Linux FrameBuffer

I found a 4" touchscreen for RPi in my cupboard the other day.  I actually purchased it 3 years ago and had forgotten about.  I thought I should try it out and see what it can do.  It turns out to be a Waveshare 480x320 touchscreen.  Initially I wanted to use it for output/display.  I certainly don't want to run Xwindows on it so I needed to find out what is possible.

Installation

I plugged it in to an RPi 2B and installed the driver software. I run RPi servers headless and I was pleasantly suprised to see that the RPi console startup messages  are displayed.  It looks nice but isn't useful as the screen / characters are very small and I don't have the ability to type in to the console or login.

Display Text

To send output to a terminal it is easiest to do if the device is logged in.  Using raspi-config we can tell linux to login to the console as pi at startup.  We can now easily display messages, e.g.:
  echo ‘Hello little screen’ > /dev/tty1

To clear the screen or display text at specific positions we use control characters.  I have a fondness for these from my DEC PDP-11 programming days when a DEC VT100 was considered an excellent screen and I used it extensively.  I can clear the screen with:
 echo -e '\e[2J\e[1;1H' > /dev/tty1

Images and videos

It is surprisingly simple to play videos on the frame buffer with vlc:
 vlc starwars.avi > /dev/fb0
VLC can actually display images such as:
 vlc AtTheBeach.jpg /dev/fb0

As everything is a file in linux I can take a screen snapshot with:
 cat /dev/fb0  > screenshot.raw
and then redisplay it later with:
 cat screenshot.raw > /dev/fb0

However the best way to display images is to install the package fbi (framebuffer image) which gives you lots of image capabilities.

C Programs using the Framebuffer

A great tutorial is provided by Raspberry Compote explaining how to use the framebuffer in a C program.  It makes the whole process fairly straightforward.

Step 1 is to interrogate Linux to obtain characteristics of the device, in our case a 480x320 32bpp screen.

This is followed by a program to create a memory map for the buffer in RAM and write pixels to it.  Again it isn't very complicated and you end up with a pattern on your screen.

After that you need to investigate formats, palettes and colours in a little detail so that you can use the best approach for your application.  In practice, for your own programs an 8 bit format is preferred.  You have 16 basic colours provided and you can define the remaining 240 colours in the palette as needed.

Image formats and processing can become very complicated so C programs are probably best suited to displaying patterns, but it is great to have a bit-mapped screen available.

As a quick test I wrote a C program to display a grid on the screen and incorporated with the colour display.

Lvgl

In this post we are concentrating mainly on outputting to the framebuffer, however, its value lies in being a touch screen.  For touch screen usage we will be using lvgl (little versatile graphics library). As an introduction I found a great demo which uses lvgl to write a number of objects ("widgets") to the screen, for example text and buttons.  A subsequent blog will describe progress on touch related functions.

Google Arts and Culture

One of the things I miss most during the Coronavirus lockdown is my visits to London Art Galleries.  A very good substitute is Google Arts and Culture GAC which provides access to the finest art from all over the world.
For some galleries, mainly in London, I have my own set of pictures, descriptons and opinions in my slideshow app.  It is interesting to visit these galleries in GAC and look at see which exhibits they highlight.
For the galleries I visit most often, Tate Britain and the National Gallery, GAC provides a street view virtual tour which includes stops at particular works.  The descriptions provided on these works are somewhat superficial but they also contain links to other works on the same theme, with similar subjects, by the same artist etc.  Linked works may be at other galleries, which can then be explored as well.

MSGEQ7 Graphic Equalizer with OLED Display

Elektor magazine gives a continual stream of great ideas for electronic projects.  About 9 months ago I saw a project which utilises MSI MSGEQ7 chips in a 7 channel spectrum analyser / graphic equaliser.  This little chip is ideal for creating a bar display showing an audio frequency response.
I tried it out with a Neopixel display and a small OLED screen.

Design

Hardware: I initially intended to use a NodeMCU ESP8266, but the software library appears not to support ESP8266, so I moved to an Arduino nano.  Neopixel and 0.96" OLED displays were used.
Input: Initially PC line out used for testing, L and R channel signals were combined. Final solution needed to work on hi-fi using RCA or Digital audio Jongo output.
Software: Nico Hood's MSGEQ7 is principal Arduino library providing the necessary functionality.
Output: Initially console output used to understand MSGEQ7 values, then dots, then bars were used for each channel.

Circuit

Nico Hood provides a lovely informal circuit diagram for the MSGEQ7.

The chip provides 7 values each time the input is sampled and these are obtained on the output pin using the strobe pin.  Frequency ranges are defined  using a 200k resistor and 30pF on pin 8.

Three pins were connected to the arduino nano: Strobe (D2), Reset (D3), Output (A0).  A further pin (D9) on the arduino was used for a neopixel display.

Testing

Nico Hood's library demo program worked well and I was quickly able to see what values are being output by showing them on the serial monitor.  There is a "noise" level of about 16 (in 8 bit value) which I subtracted.  I then scaled the remaining values and  translated them to dots on the Neopixel and got a fast responsive display.  I then replaced dots by bars and put caps on the bars.  To avoid excessive flicker on the display I only sent changes each cycle rather than re-displaying the entire screen.

The OLED B&W (£5) display provides some fine detail but it isn't as interesting as the Neopixel colours.  A 0.96" colour display is about £15 - a bit high for a little circuit.  OLED screen updates are responsive and look convincing.  

RCA analog and digital output can also be used.  To use the digital output I need to convert the signal to analog, I purchased a PROZOR 192KHz DAC convertor for about £5 to do this. Pure Jongos provide both formats so I can attach a "VU meter" at the same time as connecting Jongo to Hifi.