Bind key used to init frame CRC

README.md

@@ -4,16 +4,31 @@ _QuadMeUp Crossbow_ is a DIY project that gives 5km (at least) of RC link for UA
 
 # Current state
 
-Development, ready for testing
+Works:
+* Getting data from OpenTX radio using SBUS protocol
+* Transmitting 10 channels to RX modules
+* Frequency hopping
+* Getting basic telemetry from RX module
+* Sending 10 channels using SBUS to flight controller
+
+Needs implementation:
+* Binding
+* TX module configuration
+* Allowing to use OLED on TX to get basic data
+* RX configuration from TX module
+* Sending telemetry from TX to OpenTX radio
 
 # Protocol
 
 | Byte                  | Description | Notes |
 | ----                  | ----        | ---- |
-| 1                     | Channel ID | channel used for comunication between TX and RX |
+| 1                     | Frame type & used radio channel | bits 7-5 defines frame, bits 4-0 current radio channel |
-| 2                     | Frame type & Length | bits 7-5 defines frame, bits 4-0 current radio channel |
+| 2 - 33                | Payload | lenghth defined by frame type |
-| 3 - 34                | Payload | lenghth defined by frame type |
+| payload length + 2    | CRC | using crc8_dvb_s2 method |
-| payload length + 3    | CRC | using crc8_dvb_s2 method |
+
+## CRC
+
+CRC is computed using `crc8_dvb_s2` method. Initial CRC value for each frame CRC is equal to CRC of 4 bind bytes (unique for transmitter module). 
 
 ## Frame types
 

crossbow/config.h

@@ -15,8 +15,8 @@
  * DEVICE_MODE_TX
  * DEVICE_MODE_RX
  */
-#define DEVICE_MODE_TX
+// #define DEVICE_MODE_TX
-// #define DEVICE_MODE_RX
+#define DEVICE_MODE_RX
 
 // #define FEATURE_TX_OLED
 // #define FORCE_TX_WITHOUT_INPUT

crossbow/qsp.cpp

@@ -151,6 +151,16 @@ void qspClearPayload(QspConfiguration_t *qsp)
     qsp->payloadLength = 0;
 }
 
+/**
+ * Init CRC with salt based on 4 byte bind key
+ */
+void qspInitCrc(QspConfiguration_t *qsp) {
+    qsp->crc = 0;
+    for (uint8_t i = 0; i < 4; i++) {
+        qspComputeCrc(qsp, qsp->bindKey);
+    }
+}
+
 void qspDecodeIncomingFrame(
     QspConfiguration_t *qsp, 
     uint8_t incomingByte, 
@@ -165,24 +175,11 @@ void qspDecodeIncomingFrame(
 
     if (qsp->protocolState == QSP_STATE_IDLE)
     {
-        // Check if incomming channel ID is the same as receiver
+        qspInitCrc(qsp);
-        if (incomingByte == CHANNEL_ID)
+        qspClearPayload(qsp);
-        {
+        receivedPayload = 0;
-            qsp->frameDecodingStartedAt = millis();
+        qsp->frameDecodingStartedAt = millis();
-            qsp->protocolState = QSP_STATE_CHANNEL_RECEIVED;
-            qsp->crc = 0;
-            qspComputeCrc(qsp, incomingByte);
-            qspClearPayload(qsp);
 
-            receivedPayload = 0;
-        }
-        else
-        {
-            qsp->protocolState = QSP_STATE_IDLE;
-        }
-    }
-    else if (qsp->protocolState == QSP_STATE_CHANNEL_RECEIVED)
-    {
         //Frame ID and payload length
         qspComputeCrc(qsp, incomingByte);
 
@@ -228,11 +225,8 @@ void qspDecodeIncomingFrame(
  * Encode frame is corrent format and write to hardware
  */
 void qspEncodeFrame(QspConfiguration_t *qsp, volatile RadioState_t *radioState, uint8_t buffer[], uint8_t *size) {
-    //Zero CRC
+    //Salt CRC with bind key
-    qsp->crc = 0;
+    qspInitCrc(qsp);
-
-    qspComputeCrc(qsp, CHANNEL_ID);
-    buffer[0] = CHANNEL_ID;
 
     //Write frame type and length
     // We are no longer sending payload length, so 4 bits are now free for other usages
@@ -240,16 +234,16 @@ void qspEncodeFrame(QspConfiguration_t *qsp, volatile RadioState_t *radioState,
     uint8_t data = radioState->channel;
     data |= (qsp->frameToSend << 4) & 0xf0;
     qspComputeCrc(qsp, data);
-    buffer[1] = data;
+    buffer[0] = data;
 
     for (uint8_t i = 0; i < qsp->payloadLength; i++)
     {
         qspComputeCrc(qsp, qsp->payload[i]);
-        buffer[i + 2] = qsp->payload[i];
+        buffer[i + 1] = qsp->payload[i];
     }
 
-    buffer[qsp->payloadLength + 2] = qsp->crc;
+    buffer[qsp->payloadLength + 1] = qsp->crc;
-    *size = qsp->payloadLength + 3; //Total length of QSP frame
+    *size = qsp->payloadLength + 2; //Total length of QSP frame
 }
 
 void encodePingPayload(QspConfiguration_t *qsp, uint32_t currentMicros) {

crossbow/variables.h

@@ -48,7 +48,6 @@ static const uint8_t qspFrameLengths[QSP_FRAME_COUNT] = {
 
 enum dataStates {
     QSP_STATE_IDLE,
-    QSP_STATE_CHANNEL_RECEIVED,
     QSP_STATE_FRAME_TYPE_RECEIVED,
     QSP_STATE_PAYLOAD_RECEIVED,
     QSP_STATE_CRC_RECEIVED