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:

• /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 N=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 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