From fc088ae764aebb3535da3651b3ccd24504dfa188 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Pawe=C5=82=20Spychalski?= <pspychalski@gmail.com>
Date: Fri, 22 Sep 2017 22:36:24 +0200
Subject: [PATCH] Update README.md

---
 README.md | 38 ++++++++++++++++++++++++++++++++++++++
 1 file changed, 38 insertions(+)

diff --git a/README.md b/README.md
index 074ba16..0535a14 100644
--- a/README.md
+++ b/README.md
@@ -1,2 +1,40 @@
 # QuadMeUp_Crossbow
 Cheap DIY RC link based on LoRa 868MHz modules
+
+# Protocol
+
+| Byte | Description | Notes |
+| ---- | ---- | ---- |
+| 0    | Preamble | "Q" 0x51 |
+| 1    | Frame type & Payload Length | bits 7-5 defines frame type, bits 4-0 payload length |
+| 2    | Packet ID | |
+| 4 - 36 | Payload | 32 bytes max |
+| payload length + 3 | CRC | XOR of all previous bytes |
+
+## Frame types
+
+| Value | Description | Direction |
+| ----  | ----        | ---- |
+| 000   | RC channels data `RC_DATA` | TX -> RX |
+| 001   | Receiver health and basic telemetry `RX_HEALTH` | RX -> TX |
+| 010   | Request receiver configuration | TX -> RX |
+| 011   | Receiver configuration | RX -> TX |
+| 100   | Set receiver configuration | TX -> RX |
+
+### `RC_DATA` frame format
+
+Protocol allows to send 11 RC channels in total encoded as following
+
+* channels 1 to 6 encoded using 10 bits each
+* channels 7 to 11 encoded using 4 bits per channel
+
+Total length of `RC_DATA` payload is 10 bytes
+
+### `RX_HEALTH` frame format
+
+| Byte | Description |
+| ---- | ---- |
+| 0    | RX RSSI |
+| 1    | RX supply volatage, sent in 0,1V |
+| 2    | RX analog input 1 sent in 0,1V |
+| 3    | RX analog input 2 sent in 0,1V |