incutec-OpenFC-Lite-Mini/AGENTS.md

# OpenFC-Lite-Mini

Open-Source RP2350 based Flight Controller (FC), 20 x 20 mm mounting pattern
(FPV Drone standard). Part of OpenDrone by Incutec product lineup.

Blackbox is a microSD slot, analog OSD generated on PIO (pixel OSD)

## Repo

| Maintainer | @Just4Stan (Discord: juststan_) |
|---|---|
| Status | See the `status-*` topic on the repo. |
| Designed in | KiCad 10 |
| KiCad project | `hardware/OpenFC.kicad_pro` |
| Root schematic | `hardware/OpenFC.kicad_sch` plus sub-sheets `rp2350a`, `power`, `imu`, `osd`, `blackbox`, `pads` |
| Board | `hardware/OpenFC.kicad_pcb`, 6 layers, 1.6 mm |
| Local library | `hardware/lib.kicad_sym`, `hardware/lib.pretty/`, `hardware/lib.3dshapes/`, nickname `lib` |
| Shared library | `hardware/KiCad-Library/`, pinned submodule of [OpenDrone-hw/KiCad-Library](https://github.com/OpenDrone-hw/KiCad-Library), nickname `OpenDrone`; 3D models and exact component datasheets resolve through `OPENDRONE_LIB` |
| Design rules | `hardware/OpenFC.kicad_dru`, canonical block, no board-specific rules |
| Fab config | `hardware/fabrication-toolkit-options.json` |
| Board setup | Line standard: 6 layers, 0.09 mm clearance and track, via 0.35 on 0.20 drill |
| License | CERN-OHL-S-2.0 |

**The project is called `OpenFC` in both this repo and
[OpenFC-Lite](https://github.com/OpenDrone-hw/OpenFC-Lite).** Check which repo
you are in before importing a part or running an export: a part imported into
the wrong one looks exactly like a broken import.

## Sheets (to copy from OpenFC-Lite)

## Rules

Identical in every OpenDrone board repo. Do not edit here; edit the template.

- **Never text-edit** `.kicad_sch`, `.kicad_pcb` or `.kicad_dru`. Use KiCad, or
  kicad-skip / the pcbnew API for scripted changes. `.kicad_pro` is JSON and may
  be edited directly for metadata.
- **Metadata yes, connections no.** An agent may write BOM and documentation
  fields (MPN, Manufacturer, LCSC, Cost, Datasheet, text variables). An agent
  may not change nets, wiring, routing, placement, footprint assignment, or any
  value that changes the circuit.
- **Close KiCad before any write to a KiCad file.** KiCad caches library tables
  at process start and overwrites files on save.
- **Reuse before you draw.** Check the `OpenDrone` library and its
  `PARTS-USED.md` first. If the part is there we have already sourced,
  footprinted and shipped it, and its symbol links to the exact committed
  datasheet: place it from `OpenDrone`. Draw a new part only when the catalogue
  has nothing that fits, and import it with `easyeda2kicad` from its LCSC
  number. Pulling a newer catalogue is a deliberate, reviewed submodule commit.
- **One person holds a board layout at a time.** KiCad files do not merge. Say
  on Discord that you are taking it. See [CONTRIBUTING.md](CONTRIBUTING.md).
- **Run ERC and DRC before every pull request.** Existing approved findings
  may remain; a new type or increased count must be reviewed before merge.
  Commands below.

## Environment

```sh
# schematic and board checks
kicad-cli sch erc hardware/OpenFC.kicad_sch
kicad-cli pcb drc --schematic-parity --refill-zones hardware/OpenFC.kicad_pcb

# netlist, for scripted analysis
kicad-cli sch export netlist --format kicadsexpr -o /tmp/OpenFC.net hardware/OpenFC.kicad_sch
```

Reusable scripts (renders, STEP export, packaging art) come from Incutec
hardware tooling; the OpenDrone release standard lives in OpenDrone-hw/.github/RELEASES.md;
board-specific scripts live in hardware/tools/.

## Architecture

An RP2354A runs Betaflight against a custom target (`rp2350` sheet: MCU, USB-C,
12 MHz crystal, boot button, buzzer and LED-strip FETs). The IMU sits on SPI1
(`imu`) with its own interrupt line and its own 1.8 V analog supply, kept
separate from the 3.3 V logic rail so gyro noise is not shared with the digital
side. Blackbox logging goes to a microSD card on SPI0 (`blackbox`). Analog OSD
is generated by the MCU through PIO and mixed into the composite video path by a
discrete front end (`osd`), which is why there is no dedicated OSD chip.

Motor control is signal only: M1 to M4 leave as DShot on the 8-pin ESC connector
(Betaflight Standard Connector), alongside battery voltage sense, ESC current
sense and an ESC telemetry UART. Everything that drives current lives on the
ESC.

Serial capacity is three UARTs, two hardware and one synthesised with PIO.

