Improve frame error detection

Count frame error if frame wasn't completed within the expected time or there was a crc error

src/main/rx/crsf.c

@@ -58,7 +58,7 @@
 
 #define CRSF_LINK_STATUS_UPDATE_TIMEOUT_US  250000 // 250ms, 4 Hz mode 1 telemetry
 
-#define CRSF_FRAME_ERROR_COUNT_THRESHOLD    100
+#define CRSF_FRAME_ERROR_COUNT_THRESHOLD    3
 
 STATIC_UNIT_TESTED bool crsfFrameDone = false;
 STATIC_UNIT_TESTED crsfFrame_t crsfFrame;
@@ -362,6 +362,12 @@ STATIC_UNIT_TESTED void crsfDataReceive(uint16_t c, void *data)
     if (cmpTimeUs(currentTimeUs, crsfFrameStartAtUs) > CRSF_TIME_NEEDED_PER_FRAME_US) {
         // We've received a character after max time needed to complete a frame,
         // so this must be the start of a new frame.
+#if defined(USE_CRSF_V3)
+        if (crsfFramePosition > 0) {
+            // count an error if full valid frame not received within the allowed time.
+            crsfFrameErrorCnt++;
+        }
+#endif
         crsfFramePosition = 0;
     }
 
@@ -456,14 +462,12 @@ STATIC_UNIT_TESTED void crsfDataReceive(uint16_t c, void *data)
                     crsfFrameErrorCnt++;
 #endif
             }
-        } else {
-#if defined(USE_CRSF_V3)
-            if (crsfFrameErrorCnt < CRSF_FRAME_ERROR_COUNT_THRESHOLD)
-                crsfFrameErrorCnt++;
-#endif
         }
 #if defined(USE_CRSF_V3)
-        if (crsfFrameErrorCnt >= CRSF_FRAME_ERROR_COUNT_THRESHOLD) {
+        if (crsfBaudNegotiationInProgress()) {
+            // don't count errors when negotiation in progress.
+            crsfFrameErrorCnt = 0;
+        } else if (crsfFrameErrorCnt >= CRSF_FRAME_ERROR_COUNT_THRESHOLD) {
             // fall back to default speed if speed mismatch detected
             setCrsfDefaultSpeed();
             crsfFrameErrorCnt = 0;

src/main/telemetry/crsf.c

@@ -119,6 +119,11 @@ void setCrsfDefaultSpeed(void)
     isCrsfV3Running = false;
     crsfRxUpdateBaudrate(getCrsfDesiredSpeed());
 }
+
+bool crsfBaudNegotiationInProgress(void)
+{
+    return crsfSpeed.hasPendingReply || crsfSpeed.isNewSpeedValid;
+}
 #endif
 
 void initCrsfMspBuffer(void)

src/main/telemetry/crsf.h

@@ -48,4 +48,5 @@ int getCrsfMspFrame(uint8_t *frame, uint8_t *payload, const uint8_t payloadSize)
 #endif
 #if defined(USE_CRSF_V3)
 void speedNegotiationProcess(uint32_t currentTime);
+bool crsfBaudNegotiationInProgress(void);
 #endif