AP_RCProtocol: check for 3 good frames for CRSF

Move get_link_rate() and get_protocol_string() to CRSF protocol
allow ELRS at 420kbaud to be configured
allow CRSF to bootstrap at ELRS desired baudrate

libraries/AP_RCProtocol/AP_RCProtocol.h

@@ -101,12 +101,12 @@ public:
 #if AP_RCPROTOCOL_FPORT2_ENABLED
         case FPORT2:
 #endif
+        case CRSF:
             return true;
         case IBUS:
         case SUMD:
         case SRXL:
         case SRXL2:
-        case CRSF:
         case ST24:
         case NONE:
             return false;

libraries/AP_RCProtocol/AP_RCProtocol_CRSF.cpp

@@ -153,7 +153,11 @@ static const char* get_frame_type(uint8_t byte, uint8_t subtype = 0)
 #define CRSF_DIGITAL_CHANNEL_MAX 1811
 
 
-constexpr uint16_t AP_RCProtocol_CRSF::elrs_air_rates[8];
+const uint16_t AP_RCProtocol_CRSF::RF_MODE_RATES[RFMode::RF_MODE_MAX_MODES] = {
+    4, 50, 150, 250,    // CRSF
+    4, 25, 50, 100, 150, 200, 250, 500  // ELRS
+};
+
 AP_RCProtocol_CRSF* AP_RCProtocol_CRSF::_singleton;
 
 AP_RCProtocol_CRSF::AP_RCProtocol_CRSF(AP_RCProtocol &_frontend) : AP_RCProtocol_Backend(_frontend)
@@ -181,11 +185,23 @@ AP_RCProtocol_CRSF::~AP_RCProtocol_CRSF() {
     _singleton = nullptr;
 }
 
-void AP_RCProtocol_CRSF::process_pulse(uint32_t width_s0, uint32_t width_s1)
-{
-    uint8_t b;
-    if (ss.process_pulse(width_s0, width_s1, b)) {
-        _process_byte(ss.get_byte_timestamp_us(), b);
+// get the protocol string
+const char* AP_RCProtocol_CRSF::get_protocol_string(ProtocolType protocol) const {
+    if (protocol == ProtocolType::PROTOCOL_ELRS) {
+        return "ELRS";
+    } else if (_crsf_v3_active) {
+        return "CRSFv3";
+    } else {
+        return "CRSFv2";
+    }
+}
+
+// return the link rate as defined by the LinkStatistics
+uint16_t AP_RCProtocol_CRSF::get_link_rate(ProtocolType protocol) const {
+    if (protocol == ProtocolType::PROTOCOL_ELRS) {
+        return RF_MODE_RATES[_link_status.rf_mode + RFMode::ELRS_RF_MODE_4HZ];
+    } else {
+        return RF_MODE_RATES[_link_status.rf_mode];
     }
 }
 
@@ -386,7 +402,7 @@ bool AP_RCProtocol_CRSF::decode_crsf_packet()
             // now wait for 4ms to account for RX transmission and processing
             hal.scheduler->delay(4);
             // change the baud rate
-            uart->begin(_new_baud_rate, 128, 128);
+            uart->begin(_new_baud_rate);
         }
         _new_baud_rate = 0;
     }
@@ -559,7 +575,7 @@ void AP_RCProtocol_CRSF::start_uart()
     _uart->set_flow_control(AP_HAL::UARTDriver::FLOW_CONTROL_DISABLE);
     _uart->set_blocking_writes(false);
     _uart->set_options(_uart->get_options() & ~AP_HAL::UARTDriver::OPTION_RXINV);
-    _uart->begin(CRSF_BAUDRATE, 128, 128);
+    _uart->begin(get_bootstrap_baud_rate());
 }
 
 // change the baudrate of the protocol if we are able
@@ -570,7 +586,7 @@ bool AP_RCProtocol_CRSF::change_baud_rate(uint32_t baudrate)
         return false;
     }
 #if !defined(STM32H7)
-    if (baudrate > CRSF_BAUDRATE && !uart->is_dma_enabled()) {
+    if (baudrate > get_bootstrap_baud_rate() && !uart->is_dma_enabled()) {
         return false;
     }
 #endif
@@ -583,6 +599,23 @@ bool AP_RCProtocol_CRSF::change_baud_rate(uint32_t baudrate)
     return true;
 }
 