## Key parts

| Function | Ref | Part | LCSC | Note |
|---|---|---|---|---|
| MCU | U10 | RP2354A, QFN-60 | C41378174 | 12 MHz crystal X1, 3.3 uH buck |
| IMU | U9 | BMI270 | C2836813 | multi-source land pattern, note below |
| 10 V buck | U3 | LMR51430YFDDCR | C5219261 | 3 A, switched rail |
| 5 V buck | U4 | LMR51430YFDDCR | C5219261 | 3 A, always on |
| 3.3 V LDO | U7 | LP5912-3.3DRVR | C524780 | 500 mA, fed from +4v5 |
| 1.8 V LDO | U12 | LP5912-1.8DRVR | C2876234 | IMU analog supply |
| Power-OR | D6, D10 | DSK24 | C64890 | +5V_BUCK and +5V_USB into +4v5 |
| OSD video buffer | U1 | COS8051SOT | C7463385 | op-amp |
| OSD sync separator | U2 | TLV7031DPWR | C2876045 | comparator |
| OSD pixel switch | U18 | SN74LVC1G3157DTBR | C2673087 | SPDT analog switch |
| microSD slot | Card1 | TF-021B-H265 | C498185 | smaller footprint |

The IMU land pattern is LGA-14 3.0 x 2.5 mm with pads 2 and 3 grounded and 10
and 11 unconnected, electrically safe for Bosch BMI2xx, TDK ICM-4xxxx and ST
LSM6D parts alike: swapping the IMU is a not a layout change. Pad 9 (INT2 on the
BMI270, CLKIN on TDK parts) dead-ends on the `imu` sheet, so a TDK part would
run without external clock sync.

## Power

```
+BATT (3S-6S) ─┬─► U3 10V buck +10V ─► VTX
               └─► U4 5V buck ─► +5V ─► D6 ─┐
                                            ├─► +4v5 ─┬─► U7 3.3V LDO ─► MCU IO, IMU IO, microSD, OSD
+5V_USB ────────────────────────► D10 ──────┘         └─► U12 1.8V LDO ─► IMU

+3.3V ─► RP2354 internal switcher ─► +1.1V core
```

The 10 V rail is gated by the MCU so a VTX can be switched off in firmware. D6
and D10 diode-OR battery and USB into +4v5 with no pass element, and the
external 5 V pads hang directly on the buck output, so USB never back-feeds
them. There is no reverse-polarity protection.

## Connectors and I/O

| Connector | Ref | Part | Function |
|---|---|---|---|
| ESC, 8-pin JST SH | P1 | SM08B-SRSS-TB, C160407 | 1 +BATT, 2 GND, 3 current sense, 4 ESC telemetry (PIO UART RX), 5-8 M1-M4. Using betaflight standard. |
| VTX, 6-pin JST SH | U8 | SM06B-SRSS-TB, C160405 | 1 +10V, 2 GND, 3 UART0 TX, 4 UART0 RX, 5 GND, 6 UART1 RX |
| USB-C | USB1 | 16-pin Type-C | Configuration and flashing, USB full speed |

Everything else issolder pads (`pads` sheet)

## GPIO map

| GPIO | Function |
|---|---|
| 0, 1 | UART0 TX/RX, VTX connector |
| 2, 3 | PIO UART TX/RX; RX doubles as ESC telemetry on P1 pin 4 |
| 4, 5 | I2C0 SDA/SCL, pulled up to 3.3 V |
| 6, 7 | UART1 TX/RX, receiver; SBUS inversion at the IO mux in firmware |
| 8 | LED strip |
| 9 | IMU interrupt |
| 10-13 | SPI1 SCK/MOSI/MISO/CS, IMU |
| 14-16 | OSD_W, OSD_EN, OSD_SYNC |
| 17 | Beeper, low-side N-FET |
| 18-21 | SPI0 SCK/MOSI/MISO/CS, microSD |
| 22-25 | Motors, descending: M4, M3, M2, M1 |
| 26 | Status LED, blue |
| 27 | 10V_ENABLE (PINIO) |
| 28, 29 | ADC2 ESC current through a 1 k + 100 nF RC, ADC3 battery voltage through a 100 k / 10 k divider with 100 nF |

The QFN-60 exposes only four ADC channels, and the ADC0/ADC1 pins are spent on
the status LED and 10V_ENABLE, so battery voltage and ESC current are the only
analog inputs: the RSSI and spare ADC pads of the QFN-80 OpenFC-Lite do not
exist here.

## Firmware

Betaflight against a custom target: BOARD_NAME = OPENFC_LITE_MINI_RP2350A,
MANUFACTURER_ID = OPFC. A prebuilt uf2 lives in firmware/. First flash: hold the
boot button, plug in USB-C, and copy the uf2 onto the RP2350 UF2 mass-storage
device; after that the configurator flashes over USB.

## Layout rules

- The IMU's 1.8 V analog supply is deliberately separate from the 3.3 V logic
  rail. Do not merge them to save a regulator.
- RP2350 core buck placement follows the RP2350 datasheet, section 6.3.8.1:
  L4 and its input and output capacitors stay tight to U10, with copper pulled
  back under the switch node.

## Revisions

| Rev | Change |
|---|---|
| Rev3.3 | Current. Export `OpenFC-Lite-Mini-rev3.3`. Silkscreen rebranded OpenDrone -> incutec for export restriction reasons on flagging anything containing 'Drone'. First Incutec production run. |
| Rev3.2 | Export `OpenFC-Lite-Mini-rev3.2`. |
| Rev3.1 | TPS2116 mux replaced by the DSK24 diode-OR into +4v5. |
| Rev3 | LDO/IMU swap, pad attribute fixes. |
| Rev2 | Flown. |
| Rev1 | First prototype, bench tested. |