Seiko Epson LCD model ECM-A0526, Interfacing


Bought this funky little panel for $11 on AllElectronics. John P has a very helpful timing diagram available for this LCD panel, along with pictures of his setup using a PIC microcontroller and DRAM module. Since I'm lazy cheap a paragon of godlike infallibility, I decided that as long as I was just building a quick little circuit on a breadboard to try it out with, I'd forgo the memory entirely and just drive it directly from a microcontroller for now. Eventually, I'll want some kind of crazy FPGA contraption to blast an image at it from a SRAM at a fast framerate, but let's worry about that later.

A 12V* power supply, 7805 regulator, PIC18F4550 running with a 48MHz oscillator, and a crude little negative-volts generator for the -17V or so (contrast bias) the LCD needs, based around a LT1617 switching converter chip. Last but not least, a MAX232 chip to load code (and maybe, ultimately, the images/text...although this chip also has a builtin USB transceiver) onto the microcontroller from a PC serial port.

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The LT1617 is visible as the little black speck in the middle of the Veroboard at top right. I chose this chip because they're cheap as free ($2 direct, compared to $6.50 for MAX749 on Digikey...heck, that's half the cost of the display itself) and Linear sent me samples. It's in a 5-pin SOT-23 package though; get your magnifying glass handy if you're not used to working with surface-mount stuff. I found the four corner pins will align nicely with the copper pads on the underside of the Veroboard; once secured, you can bend the nuisance 5th pin (GND) up and solder to it directly.

* Ghetto bench supply from spare parts. 12V when the brightness on the backlight is turned way down, and considerably less than that when the brightness is cranked and the backlight inverter is trying to draw half an amp out of it.

Playing around:
Displaying an image directly from the micro's code memory. (This is what happens when you let chicks dress you up.)

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This chip only has 32KBytes of codespace (some of that's needed for actual code ;-), but 640*480 pixels is 38400 bytes, which is why the bottom of the top and bottom halves show as all black (reading from non-existant memory addresses). Bottom and middle, you say? Yep, that's a minor annoyance of this panel--in terms of shoveling data at it, it's divided into two logical 'screens'...the upper 4 bits of every byte write to one, and the lower 4 bits write to the other. Sadly, even at 48MHz controlling it with a PIC directly is a little slow (I pulled about 35fps here; there was still a bit of noticeable flicker). PIC code and a crude little image conversion script (*.raw to "DW  H'xxxx' ", written in Perl) are in this zip file...unfortunately, when wiring the LCD panel I think I flipped the D[7..0] lines around backwards, and so unflipped them in software rather than rewire the whole thing - if the results turn out funky for you, comment out the appropriate lines in the conversion script.(sorry, lazy...)

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Besides the painful 32Kbyte codespace limitation, this chip has only 2048 bytes of RAM...To see how it would work as a text-only display that could be updated in realtime, e.g. as a server process monitor, I decided to map the screen as an 80x25 character display (each character 8px wide by 16px tall, with some space left over at the bottom of the screen). That squeezes the memory requirements down to 2000 bytes to store the ASCII character at every screen position, but since this chip's driving the display directly, the characters have to be generated in realtime for every frame...additionally, since half of each byte out goes to the top screen and half to the bottom, you'd have to keep switching back and forth between two different characters every time a byte is written out (as you can see above, I didn't bother with that for this little test). Even just blasting raw bytes to it to display an image was borderline slow, all the overhead of creating characters makes things unacceptably so: flicker central. So, the next step will be to make up some kind of FPGA byte blaster / memory controller :-) Luckily, an 8x16 font table is pretty small (4k), so you could store several of them and select different fonts on the fly.

This zipfile contains the (ugly, nasty, ghetto) code for the test above. Currently, it just repeats the first line (putting code to re-index the data memory every line is just more instruction cycles I didn't want to spend), and only for half of the screen (similar excuse...see above). If I get around to making a proper memory interface, I'll fix up this code and post the end result here. Or of course, feel free to write your own :-)