AP_HAL: UARTDriver: Add new flow control option "FLOW_CONTROL_RTS_DE" for RS485 driver enable.

libraries/AP_HAL/UARTDriver.cpp

@@ -165,6 +165,20 @@ uint64_t AP_HAL::UARTDriver::receive_time_constraint_us(uint16_t nbytes)
     return AP_HAL::micros64();
 }
 
+// Helper to check if flow control is enabled given the passed setting
+bool AP_HAL::UARTDriver::flow_control_enabled(enum flow_control flow_control_setting) const
+{
+    switch(flow_control_setting) {
+        case FLOW_CONTROL_ENABLE:
+        case FLOW_CONTROL_AUTO:
+            return true;
+        case FLOW_CONTROL_DISABLE:
+        case FLOW_CONTROL_RTS_DE:
+            break;
+    }
+    return false;
+}
+
 #if HAL_UART_STATS_ENABLED
 // Take cumulative bytes and return the change since last call
 uint32_t AP_HAL::UARTDriver::StatsTracker::ByteTracker::update(uint32_t bytes)

libraries/AP_HAL/UARTDriver.h

@@ -103,10 +103,14 @@ public:
         FLOW_CONTROL_DISABLE=0,
         FLOW_CONTROL_ENABLE=1,
         FLOW_CONTROL_AUTO=2,
+        FLOW_CONTROL_RTS_DE=3, // RTS pin is used as a driver enable (used in RS-485)
     };
     virtual void set_flow_control(enum flow_control flow_control_setting) {};
     virtual enum flow_control get_flow_control(void) { return FLOW_CONTROL_DISABLE; }
 
+    // Return true if flow control is currently enabled
+    bool flow_control_enabled() { return flow_control_enabled(get_flow_control()); }
+
     virtual void configure_parity(uint8_t v){};
     virtual void set_stop_bits(int n){};
 
@@ -240,6 +244,9 @@ protected:
     // discard incoming data on the port
     virtual bool _discard_input(void) = 0;
 
+    // Helper to check if flow control is enabled given the passed setting
+    bool flow_control_enabled(enum flow_control flow_control_setting) const;
+
 #if HAL_UART_STATS_ENABLED
     // Getters for cumulative tx and rx counts
     virtual uint32_t get_total_tx_bytes() const { return 0; }