✨ Independent baud rates (#21949)

Co-authored-by: Scott Lahteine <thinkyhead@users.noreply.github.com>

Marlin/Configuration.h

 #define SERIAL_PORT 0
 
 /**
+ * Serial Port Baud Rate
+ * This is the default communication speed for all serial ports.
+ * Set the baud rate defaults for additional serial ports below.
+ *
+ * 250000 works in most cases, but you might try a lower speed if
+ * you commonly experience drop-outs during host printing.
+ * You may try up to 1000000 to speed up SD file transfer.
+ *
+ * :[2400, 9600, 19200, 38400, 57600, 115200, 250000, 500000, 1000000]
+ */
+#define BAUDRATE 250000
+//#define BAUD_RATE_GCODE     // Enable G-code M575 to set the baud rate
+
+/**
  * Select a secondary serial port on the board to use for communication with the host.
  * Currently Ethernet (-2) is only supported on Teensy 4.1 boards.
  * :[-2, -1, 0, 1, 2, 3, 4, 5, 6, 7]
  */
 //#define SERIAL_PORT_2 -1
+//#define BAUDRATE_2 250000   // Enable to override BAUDRATE
 
 /**
  * Select a third serial port on the board to use for communication with the host.
  * :[-1, 0, 1, 2, 3, 4, 5, 6, 7]
  */
 //#define SERIAL_PORT_3 1
-
-/**
- * This setting determines the communication speed of the printer.
- *
- * 250000 works in most cases, but you might try a lower speed if
- * you commonly experience drop-outs during host printing.
- * You may try up to 1000000 to speed up SD file transfer.
- *
- * :[2400, 9600, 19200, 38400, 57600, 115200, 250000, 500000, 1000000]
- */
-#define BAUDRATE 250000
+//#define BAUDRATE_3 250000   // Enable to override BAUDRATE
 
 // Enable the Bluetooth serial interface on AT90USB devices
 //#define BLUETOOTH

Marlin/Configuration_adv.h

   //#define SERIAL_XON_XOFF
 #endif
 
-// Add M575 G-code to change the baud rate
-//#define BAUD_RATE_GCODE
-
 #if ENABLED(SDSUPPORT)
   // Enable this option to collect and display the maximum
   // RX queue usage after transferring a file to SD.

Marlin/src/MarlinCore.cpp

   while (!MYSERIAL1.connected() && PENDING(millis(), serial_connect_timeout)) { /*nada*/ }
 
   #if HAS_MULTI_SERIAL && !HAS_ETHERNET
-    MYSERIAL2.begin(BAUDRATE);
+    #ifndef BAUDRATE_2
+      #define BAUDRATE_2 BAUDRATE
+    #endif
+    MYSERIAL2.begin(BAUDRATE_2);
     serial_connect_timeout = millis() + 1000UL;
     while (!MYSERIAL2.connected() && PENDING(millis(), serial_connect_timeout)) { /*nada*/ }
     #ifdef SERIAL_PORT_3
-      MYSERIAL3.begin(BAUDRATE);
+      #ifndef BAUDRATE_3
+        #define BAUDRATE_3 BAUDRATE
+      #endif
+      MYSERIAL3.begin(BAUDRATE_3);
       serial_connect_timeout = millis() + 1000UL;
       while (!MYSERIAL3.connected() && PENDING(millis(), serial_connect_timeout)) { /*nada*/ }
     #endif

buildroot/tests/STM32F103RC_btt_USB

 # Build with the default configurations
 #
 restore_configs
-opt_set MOTHERBOARD BOARD_BTT_SKR_MINI_V1_1 SERIAL_PORT 1 SERIAL_PORT_2 -1
+opt_set MOTHERBOARD BOARD_BTT_SKR_MINI_V1_1 SERIAL_PORT 1 SERIAL_PORT_2 -1 BAUDRATE_2 115200
 exec_test $1 $2 "BigTreeTech SKR Mini v1.1 - Basic Configuration" "$3"
 
 # clean up