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 with their device nodes:

• /dev/ttySMXn for i.MX processor internal ports
• /dev/ttyUSBn for USB <-> serial converters
• /dev/ttySn for 16550 compatible FPGA's IP

/dev/ttySMX0

• /dev/ttySMX0 is the default console on which you can interact with the board (U-Boot and Linux).
• So: without software modifications this port is not usable to connect a MODEM or any other RS232 device. If you want to use it 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 console=ttySMX0,115200 to console=ttySMXN,115200 in U-Boot bootargs variable (N=1 on APF9328 and 2 on APF27)

Changing port parameters

• Get current parameters:

# 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

• Change Baud rate to 115200 of port 2:

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