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1. Purpose[edit source]
This article describes how to use TTY with a user terminal. The TTY overview is described in Serial TTY overview article.
The use case of the following examples is a data transfer between a STM32 MPU board and PC, over a USB to a RS232 adapter cable.
The setup of this use case is described in details in the How to get Terminal article.
For the following examples:
- uart4 is activated by default (for the Linux console)
- usart3 is enabled by device tree
- The usart3 pins are connected to a RS232 card
- The RS232 card is connected to the PC over the USB to RS232 adapter cable.
Note: Some TTY tools are used in this article. A list of TTY tools is defined a dedicated article [TTY Tools ].
2. Print the file name of the terminal connected to standard input (with tty tool)[edit source]
tty # The console is connected to uart4 (aka ttySTM0) # /dev/ttySTM0
3. Change serial port configuration (with stty tool)[edit source]
Many serial port properties can be displayed and changed with the stty tool. The full feature list is available in stty user manual pages [1] .
stty --help
- Display the current configuration:
- The termios default configuration is specific to each Linux distribution. Before starting a serial communication between two devices, it's recommended to check that termios configurations are compatible on both devices. The termios configurations need to be aligned first.
stty -a -F /dev/ttySTM1
# Display the configuration of uart3 (aka ttySTM1) #
speed 115200 baud; rows 45; columns 169; line = 0;
intr = ^C; quit = ^\; erase = ^?; kill = ^U; eof = ^D; eol = <undef>; eol2 = <undef>; swtch = <undef>; start = ^Q;
stop = ^S; susp = ^Z; rprnt = ^R; werase = ^W; lnext = ^V;
discard = ^O; min = 1; time = 0;
-parenb -parodd -cmspar cs8 hupcl -cstopb cread clocal -crtscts
-ignbrk -brkint -ignpar -parmrk -inpck -istrip -inlcr -igncr icrnl ixon -ixoff -iuclc -ixany -imaxbel -iutf8
opost -olcuc ocrnl -onlcr -onocr onlret -ofill -ofdel nl0 cr0 tab0 bs0 vt0 ff0
isig -icanon iexten -echo echoe echok -echonl -noflsh -xcase -tostop -echoprt echoctl echoke -flusho -extproc
- Display only the current baud rate:
stty -F /dev/ttySTM1 speed
# uart3 (aka ttySTM1) baud rate is set to 115200 bps #
115200
- Change the baud rate:
- stty -F /dev/ttySTMx EXPECTED_BAUDRATE
- Example: change the baud rate to 19200
# Change uart3 (aka ttySTM1) baud rate to 19200 bps #
stty -F /dev/ttySTM1 19200
The stty tool proposes many arguments allowing many operations on a tty terminal, such as:
- special settings (various arguments such as speed, line discipline, minimum number of characters for a completed read, size, timeout, etc...)
- control settings
- input settings
- output settings
- local settings
- combination settings
Note: If you want to go further, an interesting tutorial describes termios and stty [2].
4. Send / Receive data (with stty, minicom, echo and cat tools)[edit source]
Serial counters can be very useful to debug the following use cases.
4.1. Default configuration (8 data bits frame, no parity errors detection, no framing errors detection)[edit source]
Some termios properties have a major influence on data processing, they can be deactivated for testing:
- echo: Enable echo. If ECHO is set input characters are echoed back to the terminal.
- icanon: Canonical input (erase and kill processing). If ICANON is set canonical processing is enabled. In canonical mode input processing is processed in units of lines. A line is delimited by a '\n' character or and end-of-file (EOF) character. A read request does not return until an entire line is read from the port or a signal is received..
- onlcr: Map NL to CR-NL on output. If ONLCR is set the NL character is transmitted as the CR-NL character pair.
Sending data can be simply done by opening the device as a file and writing data to it.
