10 min

Modernize RS232 connections with MAX3232 and STM32F405RG

Upgrading data transfer: The only UART-to-RS232 bridge you need

RS232 Click with UNI Clicker

Published Oct 19, 2023

Click board™

RS232 Click

Development board

UNI Clicker


NECTO Studio



Efficient solution for incorporating RS-232 communication capabilities into various electronic projects and applications, including serial data exchange and modem communication



Hardware Overview

How does it work?

RS232 Click is based on the MAX3232, a low-power, true RS-232 transceiver from Analog Devices. Several protection features improve the reliability of this device. It has up to ±15kV ESD protection, ensuring no electrical discharge damages the circuit on the input side. The MAX3232 has two receivers and two transmitter channels, and it is used to bridge the physical differences between the CMOS/TTL signal levels and RS-232 bus levels. While CMOS/TTL signal levels vary from 0V to 5V typically, RS-232 uses signal levels that range from ±5V up to ±15 V. Furthermore, the RS-232 equipment is required to withstand short circuits for any voltage, up to ±25V, during an indefinite time interval. MAX3232 IC uses two internal charge pumps to obtain required driving levels of ±5V on its transceiver sections.

This Click board™ offers two inputs and two outputs, which feature the CMOS/TTL logic levels. These lines can be used to either drive the RS-232 bus or receive the incoming data from the bus. Receivers convert the RS-232 signals to MCU-acceptable UART-type signals, while transmitters convert the MCU UART signal to RS-232 levels. Therefore, one input/output pair is routed to the UART pins of the mikroBUS™, allowing simplified operation by the host MCU, while another pair of input/output signals is routed via the J2 and J3 SMD jumpers and is used as the UART RTS and CTS. These pins are typically used for the UART communication with the hardware flow control. The jumpers are unpopulated by default. The MAX3232 device can maintain a 120kbps data rate with the worst-case scenario - load of 3kΩ in parallel with

1000pF, while the typical communication speed goes up to 232 kbps. The RS232 Click comes equipped with the SUB D connector, typically found on many devices that use the RS-232 interface, and can be used for connection directly to the RS-232 bus. RS232 uses a standard 2-Wire UART interface to communicate with the host MCU. If using it with soldered J2 and J3 jumpers, then you can use the UART RTS and CTS hardware flow control pins. This Click board™ can operate with either 3.3V or 5V logic voltage levels selected via the PWR SEL jumper. This way, both 3.3V and 5V capable MCUs can use the communication lines properly. Also, this Click board™ comes equipped with a library containing easy-to-use functions and an example code that can be used as a reference for further development.

RS232 Click hardware overview image

Features overview

Development board

UNI Clicker is a compact development board designed as a complete solution that brings the flexibility of add-on Click boards™ to your favorite microcontroller, making it a perfect starter kit for implementing your ideas. It supports a wide range of microcontrollers, such as different ARM, PIC32, dsPIC, PIC, and AVR from various vendors like Microchip, ST, NXP, and TI (regardless of their number of pins), four mikroBUS™ sockets for Click board™ connectivity, a USB connector, LED indicators, buttons, a debugger/programmer connector, and two 26-pin headers for interfacing with external electronics. Thanks to innovative manufacturing technology, it allows you to build

gadgets with unique functionalities and features quickly. Each part of the UNI Clicker development kit contains the components necessary for the most efficient operation of the same board. In addition to the possibility of choosing the UNI Clicker programming method, using a third-party programmer or CODEGRIP/mikroProg connected to onboard JTAG/SWD header, the UNI Clicker board also includes a clean and regulated power supply module for the development kit. It provides two ways of board-powering; through the USB Type-C (USB-C) connector, where onboard voltage regulators provide the appropriate voltage levels to each component on the board, or using a Li-Po/Li

Ion battery via an onboard battery connector. All communication methods that mikroBUS™ itself supports are on this board (plus USB HOST/DEVICE), including the well-established mikroBUS™ socket, a standardized socket for the MCU card (SiBRAIN standard), and several user-configurable buttons and LED indicators. UNI Clicker is an integral part of the Mikroe ecosystem, allowing you to create a new application in minutes. Natively supported by Mikroe software tools, it covers many aspects of prototyping thanks to a considerable number of different Click boards™ (over a thousand boards), the number of which is growing every day.

UNI clicker double image

Microcontroller Overview

MCU Card / MCU



8th Generation


ARM Cortex-M4

MCU Memory (KB)


Silicon Vendor


Pin count


RAM (Bytes)


You complete me!


DB9 Cable Female-to-Female (2m) cable is essential for establishing dependable serial data connections between devices. With its DB9 female connectors on both ends, this cable enables a seamless link between various equipment, such as computers, routers, switches, and other serial devices. Measuring 2 meters in length, it offers flexibility in arranging your setup without compromising data transmission quality. Crafted with precision, this cable ensures consistent and reliable data exchange, making it suitable for industrial applications, office environments, and home setups. Whether configuring networking equipment, accessing console ports, or utilizing serial peripherals, this cable's durable construction and robust connectors guarantee a stable connection. Simplify your data communication needs with the 2m DB9 female-to-female cable, an efficient solution designed to meet your serial connectivity requirements easily and efficiently.

