Re: x,y addressable LED matrix
From: John Fields (jfields_at_austininstruments.com)
Date: 09/04/04
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Date: Sat, 04 Sep 2004 13:07:01 -0500
On 4 Sep 2004 09:46:12 -0700, egreene@usc.edu (Ernest Greene) wrote:
>I am currently using a custom built 64x64 LED display in my research
>on visual perception. I am contemplating the feasibility of a
>second-generation device that avoids some of its limitations. I would
>like to have each of the 4096 LEDs be individually addressable and
>able to be toggled on or off.
>
>One option might be to use linear arrays that have a common ground
>(row) connection but which allow input to each element of the array
>from the source. But in my search for stackable linear arrays, thus
>far I have not found any that appear to be suitable for building a 2-D
>matrix. Does anyone know of a suitable array?
>
>With respect to the control logic, it may be not be practical to
>assemble the device using standard chips. I am wondering whether it
>is feasible to custom design a chip that has the needed latches and
>drivers, and can be accessed from a data buss using a binary
>addressing code. If a number of these control circuits could be put
>onto a chip, one could avoid the need to have a chip for each LED.
>Are we getting into the domain of VLSI, and do the number of contacts
>of a VLSI chip provide a limit to the clustering?
>
>I am way out of my depth here, and would appreciate comments how the
>goal might be accomplished.
---
A common scheme is to arrange the LEDs into a Cartesian 64X64 array
with, say, all of the LED anodes in each row wired together in that
row and all the cathodes in each columm wired together in that column.
Here's an example of a 4X4 array wired that way:
+-[LED>]-+ +-[LED>]-+ +-[LED>]-+ +-[LED>]-+
| | | | | | | |
ROW3> ---+--------|--+--------|--+--------|--+ |
| | | |
+-[LED>]-+ +-[LED>]-+ +-[LED>]-+ +-[LED>]-+
| | | | | | | |
ROW2> ---+--------|--+--------|--+--------|--+ |
| | | |
+-[LED>]-+ +-[LED>]-+ +-[LED>]-+ +-[LED>]-+
| | | | | | | |
ROW1> ---+--------|--+--------|--+--------|--+ |
| | | |
+-[LED>]-+ +-[LED>]-+ +-[LED>]-+ +-[LED>]-+
| | | | | | | |
ROW0> ---+--------|--+--------|--+--------|--+ |
| | | |
| | | |
COLUMN0>----------+ | | |
| | |
COLUMN1>----------------------+ | |
| |
COLUMN2>----------------------------------+ |
|
COLUMN3>----------------------------------------------+
Using this method it's easy to illuminate any single LED in the array
by proving power to that row in which the LED resides, and providing a
ground to the column in which it resides. That way, instead of 4096
separate connections which you have to make, the number is reduced to
128.
If you're only interested in accessing a single LED at a time, then
something like an array of eight 4-to-16 line decoder/demultiplexers
would work well, and if you used high-efficiency LEDs there would be
no need for LED drivers, since the demuxers could do the job.
Doing it that way would require only 6 bits of control for the row
demuxers and 6 bits for the columns, for a total of 12 bits, and your
chip count would be 9 if you wanted to use easily available, common
CMOS parts. (four HC4514s, four HC4515s, and one HC139).
With only 12 bits of control required to access any LED in the array,
it would be easy to place the array under software control and allow
you to display whatever you wanted to on it.
We'd be interested in working with you on this and building something
for you, and if there's any interest on your end please feel free to
contact me via email.
Thanks,
John Fields
President,
Austin Instruments, Inc.
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