Serial ports usage on Linux

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How to use the APF's serial ports on Linux.


On Linux, all serial ports are driven the same way. Only their device nodes are changing to reflect the port type:

  • i.MX processor internal ports:
    • on APF9328, APF27, APF51 & OPOS6UL: /dev/ttySMXN (with kernels <= 2.6.29) or /dev/ttymxcN (with kernels > 2.6.29)
    • on APF28: /dev/ttySP[0-4] (general purpose serial ports) or /dev/ttyAMxx (debug serial ports)
  • /dev/ttyUSBN for USB <-> serial converters
  • /dev/ttySnN for 16550 compatible FPGA's IP

Console port

  • Console port is the default serial port on which you can interact with the board (U-Boot and Linux):
    • it is serial port 0 (-> /dev/ttySMX0 or /dev/ttymxc0) on APF9328, APF27 & OPOS6UL
    • it is serial port 2 (-> /dev/ttymxc2) on APF51
    • it is debug serial port 0 (-> /dev/ttyAM0 (2.6.35 kernel) or /dev/ttyAMA0 (3.x kernels)) on APF28
  • So, without software modifications, this port is not available to connect a MODEM or any other RS-232 device. If you want to use it as a normal serial port on Linux then:
    • be sure that your device won't send data back in response of U-Boot boot's sequence
    • change U-Boot consoledev envt variable, to tell Linux to display console on the LCD (for example):
       BIOS> setenv consoledev tty0

Changing serial ports configuration

  • To get current configuration (adapt serial port device name depending on your board, here ttymxc0):
# stty -F /dev/ttymxc0
speed 115200 baud;
intr = ^C; quit = ^\; erase = ^?; kill = ^U; eof = ^D; eol = ^J;
eol2 = <undef>; start = ^Q; stop = ^S; susp = ^Z; rprnt = ^R; werase = ^W;
lnext = ^V; flush = ^O; min = 1; time = 0;
-brkint ixoff -imaxbel
-iexten -echoctl
  • To only get actual speed:
# stty -F /dev/ttymxc0 speed
  • By default serial ports are configured as terminal emulator (canonical mode). If you want to use them as "raw" serial port you will have to do first (example for port 1):
# stty -F /dev/ttymxc1 raw -echo -echoe -echok
  • To change baudrate of port 2 to 115200 :
# stty -F /dev/ttymxc2 115200

Sending/Receiving data

In Linux console

  • It is as simple as opening the device as a file and writing data to it:
# echo "HELLO" > /dev/ttymxc0
  • To receive data (ASCII in that case):
# cat /dev/ttymxc0

With terminal (

By default, a simple terminal named «» is installed in the BSP. To use it on OPOS6UL, launch it with /dev port name and speed :

$ /dev/ttymxc1 115200
--- Miniterm on /dev/ttymxc1  115200,8,N,1 ---
--- Quit: Ctrl+] | Menu: Ctrl+T | Help: Ctrl+T followed by Ctrl+H ---
Hello world !

On OPOS6UL_DEV if Rx/Tx are not in loop, nothing will be prompted on key input. To see something prompted the Rx/Tx must be plugged together with a jumper on RaspberryPi connector (pin 8 and 10).


  • To have some statistics on serial port usage:
# cat /proc/tty/driver/<driver_name>       (for example ttyAPP on APF28, IMX-uart on OPOS6UL)

serinfo:1.0 driver revision:
0: uart:8006a000.serial mmio:0x8006A000 irq:214 tx:0 rx:0 CTS
1: uart:8006c000.serial mmio:0x8006C000 irq:215 tx:0 rx:0 CTS
3: uart:80070000.serial mmio:0x80070000 irq:216 tx:875195 rx:4024187 CTS
4: uart:80072000.serial mmio:0x80072000 irq:217 tx:0 rx:0 CTS