Minnowboard:GPIO Programming

=Summary=

In this guide, I will describe how to do GPIO Programming on the MinnowBoard step by step. This guide is for new users who are just getting started with the MinnowBoard.

=Contents of the box=

When you purchase a MinnowBoard, the following items are included in the box as shown in Figure -1:


 * 1) MinnowBoard
 * 2) 5V Power Adaptor
 * 3) 4GB micro SD card preloaded with Angstrom Linux Distribution(Yocto Project Compatible)
 * 4) USB Cable



=Anatomy of a MinnowBoard=

Top Side
This section outlines the various components on the MinnowBoard. Figure -2 below annotates the important components on the top side of the MinnowBoard.



Below is a brief description of the micro USB and debugger ports which are annotated in Figure -2.

• Micro USB: This port can be used to access the contents of the MinnowBoard's microSD card as a removable drive on your desktop computer. Simply boot the MinnowBoard up and connect it to your desktop computer using a micro-USB cable.

• Debugger: This mini-USB port allows you to interact with the serial console of the MinnowBoard, so you can log into it and view debugging messages even when an external monitor and keyboard are not connected to the MinnowBoard. You need to use a terminal emulator on your desktop computer to access the serial console at a baud rate of 115200. This is demonstrated later in this guide in the Booting Angstrom section.

Bottom Side
Figure -3 below annotates the important components on the bottom side of the MinnowBoard.



=Specifications=

For detailed specifications of the MinnowBoard, please visit our website below:

http://www.minnowboard.org/technical-features/

=Preparing the MicroSD Card=

Please note that this section is completely optional. The MinnowBoard already comes with a microSD card that is preloaded with a working Angstrom Linux image. In case you want to use a newer image or want to program the microSD from scratch, this section covers it all. These steps were created using an Ubuntu 12.10 64-bit system.

''Note:- This section is for Linux users only. If you are using Windows, please skip to the Windows Section. If you are using Mac, please skip to the OS X section.''

Step-1:

First download the latest Angstrom Linux image for MinnowBoard using the following link:

Latest Angstrom Image for the MinnowBoard

At the time of writing this guide, the latest image available for download was 'Angstrom-development-GNOME-image-eglibc-ipk-v2012.12-minnow-2013.07.10.img.xz'

Step-2:

Insert the microSD card into a microSD compatible writer in your desktop computer. Now, identify the disk device filename for your microSD card. You can do this by opening the Disk Utility application (sometimes also called Disks) in Ubuntu and clicking on the microSD card entry. See the Device field to determine the raw device name of your microSD card. Note that this device name should specify an entire disk device (e.g, /dev/sde), and not an individual disk partition (e.g, /dev/sde1)

Step-3:

Now, make sure all possible disk partitions from the microSD card are unmounted by typing the command below into your terminal:

$ sudo umount /dev/sdX?*

Here 'sdX' stands for the raw device id of the microSD card.

Step-4:

Now unpack the image to the microSD card by typing the command below in a terminal window:

$ xzcat Angstrom-development-GNOME-image-eglibc-ipk-v2012.12-minnow-2013.07.10.img.xz | sudo dd of=/dev/sdX

=GPIO on the MinnowBoard=

The MinnowBoard provides a variety of GPIO(General Purpose Input Output), some with a dedicated purpose. For the purpose of this guide, I will be concentrating only on GPIO(s) which are on the J9 expansion header as shown in Figure - 4. Please refer the table below to find out how are the GPIO(s) actually referenced in the kernel and their default modes as well as values. Unless otherwise stated the GPIO(s) on J9 expansion header are rated at 3.3V and can source/sink a maximum of 10mA. Also, it can be seen from the table that the GPIO(s) are by default set to be used in INPUT mode and have PULL-UP resistors enabled.



As you can see from Figure -4, there are a total of ten pins on the J9 header out of which 8 pins can be used as GPIO. The remaining two pins are GND and 3.3V as mentioned in the table.

The GPIO(s) are accessible via the user space in Linux at the location below on the filesystem:

/sys/class/gpio

There is one directory per GPIO, named as shown below(as an example only two GPIO(s) are shown here):

/sys/class/gpio/gpio245

Inside each one of those directories, there are files named "direction” and “value” as shown in Figure - 5. The former is for configuring the mode of GPIO as input(“in”) or output(“out”) while the latter is for the value('1' for HIGH and '0' for LOW) if used in output mode.



