stmhal: Improvements to the STM32F4DISC UART config settings.

stmhal/boards/STM32F4DISC/mpconfigboard.h

@@ -23,18 +23,35 @@
 #define MICROPY_HW_CLK_PLLQ (7)
 
 // UART config
+#if 0
+// A9 is used for USB VBUS detect, and A10 is used for USB_FS_ID.
+// UART1 is also on PB6/7 but PB6 is tied to the Audio SCL line.
+// Without board modifications, this makes UART1 unusable on this board.
 #define MICROPY_HW_UART1_PORT (GPIOA)
 #define MICROPY_HW_UART1_PINS (GPIO_PIN_9 | GPIO_PIN_10)
+#endif
+
 #define MICROPY_HW_UART2_PORT (GPIOA)
 #define MICROPY_HW_UART2_PINS (GPIO_PIN_2 | GPIO_PIN_3)
 #define MICROPY_HW_UART2_RTS  (GPIO_PIN_1)
 #define MICROPY_HW_UART2_CTS  (GPIO_PIN_0)
+
 #define MICROPY_HW_UART3_PORT (GPIOD)
 #define MICROPY_HW_UART3_PINS (GPIO_PIN_8 | GPIO_PIN_9)
 #define MICROPY_HW_UART3_RTS  (GPIO_PIN_12)
 #define MICROPY_HW_UART3_CTS  (GPIO_PIN_11)
+
+#if MICROPY_HW_HAS_SWITCH == 0
+// NOTE: A0 also connects to the user switch. To use UART4 you should 
+//       set MICROPY_HW_HAS_SWITCH to 0, and also remove SB20 (on the back
+//       of the board near the USER switch).
 #define MICROPY_HW_UART4_PORT (GPIOA)
 #define MICROPY_HW_UART4_PINS (GPIO_PIN_0 | GPIO_PIN_1)
+#endif
+
+// NOTE: PC7 is connected to MCLK on the Audio chip. This is an input signal
+//       so I think as long as you're not using the audio chip then it should
+//       be fine to use as a UART pin.
 #define MICROPY_HW_UART6_PORT (GPIOC)
 #define MICROPY_HW_UART6_PINS (GPIO_PIN_6 | GPIO_PIN_7)