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incutec-OpenFC-Lite/AGENTS.md
JustStan 6f380f2a11 Reference shared release policy
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# OpenFC-Lite
Open-Source RP2350 based Flight Controller (FC), 30.5 x 30.5 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` with sub-sheets below |
| 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 only |
| 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-Mini](https://github.com/OpenDrone-hw/OpenFC-Lite-Mini)**, the
20 x 20 mm RP2354A sibling that shares this design. 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
| Sheet | Contents |
|---|---|
| `rp2350a.kicad_sch` | MCU, USB-C, 12 MHz crystal, boot button, buzzer and LED-strip FETs |
| `power.kicad_sch` | 10 V and 5 V bucks, USB/BATT diode-OR, 3.3 V and 1.8 V LDOs |
| `imu.kicad_sch` | IMU on SPI0 |
| `osd.kicad_sch` | Analog OSD chain |
| `blackbox.kicad_sch` | microSD slot on SPI1 |
| `pads.kicad_sch` | JST SH connectors and through-hole solder pads |
## 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
```
On macOS `kicad-cli` is at
`/Applications/KiCad/KiCad.app/Contents/MacOS/kicad-cli`, and `pcbnew` imports
only under KiCad's bundled Python. 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 RP2354B 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 SPI0 (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 SPI1 (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 four UARTs, two hardware and two synthesised with PIO.
## Key parts
| Function | Ref | Part | LCSC | Note |
|---|---|---|---|---|
| MCU | U2 | RP2354B, QFN-80 | C39843328 | 12 MHz crystal X1, 3.3 uH buck |
| IMU | U9 | BMI270 | C2836813 | multi-source land pattern, note below |
| 10 V buck | U6 | LMR51635YDDCR | C45262770 | 3 A, switched rail |
| 5 V buck | U16 | LMR51635YDDCR | C45262770 | 3 A, always on |
| Power OR | D6, D10 | DSK24 | C64890 | +5V and +5V_USB into +4v5 |
| 3.3 V LDO | U7 | LP5912-3.3DRVR | C524780 | logic rail |
| 1.8 V gyro LDO | U15 | LP5912-1.8DRVR | C2876234 | IMU analog supply |
| OSD sync comparator | U12 | TLV7031DPWR | C2876045 | sync separation from camera video |
| OSD pixel switch | U10 | SN74LVC1G3157DTBR | C2673087 | white/video mux |
| OSD video buffer | U11 | COS8051SOT | C7463385 | output driver |
| microSD slot | Card1 | TF push slot | C393941 | larger 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-8S) ─┬─► U6 10V buck +10V ─► VTX
└─► U16 5V buck ─► +5V ─► D6 ─┐
├─► +4v5 ─┬─► U7 3.3V LDO ─► MCU IO, IMU IO, microSD, OSD
+5V_USB ────────────────────────► D10 ──────┘ └─► U15 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, through-hole | P1 | SM08B-SRSS-TB | +BATT, GND, current sense, ESC telemetry, M1-M4 |
| VTX, 6-pin JST SH | U8 | SM06B-SRSS-TB | +10V, GND, UART0 TX, UART0 RX, GND, UART1 RX |
| Peripheral, 6-pin JST SH | U14 | SM06B-SRSS-TB | +4v5, GND, PIO UART1 RX/TX, I2C0 SDA/SCL |
| Receiver, 4-pin JST SH | CN1 | SM04B-SRSS-TB | +5V, GND, PIO UART0 RX/TX |
| Camera, 3-pin JST SH | U13 | SM03B-SRSS-TB | +5V, GND, video in |
| USB-C | USB1 | 16-pin | 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 UART0 TX, RX (receiver connector) |
| 4, 5, 6 | OSD_W, OSD_EN, OSD_SYNC |
| 7 | Beeper, low-side FET Q1 |
| 11 | 10 V enable |
| 12 | Status LED, blue |
| 13, 14, 15 | IMU INT, CS, CLKIN |
| 16, 17 | I2C0 SDA, SCL |
| 18, 19, 20 | SPI0 SCK, MOSI, MISO (IMU) |
| 21, 22 | UART1 RX, TX |
| 23 | LED strip, level-shifted by Q2 |
| 26, 27 | PIO UART1 TX, RX (peripheral connector; RX doubles as ESC telemetry) |
| 28, 29, 30, 31 | Motors M4, M3, M2, M1 |
| 40, 41 | ADC: ESC current, VBAT divider |
| 42, 43, 44 | SPI1 SCK, MOSI, MISO (microSD) |
| 45, 47 | ADC: RSSI, spare pad |
| 46 | microSD CS |
| Free | 8, 9, 10, 24, 25, 32-39 |
The current, RSSI and spare ADC inputs go through a 1k + 100 nF RC filter;
VBAT sits on its 100k/10k divider with the same 100 nF.
## Firmware
[Betaflight](https://github.com/betaflight/betaflight) against a custom target:
`BOARD_NAME = OPENFC_LITE_RP2350B`, `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.
- RP2354 core buck placement follows RP2350 datasheet section 6.3.8.1: L1 and
its input and output capacitors stay on the MCU side, with copper cut away
under the switch node.
## Revisions
| Rev | Date | Change |
|---|---|---|
| Rev3.3 | 2026-08-25 | Export `OpenFC-Lite-rev3.3`, current. Silkscreen rebranded OpenDrone -> incutec for export restriction reasons on flagging anything containing 'Drone'. First Incutec production run. |
| Rev 3.1 | 2026-08-14 | TPS2116 mux replaced by the DSK24 diode-OR into +4v5. |
| Rev 3 | 2026-08-11 | LMR51635 bucks raise input to 3S-8S, LP5912-1.8 gyro LDO, BMI270 IMU. |
| Rev 2 | 2026-06-06 | Flown. |
| Rev 1 | | First prototype, bench tested. Predates this repo; shared history with the Mini. |
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