AP_RCProtocol: require 3 good frames for weak CRC protocols

libraries/AP_RCProtocol/AP_RCProtocol.cpp

@@ -64,6 +64,10 @@ void AP_RCProtocol::process_pulse(uint32_t width_s0, uint32_t width_s1)
         if (backend[i] != nullptr) {
             backend[i]->process_pulse(width_s0, width_s1);
             if (backend[i]->new_input()) {
+                _good_frames[i]++;
+                if (requires_3_frames((rcprotocol_t)i) && _good_frames[i] < 3) {
+                    continue;
+                }
                 _new_input = true;
                 _detected_protocol = (enum AP_RCProtocol::rcprotocol_t)i;
                 _last_input_ms = now;
@@ -95,6 +99,10 @@ void AP_RCProtocol::process_byte(uint8_t byte, uint32_t baudrate)
         if (backend[i] != nullptr) {
             backend[i]->process_byte(byte, baudrate);
             if (backend[i]->new_input()) {
+                _good_frames[i]++;
+                if (requires_3_frames((rcprotocol_t)i) && _good_frames[i] < 3) {
+                    continue;
+                }
                 _new_input = true;
                 _detected_protocol = (enum AP_RCProtocol::rcprotocol_t)i;
                 _last_input_ms = now;

libraries/AP_RCProtocol/AP_RCProtocol.h

@@ -50,6 +50,11 @@ public:
         _disabled_for_pulses |= (1U<<(uint8_t)protocol);
     }
 
+    // for protocols without strong CRCs we require 3 good frames to lock on
+    bool requires_3_frames(enum rcprotocol_t p) {
+        return (p == DSM || p == SBUS || p == SBUS_NI || p == PPM);
+    }
+    
     enum rcprotocol_t protocol_detected()
     {
         return _detected_protocol;
@@ -72,6 +77,7 @@ private:
     bool _new_input = false;
     uint32_t _last_input_ms;
     bool _valid_serial_prot = false;
+    uint8_t _good_frames[NONE];
 
     static AP_RCProtocol *instance;
 };