=Selecting the correct LED=

• To make sure that you do not damage the GPIO pins on the MinnowBoard, please use a LED whose rating should not exceed 3.3V/10mA:

http://www.digikey.com/product-detail/en/HLMP-4700-C0002/516-2483-2-ND/1234840

• If you have an LED that is higher than the above mentioned ratings, please refer the refer the link below to select an appropriate current limiting resistor:

http://www.cmiyc.com/tutorials/led-basics

=Extra Credit=

This is an optional section which you can read for further understanding. In a nutshell, we are trying to access the on board USER LED(s) via userspace in Linux. To be more precise we are using the sysfs interface. sysfs is a virtual filesystem which translates the hardware devices and busses attached to the system(board in our case) into a file system hierarchy that can be accessed from userspace. sysfs is generated by the kernel and always mounted at /sys. As discussed earlier, trigger for D11 and D12 LEDs has to be changed to make them function as USER LEDs. I would describe 'trigger' as the API used to link a LED to an 'event' in kernel space. Here, 'event' could be microSD card or Ethernet activity, heartbeat, power etc. To understand this better, take LED D11 for example. The trigger for D11 has been set by default to heartbeat. So, to use it as an USER LED, we have to change the trigger for it to 'none' as shown in Figure - 6 below:



=Setup=

Please refer the Figure - 6 below to see the connections:



Only the pins on the J9 expansion header are used in this case. Connect the anode of the LED to the resistor(I am using a 330 ohms resistor) which in turn is connected to pin 3(GPIO) and connect the cathode of the LED to pin 2(GND). You can purchase the male-female type hookup/jumper wires used for connections from the link below:

https://www.sparkfun.com/products/9140

=Booting Angstrom=

Step-1:

First insert the card into the microSD card slot label side up as shown in Figure -7. Push the microSD card into the slot until it clicks into place.



Step-2:

Next, connect an external keyboard and mouse to the MinnowBoard. Then connect a USB mini cable to your computer and the MinnowBoard as shown in Figure -8. This connection will be used to access the serial console of the MinnowBoard.



Step-3:

Now power up the MinnowBoard, connect the 5V/2.5A power supply to it as shown in Figure -9:



Step-4:

Access the MinnowBoard's serial console via a terminal emulator:

$ sudo screen /dev/ttyUSB0 115200

''Note:- You can also use minicom, but screen is much easier to use! Also in most cases the virtual USB serial port is ttyUSB0. If it does not work, try ttyUSB1''

Step-5:

You should see a similar Angstrom login screen as shown in Figure - 10. The username is 'root' and there is no password - just press the Enter key.



=Steps=

• Now let us toggle that LED!

Step-1:

GPIO - 3 is referenced as 244 in the kernel. Since, it is set to be used in INPUT mode by default, we need to change its mode to OUTPUT. Type the following command in your terminal to accomplish this:

echo out > /sys/class/gpio/gpio244/direction

Step-2:

Now we are ready to toggle the LED. Type the following commands in your terminal as shown in Figure - 11(First one is for turning ON and latter for OFF):

echo 1 > /sys/class/gpio/gpio244/value echo 0 > /sys/class/gpio/gpio244/value



=Output=

You should get an output as shown below:





=Windows Users=

This section is for those who are using a Windows machine to interact with the MinnowBoard.

Serial Terminal
We will be using Teraterm as our terminal application. Please follow the steps below to set up Teraterm:

Step-1:

Download the latest version of Teraterm using the link below:

Teraterm

At the time of writing this guide, the latest version was '4.78'.

Step-2:

Install the Teraterm software by double clicking on the exe file downloaded earlier. If it displays a security warning, accept it and click on 'Run' as shown in Figure - 14 below:



Step-3:

Now just follow the on-screen instructions to install the software. It is advised to do the 'full' installation instead of the 'standard' one as shown in Figure - 15 below:



Step-4:

Launch Teraterm. Then go to Setup>Serial port and change the settings as shown in Figure - 16 below:



Step-5:

Then go to File>New connections and select the Serial option and chose the appropriate COM port as shown in Figure - 17 below



Step-6:

Then connect the required accessories as described earlier in this guide and power up the MinnowBoard. If all went well, you should see a login screen in Teraterm as shown in Figure - 18 below:



Preparing the microSD card
Step-1:

First, download the latest Angstrom Linux image from the link below:

Latest Angstrom Image

Step-2:

Next, download the 7-zip utility from the link below and install it:

7zip

Step-2:

Decompress the previously downloaded Angstrom image using 7-zip as shown in Figure - 19 below. The resulting file should have an extension '.img'.