- Configure a port on ttySTM1 (aka usart3). echo, icanon and onlcr properties are deactivated to handle raw data.
stty -F /dev/ttySTM1 115200 -echo -icanon -onlcr
- Display the current configuration on ttySTM1 (usart3):
# display the configuration of uart3 (aka ttySTM1) #
stty -a -F /dev/ttySTM1
speed 115200 baud; rows 45; columns 169; line = 0;
- Open a port on ttySTM1 (usart3) to receive data
cat /dev/ttySTM1 &
- On the remote PC, identify the tty terminal associated to RS232 card connected on STM32MPU USART3 pins
# Command to execute from host terminal #
ls /dev/ttyUSB*
/dev/ttyUSB0
- Open a minicom in a second terminal on the remote device connected on USART3 pins
minicom -D /dev/ttyUSB0
- Display the current configuration on ttyUSB0 (remote device):
# Display the configuration of host uart (aka ttyUSB0) #
stty -a -F /dev/ttyUSB0
speed 115200 baud; rows 45; columns 169; line = 0;
- Send data from remote PC to STM32MPU over USART3 with default termios configuration (8 frames length, no parity)
# Execute this command from host terminal #
echo "HELLO" > /dev/ttyUSB0
- send data from STM32MPU to remote PC over USART3 with default termios configuration (8 frames length, no parity)
# Execute this command from STM32 terminal #
echo "HELLO" > /dev/ttySTM1
4.2. Parity errors detection[edit source]
Some additional termios funtions allow to enable parity errors detection:
- parenb: Parity enable
- parodd: Odd parity else even
- inpck: Enable input parity or framing check
- ignpar: Ignore characters with parity or framing errors
Exemples:
- Configure a port on ttySTM1 (usart3) with even parity enabling
# STM32 parity enabling #
stty -F /dev/ttySTM1 115200 -echo -icanon -onlcr parenb -parodd inpck ignpar
- Open a port on ttySTM1 (usart3) to receive data
cat /dev/ttySTM1 &
Open a minicom in a second terminal on the remote device connected on USART3 pins minicom -D /dev/ttyUSB0
- Configure a port on ttyUSB0 (remote device) with even parity enabling:
# Remote device parity enabling #
stty -F /dev/ttyUSB0 115200 -echo -icanon -onlcr parenb -parodd inpck ignpar
- Send data from remote PC to STM32MPU over USART3
# Execute this command from host terminal #
echo "HELLO" > /dev/ttyUSB0
- send data from STM32MPU to remote PC over USART3
# Execute this command from STM32 terminal #
echo "HELLO" > /dev/ttySTM1
4.3. Framing errors detection[edit source]
Some additional termios funtions allow to enable framing errors detection:
- csize: Number of bits per byte (character size and parity bit configurations)
- inpck: Enable input framing check
- ignpar: Ignore characters with parity or framing errors
Exemples:
- Configure a port on ttySTM1 (usart3) with framing check enabling and 7 data bits length frames
# STM32 framing enabling #
stty -F /dev/ttySTM1 115200 -echo -icanon -onlcr cs7 inpck ignpar
- Open a port on ttySTM1 (usart3) to receive data
cat /dev/ttySTM1 &
Open a minicom in a second terminal on the remote device connected on USART3 pins minicom -D /dev/ttyUSB0
- Configure a port on ttyUSB0 (remote device) with framing check enabling and 7 data bits length frames
# Remote device parity enabling #
stty -a -F /dev/ttyUSB0 115200 -echo -icanon -onlcr cs7 inpck ignpar
speed 115200 baud; rows 45; columns 169; line = 0;
- Send data from remote PC to STM32MPU over USART3
# Execute this command from host terminal #
echo "HELLO" > /dev/ttyUSB0
- send data from STM32MPU to remote PC over USART3
# Execute this command from STM32 terminal #
echo "HELLO" > /dev/ttySTM1
5. Identify processes using a tty serial device (with fuser tool)[edit source]
fuser /dev/ttySTM0
# The process numbered 395, 691 and 3872 are using a tty serial device #
395 691 3872
6. Link a tty serial device with a line discipline (with ldattach tool)[edit source]
Attach ttySTM1 with line discipline number n :
ldattach n /dev/ttySTM1
7. File transfer over serial console[edit source]
Please see the dedicated article How to transfer a file over serial console.