RPi Serial Connection

From eLinux.org
Revision as of 06:58, 17 March 2013 by BerryBerry (talk | contribs) (Dedicated control of serial port)
Jump to: navigation, search

Back to RPi Advanced Setup.

The Serial Port is a low-level way to connect to the Raspberry Pi. The communication depends on byte wise data transmission and is generally available even before boot time.

First interaction with the board

Note that the Raspberry has no PREBUILT COM port. Soldering/protoyping and technical knowledge is required !

A UART is available in the Broadcom SoC, and its RxD and TxD lines are available on the GPIOs but you need a board or cable to convert 3.3V UART signals to RS-232. They do however connect directly to a USB serial legacy device, provided it uses 3V3 levels. If the device uses 5V levels you will need level converters (which could be simple resistor dividers, or could be specific IC's) See this tutorial for one example on how to build 3.3V to RS-232 levels converter with a breadboard, MAX3232CPE and five 0.1 uF capacitors. A 5V to 3V resistor divider can be built from a 2K2 (top) and a 3K3 (bottom) resistor.

Connect the serial cable to your COM circuitry , and connect the other end to the COM port or USB Serial Adapter in the computer.

The TxD and TXD signals can also be connected directly to similar signals on a microcontroller board like the arduino provided the signals are all at 3V3 levels. It's still a good idea to put 2K2 series resistors in the lines to prevent damage when two outputs are connected together, which could also happen if a GPIO input pin is accidentally programmed as output.

Serial Parameters

The following parameters are needed to connect to the Raspberry. All parameters except Port_Name and Speed are default values and may not need to be set.

  • Port_Name: Linux automatically assigns different names for different types of serial connectors. Choose your option:
    • Standard Serial Port: ttyS0 ... ttySn
    • USB Serial Port Adapter: ttyUSB0 ... ttyUSBn
  • Speed: 115200
  • Bits: 8
  • Parity: None
  • Stop Bits: 1
  • Flow Control: None

The Serial Port is generally usable by the users in the group dialout. To add oneself to the group dialout the the following command needs to be executed with root privileges:

 $useradd -G {dialout} your_name 
  • Super Easy Way Using GNU Screen

Enter the command below into a terminal window

 screen Port_Name 115200
  • Super Easy Way Using Minicom

Run minicom with the following parameters:

minicom -b 115200 -o -D Port_Name
  • Tedious Old-Fashioned Way Using Minicom

Another method to setup minicom is described in the Tincantools Minicom Tutorial

  • GUI method with GtkTerm

Start GtkTerm, select Configuration->Port and enter the values above in the labeled fields.

  • Windows Users

Windows Users above Windows XP must download putty, TeraTerm or a comparable terminal program. Users of XP and below can choose between using putty and Hyperterminal.

First Dialog

If you get the prompt below, you are connected to the Raspberry Pi shell!

 prompt> #

First command you might want try is "help":

 prompt> # help

If you get some output, you are correctly connected to the Raspberry Pi! Congratulations!

Unwanted serial garbage input

Note that on older software by accident the internal pullups of the RxD GPIO pins were not enabled, this could lead to lots of serial garbage being picked up if the GPIO pin was touched, or even if a finger was nearby. In extreme case this could lead to kernel warnings and other problems.

Dedicated control of serial port

there are several minor things in the way if you want to have dedicated control of the serial port on a raspberry pi

the first is kernel boot messages at startup, those are caused by the kernel parameter console=ttyAMA0,115200 set in /boot/cmdline.txt

next is the login prompt, set in /etc/inittab with the line T0:23:respawn:/sbin/getty -L ttyAMA0 115200 vt100

kernel debuging is also on the serial port and i dont know what effect it may have on the end-use, so you may want to turn that off also in cmdline.txt, just remove kgdboc=ttyAMA0,115200

once all of those are off, you can use /dev/ttyAMA0 like any normal linux serial port, and you wont get any unwanted traffic confusing the attached devices

some distro's may be setup differently, just double check 'ps aux|grep ttyAMA0' and 'cat /proc/cmdline' to verify ttyAMA0 is not in use anywhere

You can have the RTS0 signal on GPIO 17 (P1-11) if you set it to ALT function 3. Likewise, the CTS0 is available on GPIO 30 (P5-05), if it is set to ALT function 3. You can control the settings of IOs with gpio_setfunc