Serial ports usage on Linux

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

Generalities

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 port parameters

• To get current parameters (change serial port device name depending on your board, here ttySMX0):

# stty -F /dev/ttySMX0
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/ttySMX0 speed
115200

• By default serial ports are configured as terminal emulator. If you want to use them as "raw" serial port you will have to do first (example for port 1):

# stty -F /dev/ttySMX1 raw
# stty -F /dev/ttySMX1 -echo -echoe -echok

• To change baudrate of port 2 to 115200 :

# stty -F /dev/ttySMX2 115200

Sending/Receiving data

• It is simple as opening the device as a file and writing data to it:

# echo "HELLO" > /dev/ttySMX0

• To receive data (ASCII in that case):

# cat /dev/ttySMX0

Links

• Serial uart manual in Linux