protoflight 0.1.4

Protoflight flight controller.
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# Boards

The boards directory contains the Board Support Packages (BSP) for the boards supported by Protoflight.

For example the Speedy Bee F404 V4 board is initialized in the `speedybee_f405_v4.rs` BSP.

It is there that the pins are assigned, the SPI and I2C devices, the UARTs etc created,
and they are assigned to the Protoflight objects.

There is no visibility to the Protoflight app of this low level hardware, all Protoflight
sees is the higher level objects, ie `Imu`, `MotorDriver` etc.

The low level hardware is abstracted away in the BSP, partly by Protoflight and partly by the `embassy` framework.

## STM32 hardware allocation

| Hardware | Assignment                               |
| -------- | ---------------------------------------- |
| SPI1     | IMU (Acc/Gyro)                           |
| SPI2     | Blackbox                                 |
| SPI3     | Analog OSD (eg MAX7456)                  |
| I2C1     | Barometer / Magnetometer                 |
| VCP      | MSP configuration                        |
| UART1    | Digital VTX / OSD                        |
| UART2    | Serial Radio Receiver (ELRS / Crossfire) |
| UART3    | GPS Module                               |
| UART4/6  | ESC Telemetry                            |

## Raspberry Pi Pico hardware allocation

```text
            ┌────────────────────┐
            │ RP Microcontroller │
            └────────────────────┘
                    │              
        ┌───────────┴─────────────────┐─────────────────────────┐─────────────────────┐
        ▼                             ▼                         ▼                     ▼
 ┌───────────────┐              ┌─────────────┐           ┌─────────────┐       ┌─────────────┐
 │ Hardware Core │              │ PIO 0 Block │           │ PIO 1 Block │       │ PIO 2 Block │
 └───────────────┘              └─────────────┘           └─────────────┘       └─────────────┘
    │                              │                         │                     │
    ├── USB VCP (Configurator)     ├── SM 0 (GPS TX)         ├── SM 0 (Motor 1)    ├── SM 0 (Motor 5)
    ├── UART1 (Radio)              ├── SM 1 (GPS RX)         ├── SM 1 (Motor 2)    ├── SM 1 (Motor 6)
    ├── UART0 (VTX/Digital OSD)    ├── SM 2 (SPI Analog OSD) ├── SM 2 (Motor 3)    ├── SM 2 (Motor 7)
    ├── SPI0 (IMU)                └── SM 3 (Spare)          └── SM 3 (Motor 4)    └── SM 3 (Motor 8)
    ├── SPI1 (Blackbox)
    └── I2C0 (Baro/Mag)
```

--- Device 1: Hardware SPI0 (IMU) ---.

Tied to SPI0 running asynchronously via the DMA system.

--- Device 2: Hardware SPI1 (Blackbox SD Card) ---.

Tied to SPI1 running asynchronously via the DMA system.

--- Device 3: PIO0 Backed SPI (Auxiliary Peripheral - MAX7456) ---.

Fully concrete representation using State Machine 0 on the PIO0 block.