Function added to detect baudrate

cores/esp8266/HardwareSerial.cpp

@@ -108,6 +108,30 @@ void HardwareSerial::flush()
     delayMicroseconds(11000000 / uart_get_baudrate(_uart) + 1);
 }
 
+void HardwareSerial::startDetectBaudrate()
+{
+    uart_start_detect_baudrate(_uart_nr);
+}
+
+unsigned long HardwareSerial::testBaudrate()
+{
+    return uart_detect_baudrate(_uart_nr);
+}
+
+unsigned long HardwareSerial::detectBaudrate(time_t timeoutMillis)
+{
+    time_t startMillis = millis();
+    unsigned long detectedBaudrate;
+    while ((time_t) millis() - startMillis < timeoutMillis) {
+        if ((detectedBaudrate = testBaudrate())) {
+          break;
+        }
+        yield();
+        delay(100);
+    }    
+    return detectedBaudrate;
+}
+
 #if !defined(NO_GLOBAL_INSTANCES) && !defined(NO_GLOBAL_SERIAL)
 HardwareSerial Serial(UART0);
 #endif

cores/esp8266/HardwareSerial.h

@@ -184,6 +184,12 @@ class HardwareSerial: public Stream
         return uart_has_overrun(_uart);
     }
 
+    void startDetectBaudrate();
+
+    unsigned long testBaudrate();
+
+    unsigned long detectBaudrate(time_t timeoutMillis);
+
 protected:
     int _uart_nr;
     uart_t* _uart = nullptr;

cores/esp8266/uart.c

@@ -44,6 +44,7 @@
 #include "uart.h"
 #include "esp8266_peri.h"
 #include "user_interface.h"
+#include "uart_register.h"
 
 const char overrun_str [] ICACHE_RODATA_ATTR STORE_ATTR = "uart input full!\r\n";
 static int s_uart_debug_nr = UART0;
@@ -739,3 +740,49 @@ uart_get_debug()
 {
     return s_uart_debug_nr;
 }
+
+/*
+To start detection of baud rate with the UART the UART_AUTOBAUD_EN bit needs to be cleared and set. The ROM function uart_baudrate_detect() does this only once, so on a next call the UartDev.rcv_state is not equal to BAUD_RATE_DET. Instead of poking around in the UartDev struct with unknown effect, the UART_AUTOBAUD_EN bit is directly triggered by the function uart_detect_baudrate().
+*/
+void
+uart_start_detect_baudrate(int uart_nr)
+{
+    USA(uart_nr) &= ~(UART_GLITCH_FILT << UART_GLITCH_FILT_S | UART_AUTOBAUD_EN);
+    USA(uart_nr) = 0x08 << UART_GLITCH_FILT_S | UART_AUTOBAUD_EN;
+}
+
+int
+uart_detect_baudrate(int uart_nr)
+{
+    static bool doTrigger = true;
+
+    if (doTrigger)
+    {
+        uart_start_detect_baudrate(uart_nr);
+        doTrigger = false;
+    }
+
+    int32_t divisor = uart_baudrate_detect(uart_nr, 1);
+    if (!divisor) {
+        return 0;
+    }
+
+    doTrigger = true;    // Initialize for a next round
+    int32_t baudrate = UART_CLK_FREQ / divisor;
+
+    static const int default_rates[] = {300, 600, 1200, 2400, 4800, 9600, 19200, 38400, 57600, 74880, 115200, 230400, 256000, 460800, 921600, 1843200, 3686400};
+
+    size_t i;
+    for (i = 1; i < sizeof(default_rates) / sizeof(default_rates[0]) - 1; i++)	// find the nearest real baudrate
+    {
+        if (baudrate <= default_rates[i])
+        {
+            if (baudrate - default_rates[i - 1] < default_rates[i] - baudrate) {
+                i--;
+            }
+            break;
+        }
+    }
+
+    return default_rates[i];
+}

cores/esp8266/uart.h

@@ -141,6 +141,9 @@ bool uart_has_overrun (uart_t* uart); // returns then clear overrun flag
 void uart_set_debug(int uart_nr);
 int uart_get_debug();
 
+void uart_start_detect_baudrate(int uart_nr);
+int uart_detect_baudrate(int uart_nr);
+
 
 #if defined (__cplusplus)
 } // extern "C"

doc/reference.rst

@@ -155,6 +155,17 @@ current speed. For example
 | Note that this implementation is **only for ESP8266 based boards**,
   and will not works with other Arduino boards.
 
+
+To detect an unknown baudrate of data coming into Serial use ``Serial.detectBaudrate(time_t timeoutMillis)``. This method tries to detect the baudrate for a maximum of timeoutMillis ms. It returns zero if no baudrate was detected, or the detected baudrate otherwise. The ``detectBaudrate()`` function may be called before ``Serial.begin()`` is called, because it does not need the receive buffer nor the SerialConfig parameters.
+
+The uart can not detect other parameters like number of start- or stopbits, number of data bits or parity.
+
+The detection itself does not change the baudrate, after detection it should be set as usual using ``Serial.begin(detectedBaudrate)``.
+
+Detection is very fast, it takes only a few incoming bytes.
+
+SerialDetectBaudrate.ino is a full example of usage.
+
 Progmem
 -------
 

libraries/esp8266/examples/SerialDetectBaudrate/SerialDetectBaudrate.ino

@@ -0,0 +1,34 @@
+#define TIMEOUT (10000UL)     // Maximum time to wait for serial activity to start
+
+void setup() {
+  // put your setup code here, to run once:
+
+  Serial.begin(115200);
+
+  // Serial.detectBaudrate() may also be called before Serial.begin()
+  // There must be activity on the serial port for the baudrate to be detected
+  unsigned long detectedBaudrate = Serial.detectBaudrate(TIMEOUT);
+
+  if (detectedBaudrate) {
+    Serial.printf("\nDetected baudrate is %lu, switching to that baudrate now...\n", detectedBaudrate);
+
+    // Wait for printf to finish
+    while (Serial.availableForWrite() != UART_TX_FIFO_SIZE) {
+      yield();
+    }
+
+    // Clear Tx buffer to avoid extra characters being printed
+    Serial.flush();
+
+    // After this, any writing to Serial will print gibberish on the serial monitor if the baudrate doesn't match
+    Serial.begin(detectedBaudrate);
+  } else {
+    Serial.println("\nNothing detected");
+  }
+}
+
+void loop() {
+  // put your main code here, to run repeatedly:
+
+}
+