RS232 Click accessories image

Used MCU Pins

mikroBUS™ mapper

Power Supply
Power Supply

Take a closer look


RS232 Click Schematic schematic

Step by step

Project assembly

UNI Clicker front image hardware assembly

Start by selecting your development board and Click board™. Begin with the UNI Clicker as your development board.

UNI Clicker front image hardware assembly
GNSS2 Click front image hardware assembly
SiBRAIN for STM32F745VG front image hardware assembly
Prog-cut hardware assembly
GNSS2 Click complete accessories setup image hardware assembly
UNI Clicker Access MB 1 - upright/background hardware assembly
Necto image step 2 hardware assembly
Necto image step 3 hardware assembly
Necto image step 4 hardware assembly
Necto image step 5 hardware assembly
Necto image step 6 hardware assembly
Necto image step 7 hardware assembly
Necto No Display image step 8 hardware assembly
Necto image step 9 hardware assembly
Necto image step 10 hardware assembly
Debug Image Necto Step hardware assembly

Track your results in real time

Application Output

After loading the code example, pressing the "DEBUG" button builds and programs it on the selected setup.

Application Output Step 1

After programming is completed, a header with buttons for various actions available in the IDE appears. By clicking the green "PLAY "button, we start reading the results achieved with Click board™.

Application Output Step 3

Upon completion of programming, the Application Output tab is automatically opened, where the achieved result can be read. In case of an inability to perform the Debug function, check if a proper connection between the MCU used by the setup and the CODEGRIP programmer has been established. A detailed explanation of the CODEGRIP-board connection can be found in the CODEGRIP User Manual. Please find it in the RESOURCES section.

Application Output Step 4

Software Support

Library Description

This library contains API for RS232 Click driver.

Key functions:

  • rs232_generic_write - Generic write function.

  • rs232_generic_read - Generic read function.

Open Source

Code example

This example can be found in NECTO Studio. Feel free to download the code, or you can copy the code below.

 * \file 
 * \brief Rs232 Click example
 * # Description
 * This example reads and processes data from RS232 clicks.
 * The demo application is composed of two sections :
 * ## Application Init 
 * Initializes driver.
 * ## Application Task  
 * Depending on the selected mode (receiver/transmitter) this function reads/sends an appropriate message.
 * All data is displayed on USB UART.
 * \author MikroE Team
// ------------------------------------------------------------------- INCLUDES

#include "board.h"
#include "log.h"
#include "rs232.h"
#include "string.h"


// #define RS232_RECEIVER

// ------------------------------------------------------------------ VARIABLES

static rs232_t rs232;
static log_t logger;

static int32_t rsp_size;
static char uart_rx_buffer[ PROCESS_RX_BUFFER_SIZE ] = { 0 };
static char message[ ] = "MikroE";

// ------------------------------------------------------ APPLICATION FUNCTIONS

void application_init ( void )
    log_cfg_t log_cfg;
    rs232_cfg_t cfg;

     * Logger initialization.
     * Default baud rate: 115200
     * Default log level: LOG_LEVEL_DEBUG
     * @note If USB_UART_RX and USB_UART_TX 
     * are defined as HAL_PIN_NC, you will 
     * need to define them manually for log to work. 
     * See @b LOG_MAP_USB_UART macro definition for detailed explanation.
    LOG_MAP_USB_UART( log_cfg );
    log_init( &logger, &log_cfg );
    log_info( &logger, "---- Application Init ----" );

    //  Click initialization.

    rs232_cfg_setup( &cfg );
    RS232_MAP_MIKROBUS( cfg, MIKROBUS_1 );
    rs232_init( &rs232, &cfg );
    Delay_ms( 100 );
#ifdef RS232_RECEIVER
    log_printf( &logger, " ** RS232 Receiver **\r\n" );
    log_printf( &logger, " ** RS232 Transmitter **\r\n" );

void application_task ( void )
#ifdef RS232_RECEIVER
    rsp_size = rs232_generic_read( &rs232, uart_rx_buffer, PROCESS_RX_BUFFER_SIZE );

    if ( rsp_size == strlen( message ) )
        log_printf( &logger, "Message received: %s", uart_rx_buffer );
        log_printf( &logger, "\r\n-------------------------\r\n" );
        memset( uart_rx_buffer, 0, rsp_size );
    Delay_ms( 100 );
    rs232_generic_write( &rs232, message, strlen( message ) );
    log_printf( &logger, "Message sent: %s", message );
    log_printf( &logger, "\r\n-------------------------\r\n" );
    Delay_ms( 1000 );

void main ( void )
    application_init( );

    for ( ; ; )
        application_task( );

// ------------------------------------------------------------------------ END

Additional Support