# QuadMeUp Crossbow

_QuadMeUp Crossbow_ is a DIY project that gives 5km (at least) of RC link for UAV (airplanes and drones) for a price below $40. I uses SX1278 (LoRa 868MHz/915MHz) compatible (like HopeRF RFM95W) radio modules connected to Arduino compatible boards. It can be regular Arduino connected via SPI to SX1278 or dedicated board like _Adafruit Feather 32u4 RFM LoRa_ or _LoRa32u4 II_

# Current state

Development, not yet functional

# Protocol

| Byte                  | Description | Notes |
| ----                  | ----        | ---- |
| 1                     | Preamble | "Q" 0x51 |
| 2                     | Channel ID | channel used for comunication between TX and RX |
| 3                     | Frame type & Length | bits 7-5 defines frame, bits 4-0 payload length |
| 4                     | Packet ID | |
| 5 - 36                | Payload | 32 bytes max |
| payload length + 5    | CRC | XOR of all previous bytes |

## Frame types

| Value  | Value hex    | Description                      | Direction  |
| ----   | ----         |----                              | ---- |
| 0000   | 0x0          | RC channels data `RC_DATA` | TX -> RX |
| 0001   | 0x1          | Receiver health and basic telemetry `RX_HEALTH` | RX -> TX |
| 0010   | 0x2          | Request receiver configuration | TX -> RX |
| 0011   | 0x3          | Receiver configuration | RX -> TX |
| 0100   | 0x4          | Set receiver configuration | TX -> RX |

### `RC_DATA` frame format

Protocol allows to send 10 RC channels in total encoded as following

* channels 1 to 4 encoded using 10 bits each (5 bytes)
* channels 5 to 6 encoded using 8 bits each (2 bytes)
* channels 7 to 10 encoded using 4 bits per channel (2 bytes)

Total length of `RC_DATA` payload is 9 bytes

### `RX_HEALTH` frame format

| Byte | Description                        |
| ---- | ----                               |
| 0    | RX RSSI                            |
| 1    | RX SNR                            |
| 2    | RX supply volatage, sent in 0,1V   |
| 3    | RX analog input 1 sent in 0,1V     |
| 4    | RX analog input 2 sent in 0,1V     |