+// change the bootstrap baud rate to ELRS standard if configured
+void AP_RCProtocol_CRSF::process_handshake(uint32_t baudrate)
+{
+    AP_HAL::UARTDriver *uart = get_current_UART();
+
+    // only change the baudrate if we are bootstrapping CRSF
+    if (uart == nullptr
+        || baudrate != CRSF_BAUDRATE
+        || baudrate == get_bootstrap_baud_rate()
+        || uart->get_baud_rate() == get_bootstrap_baud_rate()
+        || (get_rc_protocols_mask() & ((1U<<(uint8_t(AP_RCProtocol::CRSF)+1))+1)) == 0) {
+        return;
+    }
+
+    uart->begin(get_bootstrap_baud_rate());
+}
+
 //returns uplink link quality on 0-255 scale
 int16_t AP_RCProtocol_CRSF::derive_scaled_lq_value(uint8_t uplink_lq)
 {

libraries/AP_RCProtocol/AP_RCProtocol_CRSF.h

@@ -20,6 +20,7 @@
 
 #include "AP_RCProtocol.h"
 #include <AP_Math/AP_Math.h>
+#include <RC_Channel/RC_Channel.h>
 #include "SoftSerial.h"
 
 #define CRSF_MAX_CHANNELS   24U      // Maximum number of channels from crsf datastream
@@ -27,6 +28,7 @@
 #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      416666U
+#define ELRS_BAUDRATE      420000U
 #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)
 
@@ -35,11 +37,14 @@ public:
     AP_RCProtocol_CRSF(AP_RCProtocol &_frontend);
     virtual ~AP_RCProtocol_CRSF();
     void process_byte(uint8_t byte, uint32_t baudrate) override;
-    void process_pulse(uint32_t width_s0, uint32_t width_s1) override;
+    void process_handshake(uint32_t baudrate) override;
     void update(void) override;
     // support for CRSF v3
     bool change_baud_rate(uint32_t baudrate);
-    bool is_crsf_v3_active() const { return _crsf_v3_active; }
+    // bootstrap baudrate
+    uint32_t get_bootstrap_baud_rate() const {
+        return rc().use_420kbaud_for_elrs() ? ELRS_BAUDRATE : CRSF_BAUDRATE;
+    }
 
     // is the receiver active, used to detect power loss and baudrate changes
     bool is_rx_active() const override {
@@ -233,7 +238,13 @@ public:
         // uint16_t digital_switch_channel[]:10; // digital switch channel
     } PACKED;
 
-    enum class RFMode : uint8_t {
+    enum class ProtocolType {
+        PROTOCOL_CRSF,
+        PROTOCOL_TRACER,
+        PROTOCOL_ELRS
+    };
+
+    enum RFMode {
         CRSF_RF_MODE_4HZ = 0,
         CRSF_RF_MODE_50HZ,
         CRSF_RF_MODE_150HZ,
@@ -246,10 +257,9 @@ public:
         ELRS_RF_MODE_200HZ,
         ELRS_RF_MODE_250HZ,
         ELRS_RF_MODE_500HZ,
+        RF_MODE_MAX_MODES,
         RF_MODE_UNKNOWN,
     };
-    // nominal ELRS air rates
-    static constexpr uint16_t elrs_air_rates[8] = {4, 25, 50, 100, 150, 200, 250, 500};
 
     struct LinkStatus {
         int16_t rssi = -1;
@@ -263,6 +273,12 @@ public:
         return _link_status;
     }
 
+    // return the link rate as defined by the LinkStatistics
+    uint16_t get_link_rate(ProtocolType protocol) const;
+
+    // return the protocol string
+    const char* get_protocol_string(ProtocolType protocol) const;
+
 private:
     struct Frame _frame;
     struct Frame _telemetry_frame;
@@ -304,9 +320,9 @@ private:
 
     volatile struct LinkStatus _link_status;
 
-    AP_HAL::UARTDriver *_uart;
+    static const uint16_t RF_MODE_RATES[RFMode::RF_MODE_MAX_MODES];
 
-    SoftSerial ss{CRSF_BAUDRATE, SoftSerial::SERIAL_CONFIG_8N1};
+    AP_HAL::UARTDriver *_uart;
 };
 
 namespace AP {