HAnix-diy - Public: Raspberry Pi - connecting a traditional LCD display (Update)

Raspberry Pi is a powerful enough platform and it is possible to use many recent touchscreen with a nice graphic on it.
However often a traditional LCD is more useful, and surely is more cheap than a touchscreen solution.

Here one way to connect a traditional LCD and use it in projects, like the TeirmiLab one.
This article describes how to connect a traditional LCD but with RGB capabilities to the Raspberry Pi B.

We need :

It is possible to hook a display without the need of the GPIO expander, however a normal LCD uses at least 6 signals. 9 if the LCD display is an RGB one like the one used for this project.
Considering that the Raspberry Pi B doesn't have many GPIO available and that usually the speed required to handle the display is not critical, using a chip like the MCP23017 make sense.
In this way it will be enough to "hook" the chip to the I2C bus, leaving all the Raspberry Pi GPIO free for other use.

Let see a schematic.

Software

There are many ways to control the hardware.
For now I found a ready-to-use Python library published by Adafruit that is working nicely.
Again, the important thing is that control an LCD is not a critical task, so a python script is Ok.
Maybe in future I'll explore the possibility to control it directly in C.

Python

To use our display we need to install some code in our Raspberry.
Here a quick step-by-step guide (see Adafruit tutorials for a more detailed and easy instructions) :

sudo apt-get update
sudo apt-get install build-essential python-dev python-smbus python-pip
sudo apt-get install i2c-tools
sudo pip install RPi.GPIO
cd ~
git clone https://github.com/adafruit/Adafruit_Python_CharLCD.git
cd Adafruit_Python_CharLCD
sudo python setup.py install

Before to be able to use it, we need to be sure a configuration file is correctly set-up.

Execute : sudo nano /etc/modules

and if the file exists, add these two lines in the end.

i2c-bcm2708

i2c-dev

Save the file then open another one in edit.
Execute : sudo nano /etc/modprobe.d/raspi-blacklist.conf

In this file comment out the two lines showed below, adding # at the beginning:

#blacklist spi-bcm2708

#blacklist i2c-bcm2708

Reboot Raspberry : sudo shutdown -r now

or : sudo reboot

At this point everything should be installed in order to use the display.

First is better to run an utility to see if the I2C GPIO expander is seen correctly.

sudo i2cdetect -y 1

The command should display a table and in the table should be present a 0x20 address.

pi@raspberrypi ~ $ sudo i2cdetect -y 1
0 1 2 3 4 5 6 7 8 9 a b c d e f
00: -- -- -- -- -- -- -- -- -- -- -- -- --
10: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
20: 20 -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
30: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
40: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
50: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
60: -- -- -- -- -- -- -- -- 68 -- -- -- -- -- -- --
70: -- -- -- -- -- -- -- --
pi@raspberrypi ~ $

If so, the chip is correctly seen by Raspberry.

To test if the display is working you can execute a test script :

cd Adafruit_Python_CharLCD/examples
sudo ./char_lcd_mcp.py

Some test messages should be displayed on the LCD display.

Troubleshooting

It is possible to have some problems installing the libraries.
Here a couple of tips and resolutions.

apt-get missing

If you start from a fresh installation of Raspbian, especially a lite version, before to start to follow the instructions above, be sure to execute :

sudo apt-get update
sudo apt-get upgrade
sudo apt-get update

It is important because updating the first time, include more repo on apt-get.

python build fail

If during the python building of the library you have this errors :

python ./setup.py build

Downloading https://pypi.python.org/packages/source/s/setuptools/setuptools-3.5.1.zip

Extracting in /tmp/tmpw6UbTx

Traceback (most recent call last):

File "./setup.py", line 4, in

use_setuptools()

File "/home/pi/Adafruit_Python_CharLCD/ez_setup.py", line 128, in use_setuptools

return _do_download(version, download_base, to_dir, download_delay)

File "/home/pi/Adafruit_Python_CharLCD/ez_setup.py", line 108, in _do_download

_build_egg(egg, archive, to_dir)

File "/home/pi/Adafruit_Python_CharLCD/ez_setup.py", line 57, in _build_egg

with archive_context(archive_filename):

File "/usr/lib/python2.7/contextlib.py", line 17, in __enter__

return self.gen.next()

File "/home/pi/Adafruit_Python_CharLCD/ez_setup.py", line 88, in archive_context

with get_zip_class()(filename) as archive:

File "/usr/lib/python2.7/zipfile.py", line 770, in __init__

self._RealGetContents()

File "/usr/lib/python2.7/zipfile.py", line 813, in _RealGetContents

raise BadZipfile, "File is not a zip file"

zipfile.BadZipfile: File is not a zip file

this happens because for some magic reason :) the python script screw up the download of a zipped file.

In my case the file was setuptools-3.5.1.zip.

Doing an ls -l showed up to be 122 byte big !! Definitively NOT a zip file.
The workaround was quite easy actually.

remove the file : sudo rm setuptools-3.5.1.zip
manually download the file :
wget https://pypi.python.org/packages/source/s/setuptools/setuptools-3.5.1.zip
Re-execute the build : sudo python ./setup.py build

Examples

Be aware about the type of kit you are using.
On the examples area there are different examples depending the type of display in use.

To test the basic display, use : char_lcd_mcp.py
To test the kit with RGB display and pushbutton use : char_lcd_plate.py

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Saturday, October 11, 2014

Raspberry Pi - connecting a traditional LCD display (Update)