Wiring

STM32F405 Feather Express

The STM32F405 is the primary MCU. It reads 4 BNO055 IMUs (I2C1 + I2C2), 6 ADC channels (2 MyoWare + 4 FSR), receives the KB2040 relay frame over USART6, and streams the unified 214-byte packet over USB CDC to the PC.

I2C buses

Bus	Port pins	Silkscreen	Devices
I2C1	PB7 (SDA), PB6 (SCL)	SDA / SCL pads	BNO055 #0 L wrist (0x28), BNO055 #1 R wrist (0x29)
I2C2	PB11 (SDA), PB10 (SCL)	D0 / D1	BNO055 #2 L arm (0x28), BNO055 #3 R arm (0x29)

Each bus supports two addresses (0x28 and 0x29), selected by the ADR pin on the BNO055: - ADR = GND → 0x28 - ADR = 3.3 V → 0x29

Pull-up resistors

BNO055 can stretch the I²C clock up to ~700 µs during fusion. Use 4.7 kΩ pull-ups on SDA/SCL. The STM32 hardware I²C handles clock stretching; if reads fail intermittently, check the pull-ups first.

ADC channels

Pin	Sensor	Signal
A0	MyoWare forearm flexor	0–3.3 V (envelope)
A1	MyoWare lumbar erector	0–3.3 V (envelope)
A2	FSR left heel	voltage divider, 0–3.3 V
A3	FSR left toe	voltage divider, 0–3.3 V
A4	FSR right heel	voltage divider, 0–3.3 V
A5	FSR right toe	voltage divider, 0–3.3 V

FSR voltage divider: 10 kΩ pull-down per sensor. Connect one FSR leg to 3.3 V, the other leg to ADC pin + 10 kΩ to GND.

USART6 (KB2040 bridge)

STM32 pin	Direction	KB2040 pin
PC6 (TX)	→	GP5 (RX)
PC7 (RX)	←	GP4 (TX)

921600 baud, 8N1. The KB2040 sends a 70-byte frame every 10 ms.

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KB2040

The KB2040 is the I²C co-processor. It reads 2 BNO055 IMUs (chest, thigh) and sends the 70-byte frame to the STM32 over Serial1 (UART1).

I2C buses

Bus	Port pins	Silkscreen	Devices
I2C0	GP12 (SDA), GP13 (SCL)	D12 / D13	BNO055 #4 chest (0x28)
I2C1	GP2 (SDA), GP3 (SCL)	D2 / D3	BNO055 #5 thigh (0x28)

Both IMUs at address 0x28 (ADR = GND) since they are on separate buses.

Serial1 (UART to STM32)

KB2040 pin	Direction	STM32 pin
GP4 (TX)	→	PC7 (RX)
GP5 (RX)	←	PC6 (TX)

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Power

Both MCUs power up via USB-C. The STM32 USB-C also carries data (CDC serial). The KB2040 USB-C is for power only if connected directly to a battery; it can also be powered via the STM32's 3.3 V rail through the UART connection (GND + VIN).

All BNO055 sensors are powered from the 3.3 V rail on their respective MCU. MyoWare sensors need 3.3–5 V (STM32 3.3 V is sufficient). FSR voltage dividers use the 3.3 V rail.

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Quick connectivity check

After flashing both MCUs (see Flashing):

1. Connect STM32 USB-C to PC
2. Open serial monitor at 115200 baud — pio device monitor -b 115200 from firmware/stm32f405/
3. Verify: [IMU] 0:ok 1:ok 2:ok 3:ok (initialization logs)
4. Run: .venv/bin/python -m host.serial_reader
5. Verify: frames with seq=N and valid CRC appear at ~100 Hz