Step-3:

Image Writer for Windows is needed to write the .img file to a microSD card. Download and install Image Writer using the link below: :

Image Writer for Windows

Step-4:

Insert the mircoSD card into the computer using an appropriate adapter. Next, launch the image writer software we just installed earlier. Select the Angstrom image and write it to the microSD card as shown in Figure - 20 below.



=Mac Users=

This section is for those who are using OS X to interact with the MinnowBoard.

Serial Terminal
We will be using the 'screen' command which is by default a part of the bash shell on OS X. To use screen command, the FTDI driver(it enables communication between the development board and computer over USB) has to be installed. Please follow the steps below to install it:

Step-1:

Download the latest ftdi driver for Mac OS X from the link below and make sure that you select the correct file corresponding to your computer architecture(32/64 bit):

FTDI Drivers

Step-2:

Double click on the downloaded file to start the installation procedure.

Step-3:

You will have two files inside the downloaded driver package as shown below in Figure - 21. The first file, ending with 10_3 is specific for Mac OS X version 10.3. The second file is for all other versions, newer than 10.3.



Step-4:

Based on your Max OS X version, select the appropriate and double click it to proceed further.

Step-5:

You will now be greeted with an ftdi installer screen as shown in Figure - 22.



Step-6:

Proceed forward by pressing the 'continue' button till you reach the “installation type” screen as shown below in Figure -23.



Step-7:

Now press the 'install' button and wait for it to finish as shown in Figure - 24.



Step-8:

If the installation was successful, you should be greeted by a screen similar to the one shown in Figure - 25



Step-9:

Then connect the required accessories as described earlier in this guide and power up the MinnowBoard.

If everything went well, you will see new entries in the /dev directory:

/dev/cu.usbserial-xxxxxxxx /dev/tty.usbserial-xxxxxxxx

Here, xxxxxxxx is either the device's serial number or, for unserialized devices, a location string that depends on which USB port your device is connected to. /dev can be accessed through the Terminal application. The Terminal application can be launched by selecting Go > Applications > Utilities > Terminal. Now fire up the terminal in Mac OS X and type the following command in it:

$ cd /dev

Step-10:

Access the MinnowBoard's serial console via a terminal emulator and you should be greeted by the Angstrom screen as shown in Figure - 26:

$ sudo screen /dev/tty.usbserial-xxxxxxxx 115200



Preparing the microSD card
Step-1:

First, download the latest Angstrom Linux image from the link below:

Latest Angstrom Image

Step-2:

Next, install the “The Unarchiver” application(it is free) from the app store on Mac OS X as shown in Figure - 27



Step-3:

Then unpack the downloaded angstrom image using the unarchiver application as shown below in Figure -28



Step-4:

Insert the microSD card into a microSD compatible writer in your computer. Now, identify the disk device filename for your microSD card. You can do this by typing the command below, before and after you insert the microSD card and comparing the outputs to see the newly added device:

$ df-h

''Note:- If you notice closely, the microSD card entry will be in the form of /dev/diskNsX where 'N' and 'X' are numbers. diskNsX is basically the device id of your microSD card. So, to obtain the raw device id from this, remove the 's' and 'X' from the device id. The raw device id should be in the form of diskN .''

Step-5:

Now, make sure all possible disk partitions from the microSD card are unmounted by typing the command below into your terminal: $ sudo umount /dev/diskN?* Here 'diskN' stands for the raw device id of the microSD card.

Step-6:

Now unpack the image to the microSD card by typing the command below in a terminal window:

$ sudo dd if=Angstrom-development-GNOME-image-eglibc-ipk-v2012.12-minnow-2013.07.10.img of=/dev/rdiskN

Note:- In the above command we subtly replaced “diskN” with “rdiskN” because in the latter case, you are writing to a buffered device and hence the process becomes much faster!

=Resources=

Please refer the websites below for more videos, tutorials, and projects you can do with your MinnowBoard:


 * 1) http://www.minnowboard.org/
 * 2) http://www.elinux.org/Minnowboard
 * 3) http://www.angstrom-distribution.org/