AP_RCProtocol: process CRSF crc per-byte

libraries/AP_RCProtocol/AP_RCProtocol_CRSF.cpp

@@ -135,7 +135,8 @@ static const char* get_frame_type(uint8_t byte)
 #define CRSF_INTER_FRAME_TIME_US_250HZ    4000U // At fastest, frames are sent by the transmitter every 4 ms, 250 Hz
 #define CRSF_INTER_FRAME_TIME_US_150HZ    6667U // At medium, frames are sent by the transmitter every 6.667 ms, 150 Hz
 #define CRSF_INTER_FRAME_TIME_US_50HZ    20000U // At slowest, frames are sent by the transmitter every 20ms, 50 Hz
-#define CSRF_HEADER_LEN     2
+#define CSRF_HEADER_LEN     2                   // header length
+#define CSRF_HEADER_TYPE_LEN     (CSRF_HEADER_LEN + 1)           // header length including type
 
 #define CRSF_DIGITAL_CHANNEL_MIN 172
 #define CRSF_DIGITAL_CHANNEL_MAX 1811
@@ -200,12 +201,13 @@ void AP_RCProtocol_CRSF::_process_byte(uint32_t timestamp_us, uint8_t byte)
     add_to_buffer(_frame_ofs++, byte);
 
     // need a header to get the length
-    if (_frame_ofs < CSRF_HEADER_LEN) {
+    if (_frame_ofs < CSRF_HEADER_TYPE_LEN) {
         return;
     }
 
     // parse the length
-    if (_frame_ofs == CSRF_HEADER_LEN) {
+    if (_frame_ofs == CSRF_HEADER_TYPE_LEN) {
+        _frame_crc = crc8_dvb_s2(0, _frame.type);
         // check for garbage frame
         if (_frame.length > CRSF_FRAMELEN_MAX) {
             _frame_ofs = 0;
@@ -213,6 +215,11 @@ void AP_RCProtocol_CRSF::_process_byte(uint32_t timestamp_us, uint8_t byte)
         return;
     }
 
+    // update crc
+    if (_frame_ofs < _frame.length + CSRF_HEADER_LEN) {
+        _frame_crc = crc8_dvb_s2(_frame_crc, byte);
+    }
+
     // overflow check
     if (_frame_ofs > _frame.length + CSRF_HEADER_LEN) {
         _frame_ofs = 0;
@@ -226,13 +233,8 @@ void AP_RCProtocol_CRSF::_process_byte(uint32_t timestamp_us, uint8_t byte)
         // we consumed the partial frame, reset
         _frame_ofs = 0;
 
-        uint8_t crc = crc8_dvb_s2(0, _frame.type);
-        for (uint8_t i = 0; i < _frame.length - 2; i++) {
-            crc = crc8_dvb_s2(crc, _frame.payload[i]);
-        }
-
         // bad CRC
-        if (crc != _frame.payload[_frame.length - CSRF_HEADER_LEN]) {
+        if (_frame_crc != _frame.payload[_frame.length - CSRF_HEADER_LEN]) {
             return;
         }
 

libraries/AP_RCProtocol/AP_RCProtocol_CRSF.h

@@ -239,6 +239,7 @@ private:
     struct Frame _frame;
     struct Frame _telemetry_frame;
     uint8_t _frame_ofs;
+    uint8_t _frame_crc;
 
     const uint8_t MAX_CHANNELS = MIN((uint8_t)CRSF_MAX_CHANNELS, (uint8_t)MAX_RCIN_CHANNELS);