, DRV8461, and J11.
• UNIT1_R1.kicad_pcb — four-layer integrated UNIT1 physical-design checkpoint.
• UNIT1_R1_placement_preview.png — current top-side 3D placement render.
• UNIT1_R1_back_preview.png — back-side render; regenerate after identity or layout changes.
• UNIT1_R1_DRC.json — current full-board KiCad DRC result.
• UNIT1_R1_QA.md — fabrication, firmware, thermal, connector, and multi-week qualification gates.
• UNIT1_R1.kicad_pro — KiCad project settings.
• UNIT1_R1_schematic.pdf — rendered two-page review copy.
• UNIT1_R1_ERC.json — KiCad 10 ERC result.
• tools/generate_schematic.py — reproducible hierarchy generator.
• tools/build_pcb.py — reproducible outline, placement, zone, and critical-route builder.
• tools/check_release.py — fail-closed automated DRC/connectivity/identity gate.
• tools/check_netlist.py — asserts the 16 critical control, motor, and power nets.
• artwork/cell_geometry_lab_logo.webp — archived source artwork; deliberately not placed on the PCB.
• RASPBERRY_PI_REFERENCE_LICENSE.txt — upstream MIT license and disclaimer.

Electrical path now captured

J10 barrel tip
  -> F10 2.5 A time-lag fuse
  -> Q10 IRF4905S reverse-polarity stage + D10 VGS clamp
  -> VM_RAW + D11 SMBJ18CA + C20/C21 bulk/high-frequency bypass
       |-> U10 AP63205 5 V buck -> D21 --+
       |                                  +-> +5V_SYS -> U2 3.3 V LDO -> RP2350A
USB VBUS -------------------------> D20 --+
       |
       +-> U11 DRV8461 VM -> J11 pins A1, A2, B1, B2

The Schottky source OR prevents the 12 V-derived supply from feeding the USB port and prevents USB from back-powering the buck. U11 VREF is grounded because the firmware enables the DRV8461 internal 3.3 V reference before motion.

Firmware-to-driver net contract

J11 is a board-mounted 1x4, 2.54 mm male header so the photographed one-piece 4-position Harwin-style female motor housing remains intact:

Confirm both coil pairs and the physical housing order with an ohmmeter before connection. Never insert or remove J11 with power applied.

Verification result

KiCad 10.0.5 loads both pages, exports the netlist/PDF, and reports no new ERC violations from the UNIT1 motor/power sheet. The full hierarchy reports the same 135 items as the untouched Raspberry Pi reference: three reference-design ERC errors, 130 warnings for unavailable Raspberry Pi private library names (the symbols are embedded), and two off-grid warnings. The three inherited errors are two power-drive declarations and duplicate LDO output pins; none is introduced by the UNIT1 circuitry.

The exported netlist was also asserted for all nine MCU/DRV control nets, all four motor outputs, VM_RAW, Q1_GATE, and +5V_SYS. Root and child sheets have no reference-designator collisions.

The full R1-LAYOUT PCB DRC checkpoint has zero shorts, zero track crossings, and zero clearance violations. It reports 17 unconnected items: one inherited GPIO0 disconnect in the imported reference board and 16 deliberate UNIT1 ratsnest items covering the nine MCU/driver control paths, their boot-state pull branches, and VCP/C40. The remaining non-connectivity DRC entries are library/silkscreen cleanup, dangling ends created by those open routes, four starved thermals, and one C44/L20 courtyard overlap. This is a clean routing checkpoint, not an order-ready DRC result.

Schematic-parity mode at the preceding checkpoint reported 50 footprint identifier/value/field mismatches introduced by the physical-board import/generator. The generator now preserves library-qualified footprint IDs; the remaining field and mounting-hole parity items still require regeneration and review before release.

The back silkscreen identity is text-only and must read, exactly:

UNIT1 by The Laboratory of Cell Geometry
University of California, San Francisco
Designed & Manufactured in California

To regenerate the sheets:

python3 -m venv .venv-hardware
.venv-hardware/bin/pip install -r tools/requirements.txt
.venv-hardware/bin/python tools/generate_schematic.py \
  --symbol-dir /path/to/KiCad/10.0/symbols
kicad-cli sch erc --format json --severity-all \
  --output UNIT1_R1_ERC.json UNIT1_R1.kicad_sch
kicad-cli sch export pdf --output UNIT1_R1_schematic.pdf UNIT1_R1.kicad_sch
.venv-hardware/bin/python tools/check_netlist.py --kicad-cli /path/to/kicad-cli
kicad-cli sch export netlist --format kicadxml \
  --output /tmp/UNIT1_R1.xml UNIT1_R1.kicad_sch
/path/to/kicad-python tools/build_pcb.py \
  --netlist /tmp/UNIT1_R1.xml --footprint-root /path/to/KiCad/footprints
kicad-cli pcb drc --refill-zones --save-board --format json \
  --output UNIT1_R1_DRC.json UNIT1_R1.kicad_pcb
python3 tools/check_release.py

check_release.py is expected to fail at the current R1-LAYOUT checkpoint. It must not be bypassed to order a mission board; manual gates and qualification evidence are tracked in UNIT1_R1_QA.md.

Fabrication release gates

The rest of schematic capture and placement can continue now. These items must be resolved before ordering boards:

1. Trace the YU1204 12 V / 4 A and XSJ-1205000HUS 12 V / 5 A supplies to their legacy loads; then gauge both plugs and verify a real plug against J10 before retaining the provisional Würth footprint. Both labels indicate center-positive.
2. Measure TP1 on the working Big Easy Driver rig and confirm the target phase current. Current equivalence cannot be inferred from the knob position.
3. Verify the installed motor's two coil pairs with an ohmmeter. The photographed 17HS19-2004S1 housing and black/green/red/blue order match the published pinout.
4. Add USB ESD protection and decide whether R1 keeps the reference Micro-B receptacle or migrates to USB-C.
5. Finish the 16 visible UNIT1 ratsnest connections, relocate the pull network, close VCP/C40, fix the C44/L20 courtyard, complete ground thermals, and clean silkscreen/library-rule noise without reintroducing crossings.
6. Validate the PWP-28 exposed-pad land pattern and 2 oz stackup/thermal target, then run full ERC/DRC, fabrication-output inspection, current-limited bring-up, and the one-to-one golden-rig calibration before experimental use.

Primary source designs

• Raspberry Pi, RP2350A Minimal KiCAD, R4/S1, 2026-07-03.
• Raspberry Pi, Hardware design with RP2350.
• Texas Instruments, DRV8461 65-V, 3-A stepper motor driver.
• Diodes Incorporated, AP63200/AP63201/AP63203/AP63205.
• SparkFun ROB-12859 Big Easy Driver v1.6 schematic and current guidance.