TFC1: Designing a custom RP2354B flight controller

The MadFlight FC3 discovery

In late February 2026, a new BetaFlight configuration appeared in the upstream repo: MADFLIGHT_FC3. This was a flight controller board designed specifically for the RP2350B, and its existence confirmed that the community was actively exploring RP2350-class MCUs for FPV applications.

Compiling BetaFlight for the MADFLIGHT_FC3 target was successful — 401 KB of flash usage (9.73% of 4 MB) and 453 KB of RAM (86.57% of 512 KB). The MadFlight FC3v2 schematic, available from the manufacturer, provided a concrete reference for our own design.

Component availability check

Before finalizing the design, we conducted a thorough component availability survey across multiple distributors:

The IMU situation was the biggest concern. Both the ICM-45686 and its predecessor ICM-42688 were unavailable from major distributors — only LCSC and Taobao had stock. The ICM-42688 and ICM-45686 are not pin-compatible, so substitution wasn’t straightforward.

For the barometer, the BMP580 (matching MadFlight’s original design) was available through Chinese distributors, while the slightly higher-spec BMP581 was available from DigiKey. Since they’re pin-compatible, we designed for BMP580 first with BMP581 as a drop-in replacement for future production runs.

The RP2354B was in normal supply — an important advantage over the RP2350B, which had been difficult to source globally. The RP2354B includes external QSPI flash onboard, eliminating the need for the separate W25Q16 chip.

Power tree architecture

Two distinct hardware configurations were designed, each suited to different use cases:

Gen1 (Simple): Direct power from battery through ESC to FMU. No separate power module needed.

• 2-4S LiPo without VTX
• 3-4S LiPo with VTX (due to buck LDO input requirements)
• Battery voltage monitoring via ADC
• Current monitoring only available with compatible ESCs that provide telemetry data

Gen2 (Advanced): Using a Holybro PM06-style power distribution module.

• BEC splits power: one path to ESCs, one to FMU, one to companion computer
• Full system current monitoring through the power module
• FMU stays powered even if companion computer crashes
• 5V/3A (15W) output from BEC to FMU via standard 6-pin JST-GH connector

The Gen2 architecture is the standard Pixhawk approach and provides better isolation and monitoring. For our initial boards, we included both Gen1 and Gen2-compatible connectors, with solder jumpers to select between configurations.

Design decisions

Several key decisions shaped the TFC1 layout:

1. J7 pin2 (+BATT_IN): The critical power input from ESC for Gen1 operation
2. J7 pin7: Mapped to FMU_UART0_RX_RC with a 0-ohm resistor or solder jumper for flexibility
3. J1 M2_DP/M2_DN: Changed to RP_DP/RP_DN for proper RP2354B USB signaling
4. U1 set to DNP: Leave the footprint empty so RP2350B can be used as a fallback (requires external QSPI flash)
5. JST-GH 6-pin connector: Added for Gen2 compatibility with standard Pixhawk power modules

Three hardware variants were ordered simultaneously, each with a different IMU: ICM-45686, ICM-42688, and MPU-6000. This allowed parallel testing to determine which sensor worked best with our BetaFlight configuration.

Board fabrication

The order was placed and paid in early March 2026. With the PCB fabrication timeline, the boards were expected to arrive by mid-March, at which point assembly and firmware bring-up could begin.