Organic display, part 2

02.06.2012 21:06

Here is the follow up to the organic LED display I was reviewing back in February and my recently manufactured Arduino shield for it.

There is not much more to tell about the hardware. I basically went with the design I described in my first post. Display controller is powered from the 3.3 V line supplied by Arduino while the display itself uses a LM2703 step-up converter connected to Arduino's 5 V supply. This micropower switcher is sufficient to power the OLED array, but the reference design proved to be a bit slow to react to fast changes in display current. For instance, with a checkerboard pattern on the display, the supply voltage will drop for almost 1 V when display scans the high-brightness areas and this causes some visible shadowing. If you look closely at the photo below, you can see that the centers of white squares are darker than the corners.

Arduino OLED shield showing a checkerboard pattern

The reference LM2703 design I followed uses a single ceramic 4.7 µF capacitor at the switcher output, so this problem could most likely be fixed by adding some larger output capacitors.

VDDH supply when displaying a checkerboard pattern

Voltage drop on the VDDH line when displaying the 45° checkerboard pattern.

From the software side, SEPS525 controller turned out to be an interesting challenge. Densitron's datasheet warns that you need to correctly program the controller (like pixel driving current and timings) before turning on the high-voltage supply or risk damaging the display. This is somewhat of a gamble, since the SPI interface only allows for one-way communication and except for turning the display on you can't know whether you actually managed to set up SPI correctly. In my case this resulted in a few mishaps where the switcher's coil loudly complained about currents that were probably well beyond display's maximum ratings. Luckily, the display appears to have survived all of them.

SEPS525 datasheet is quite long and vague in places, but once you figure out the SPI protocol and set up the configuration registers correctly the use is quite straightforward: you give it a drawing area (either full screen or a part of it) and burst the pixel data in row-major order while the controller takes care of incrementing the framebuffer pointer and line wrap. Display supports either 6/6/6 or 5/6/5 RGB color formats, however I'm not aware of any microcontroller SPI peripheral that is actually capable of transmitting 18 bit words through SPI. Arduino's certainly isn't, which means you must either sacrifice 2 bits of color depth or bit-bang the SPI protocol which is horribly slow.

Talking about speed, bursting the whole screen worth of data from ATmega328 using hardware SPI through Arduino's library takes around 100 ms. If you want to do some calculations on the pixels (like the checkerboard pattern above) this might quickly become 1000 ms, so don't expect high frame rates. Another problem is that the framebuffer takes 40960 bytes, which is too much to store even a single full-screen image in ATmega328's 32 kB flash. I cooked up a simple RLE-compressor for the example you see below, which approximately halves the size of the stored images for the price of some extra CPU cycles (drawing this particular one takes 180 ms).

Arduino OLED shield showing Preening RD by MochaDelight

Preening RD by MochaDelight

Hardware design and software is available under CC-BY-SA 3.0 and GNU GPL 3 licenses respectively. The repository below contains a SEPS525 driver and an Arduino sketch that demonstrates its use with the three push-buttons that are also on the shield:

I have a few of these boards extra. So if you're interested in getting a bare PCB or a kit, please let me know. Be warned though that soldering the OLED connector makes for a good exercise in precision SMD soldering.

Posted by Tomaž | Categories: Digital


Kudos. :D

Posted by brodul

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