Documentation: add documentation for STM32F401RC-RS485 board

Documentation/platforms/arm/stm32f4/boards/stm32f401rc-rs485/index.rst

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+=================
+stm32f401rc-rs485
+=================
+
+This page discusses issues unique to NuttX configurations for the
+NuttX STM32F4-RS485 development board.
+
+.. figure:: stm32f401rc_rs485.jpg
+   :align: center
+
+Board information
+=================
+
+This board was release on NuttX International Workshop 2023 and developed based on
+STM32F401RCT6 microcontroller.
+
+STM32F401RCT6 microcontroller features:
+ - Arm 32-bit Cortex®-M4 CPU with FPU
+ - 128 Kbytes of Flash memory
+ - 64 Kbytes of SRAM
+ - Serial wire debug (SWD) & JTAG interfaces
+ - Up to 81 I/O ports with interrupt capability
+ - Up to 11 communication interfaces
+ - Up to 3 I2C interfaces
+ - Up to 3 USARTs
+ - Up to 4 SPIs
+ - SDIO interface
+ - USB 2.0 full-speed device/host/OTG controller with on-chip PHY
+
+
+The board features:
+
+- Digital I2C Temperature Sensor (TMP75)
+- 2K bits (256x8) I2C EEPROM
+- On-board RS485 Transceiver
+- Two Analog Input Stages with Amplifier Buffer
+- Two Analog Output Stages with Amplifier Buffer
+- MicroSD Connector supporting 1 or 4-bit bus
+- Four User LEDs
+- Four User Buttons
+- USB for DFU (Device Firmware Update) and USB device functionality, as well as powering the board
+- Onboard voltage regulator from 5V to 3.3V
+- SWD Pins for use as STLink (Pin header) and TC2030-IDC 6-Pin Tag-Connect Plug-of-Nails™ Connector
+- Crystal for HS 8MHz
+- Crystal for RTC 32.768KHz
+
+Board documentation:
+https://github.com/lucaszampar/NuttX_STM32F4_RS485_DevBoard
+
+As F4 series have a USB DFuSe-capable BootROM [AN2606], the board can be flashed
+via `dfu-util` over USB, or via `stm32flash` over UART without any debuggers.
+
+LEDs
+====
+
+The STM32F4-RS485 has 4 software controllable LEDs.
+
+  =====  =====
+  LED    PINS
+  =====  =====
+  LED_1  PC0
+  LED_2  PC1
+  LED_4  PC2
+  LED_5  PC3
+  =====  =====
+
+User Buttons
+============
+
+The STM32F4-RS485 has 4 user switches.
+
+  ======= =====
+  SWITCH  PINS
+  ======  =====
+  SWIO_1  PB13
+  SWIO_2  PB14
+  SWIO_3  PB15
+  SWIO_4  PC6
+  ======  =====
+
+UARTs
+=====
+
+The STM32F4-RS485 has 1 USART available for user.
+
+USART2
+------
+
+  ========== =====
+  UART/USART PINS
+  ========== =====
+  CTS        PA0
+  RTS        PA1
+  TX         PA2  *Warning you make need to reverse RX/TX on
+  RX         PA3   some RS-232 converters
+  CK         PA4
+  ========== =====
+
+
+SDCARD
+=====
+
+The STM32F4-RS485 has 1 SDCard slot connected as below:
+
+  ========== =====
+  SDIO       PINS
+  ========== =====
+  SDIO_D0    PC8
+  SDIO_D1    PC9
+  SDIO_D2    PC10
+  SDIO_D3    PC11
+  SDIO_DK    PC12
+  ========== =====
+
+
+EEPROM
+======
+
+The STM32F4-RS485 development board has serial EEPROM HX24LC02B, with 2k bits (256x8) and internally
+organized with 32 pages of 8 bytes each. It is connected through I2C as below:
+  ====== =====
+  I2C    PINS
+  ====== =====
+  SDA    PB7
+  SCL    PB8
+  ====== =====
+
+
+Temperature Sensor
+==================
+
+The STM32F4-RS485 development board has a temperature sensor TMP75 (TMP75AIDR) connected through I2C as below:
+
+  ====== =====
+  I2C    PINS
+  ====== =====
+  SDA    PB7
+  SCL    PB8
+  ====== =====
+
+
+RS485 Transceiver
+=====
+The STM32F4-RS485 development board has a half-duplex RS-485 transceiver, the BL3085B it is connected
+through USART2 as below:
+  ==========   =====
+  USART2         PINS
+  ==========   =====
+  USART2_RX    RO
+  USART2_RTS   DE, /RE
+  USART2_RX    DI
+  ==========   =====
+
+A/D Converter
+=====
+The STM32F4-RS485 development board has two Analog to Digital converters with Amplifier Buffer (1COS724SR)
+and connected as below:
+  ======= =====
+  PWM     PINS
+  ======= =====
+  PWM_1   PB6
+  PWM_2   PA6
+  ======= =====
+
+D/C Converter
+=====
+The STM32F4-RS485 development board has two Digital to Analog converters with Amplifier Buffer (1COS724SR)
+and connected as below:
+  ======= =====
+  ADC     PINS
+  ======= =====
+  ADC_1   PA0
+  ADC_2   PA4
+  ======= =====
+
+Configurations
+==============
+
+Each stm32f401rc-rs485 configuration is maintained in a sub-directory and
+can be selected as follow::
+
+    tools/configure.sh stm32f401rc-rs485:<subdir>
+
+  Where <subdir> is one of the following:
+
+
+Configuration Directories
+-------------------------
+
+nsh
+---
+
+Configures the NuttShell (nsh) located at apps/examples/nsh. This
+configuration enables a serial console on UART2.