Serial Communication

This page describes how the serial drivers work and how to access them.

Using stdio.h (printf(), scanf(), ...)

When you use functions like printf(), scanf(), puts(), gets() etc... inside your code, MIOSIX redirects them to one of the microcontroller's serial ports, called the default serial port. This is defined in miosix/config/arch/<your_arch>/<your_board>/board_settings.h, along with the default baudrate and other related configurations. The file looks like this:

const unsigned int defaultSerial=1;             //default serial = USART1
const unsigned int defaultSerialSpeed=19200;    //baudrate
const bool defaultSerialFlowctrl=false;         //use rts/cts

#define SERIAL_1_DMA                            //use the DMA

WARNING: printfs are very heavy and can significantly modify the timings if used inside drivers. For this reason, we suggest instead the use of TRACE() throughout the code.

Using fnctl.h (other serial ports)

A part from the default serial port, Miosix offers access to up to 3 USARTs in STM32 microcontrollers. In arch/common/drivers/serial_stm32.cpp you can find the 3 USARTs enabled by default. Here is the pinout:

Miosix gives the possibility to access these USART ports through Unix-like device files. In particular, to access a particular USART of your microcontroller (e.g. USART2) you should modify the bspInit2() method inside miosix/arch/<your arch>/<your board>/interfaces-impl/bsp.cpp as follows:

void bspInit2()
{
    #ifdef WITH_FILESYSTEM
    /**
     * Pointer to file system.
     */
    intrusive_ref_ptr<DevFs> devFs = basicFilesystemSetup(SDIODriver::instance())
    
    /**
     * Open serial port, accessible as '/dev/gps', on USART2 with baudrate 115200.
     */
    devFs->addDevice("gps", intrusive_ref_ptr<Device>(new STM32Serial(2,115200)));
    #endif //WITH_FILESYSTEM
}

After that, you can easily use the device as follows:

#include <fcntl.h>
//...
int fd=open("/dev/gps",O_RDWR);      //open serial port named gps
if(fd<0) printf("Cannot open /dev/auxtty");

char buffer[11];         //Buffer is 10 chars + '\0'
read(fd, buffer, 10);    //Read 10 chars
write(fd, buffer, 10);   //Write 10 chars

Pay attention to the baudrate at which the used port is set!

Reading/Writing from a PC

To communicate with a board from your PC you will need a USB-TTL driver, or you can do it via the Discovery's USB cable if the feature is enabled in your Discovery board.

Once your USB-TTL dongle is attached to the PC, you will need to connect the RX and TX of the dongle to the TX and RX of the decided serial port in your Discovery (ATTENTION: RX discovery<-->TX usb dongle and vice-versa).

Then you can open a communication using:

• Putty in Windows: Select the Session tab on the left panel and select Serial as "Connection type". You will need to know the COM port to which the Discovery is attached (you can find it out in Control Pane > Device Manager).
• GTKTerm on Linux: Configuration > Port > /dev/ttyUSB0 and select baudrate (19200 for the default serial). You can also use screen or minicom.

In both cases, you will need to set the correct port and baudrate, and you may need install some drivers for the USB dongle to make things work.

Auxtty USART (STM32F407VG only)

For the f407vg board, there's also another serial port, called auxtty: you need to remove the comment from the board_settings.h in order to use it:

//#define AUX_SERIAL "auxtty"    UNCOMMENT THIS TO OPEN THE SERIAL
const unsigned int auxSerialSpeed=9600;
const bool auxSerialFlowctrl=false;