AP_RCProtocol: reset UART on RX failure for CRSF

update CRSF timestamps to use microseconds
implement RX liveness protocol and implement for CRSF

libraries/AP_RCProtocol/AP_RCProtocol.cpp

@@ -309,6 +309,13 @@ void AP_RCProtocol::check_added_uart(void)
             }
             added.opened = false;
         }
+    // power loss on CRSF requires re-bootstrap because the baudrate is reset to the default. The CRSF side will
+    // drop back down to 416k if it has received 200 incorrect characters (or none at all)
+    } else if (_detected_protocol != AP_RCProtocol::NONE
+        // protocols that want to be able to renegotiate should return false in is_rx_active()
+        && !backend[_detected_protocol]->is_rx_active()
+        && now - added.last_config_change_ms > 1000) {
+        added.opened = false;
     }
 }
 

libraries/AP_RCProtocol/AP_RCProtocol_Backend.h

@@ -85,6 +85,11 @@ public:
     bool have_UART(void) const {
         return frontend.added.uart != nullptr;
     }
+
+    // is the receiver active, used to detect power loss and baudrate changes
+    virtual bool is_rx_active() const {
+        return true;
+    }
     
 protected:
     struct Channels11Bit_8Chan {

libraries/AP_RCProtocol/AP_RCProtocol_CRSF.cpp

@@ -198,8 +198,6 @@ void AP_RCProtocol_CRSF::_process_byte(uint32_t timestamp_us, uint8_t byte)
         _frame_ofs = 0;
     }
 
-    _last_rx_time_us = timestamp_us;
-
     // overflow check
     if (_frame_ofs >= CRSF_FRAMELEN_MAX) {
         _frame_ofs = 0;
@@ -250,9 +248,10 @@ void AP_RCProtocol_CRSF::_process_byte(uint32_t timestamp_us, uint8_t byte)
             return;
         }
 
-        _last_frame_time_us = timestamp_us;
+        _last_frame_time_us = _last_rx_frame_time_us = timestamp_us;
         // decode here
         if (decode_crsf_packet()) {
+            _last_tx_frame_time_us = timestamp_us;  // we have received a frame from the transmitter
             add_input(MAX_CHANNELS, _channels, false, _link_status.rssi, _link_status.link_quality);
         }
     }
@@ -273,14 +272,15 @@ void AP_RCProtocol_CRSF::update(void)
         for (uint8_t i = 0; i < n; i++) {
             int16_t b = _uart->read();
             if (b >= 0) {
-                _process_byte(now, uint8_t(b));
+                process_byte(AP_HAL::micros(), uint8_t(b));
             }
         }
     }
 
     // never received RC frames, but have received CRSF frames so make sure we give the telemetry opportunity to run
     uint32_t now = AP_HAL::micros();
-    if (_last_frame_time_us > 0 && !get_rc_frame_count() && now - _last_frame_time_us > CRSF_INTER_FRAME_TIME_US_250HZ) {
+    if (_last_frame_time_us > 0 && (!get_rc_frame_count() || !is_tx_active())
+        && now - _last_frame_time_us > CRSF_INTER_FRAME_TIME_US_250HZ) {
         process_telemetry(false);
         _last_frame_time_us = now;
     }
@@ -471,7 +471,7 @@ bool AP_RCProtocol_CRSF::process_telemetry(bool check_constraint)
 
     if (!telem_available) {
 #if HAL_CRSF_TELEM_ENABLED && !APM_BUILD_TYPE(APM_BUILD_iofirmware)
-        if (AP_CRSF_Telem::get_telem_data(&_telemetry_frame)) {
+        if (AP_CRSF_Telem::get_telem_data(&_telemetry_frame, is_tx_active())) {
             telem_available = true;
         } else {
             return false;

libraries/AP_RCProtocol/AP_RCProtocol_CRSF.h

@@ -26,7 +26,9 @@
 #define CRSF_FRAMELEN_MAX   64U      // maximum possible framelength
 #define CSRF_HEADER_LEN     2U       // header length
 #define CRSF_FRAME_PAYLOAD_MAX (CRSF_FRAMELEN_MAX - CSRF_HEADER_LEN)     // maximum size of the frame length field in a packet
-#define CRSF_BAUDRATE       416666
+#define CRSF_BAUDRATE      416666U
+#define CRSF_TX_TIMEOUT    500000U   // the period after which the transmitter is considered disconnected (matches copters failsafe)
+#define CRSF_RX_TIMEOUT    150000U   // the period after which the receiver is considered disconnected (>ping frequency)
 
 class AP_RCProtocol_CRSF : public AP_RCProtocol_Backend {
 public:
@@ -38,6 +40,20 @@ public:
     // support for CRSF v3
     bool change_baud_rate(uint32_t baudrate);
     bool is_crsf_v3_active() const { return _crsf_v3_active; }
+
+    // is the receiver active, used to detect power loss and baudrate changes
+    bool is_rx_active() const override {
+        // later versions of CRSFv3 will send link rate frames every 200ms
+        // but only before an initial failsafe
+        return AP_HAL::micros() < _last_rx_frame_time_us + CRSF_RX_TIMEOUT;
+    }
+
+    // is the transmitter active, used to adjust telemetry data
+    bool is_tx_active() const {
+        // this is the same as the Copter failsafe timeout
+        return AP_HAL::micros() < _last_tx_frame_time_us + CRSF_TX_TIMEOUT;
+    }
+
     // get singleton instance
     static AP_RCProtocol_CRSF* get_singleton() {
         return _singleton;
@@ -275,8 +291,9 @@ private:
     void add_to_buffer(uint8_t index, uint8_t b) { ((uint8_t*)&_frame)[index] = b; }
 
     uint32_t _last_frame_time_us;
+    uint32_t _last_tx_frame_time_us;
     uint32_t _last_uart_start_time_ms;
-    uint32_t _last_rx_time_us;
+    uint32_t _last_rx_frame_time_us;
     uint32_t _start_frame_time_us;
     bool telem_available;
     uint32_t _new_baud_rate;