piclist 2005\01\17\183722a >
Thread: LED Matrix
www.piclist.com/techref/displays.htm?key=led
picon face BY : Hernán Freschiemail (remove spam text)



Sorry everyone for starting a new article, but I'm new to this list
and I'm just answering to some archived questions.
Regarding to the matrixes, I've always noticed that most of those led
signs are usually 5x7 characters. I have no idea why. even 44780-based
LCDs
When I was 13 or so, I made a program in QBasic which controls the
parallel port and drives a 5x5 matrix. There are many approaches to
this, but at the time i used this one: The columns are driven by the
port bits, and the row is selected with a 4017 (this IC does a
"rrf"-like operation on its outputs, that is, it selects 1 of 10
outputs, in sequence). with this approach you need 5 bits for the
columns and 2 bits for the rows: one pulse strobes the IC and switches
rows and the other pulse resets the output to 1 -- in case it losses
sync, which it does quite often).

But there is a more interesting approach, and I think this is the
reason why characters are usually 5x7: If you use a 3-to-8 line
decoder, such as the 74HC137, you use 3 bits (upper 3, for example)
for selecting the row, and you have 5 bits left for your character!,
so using a simple table lookup which looks like, say:

00000100
00101010
01010001
01110001
10011111
10110001
11010001

you can get a nice "A" character: notice the first 3 characters are
just a binary count, and the other 5 are the data to be sent. first 3
bits are sent to the 74HC and the rest to the leds.
if you sweep occidental-handwriting style (left-right, top-down) you
can have more displays and room for whole words...). but i'm guessing,
if the line is too long, you may need latches to avoid flickering.

take a look at some of those ticker-style 1-line led signs, especially
popular in the late 80s, and you will notice:
characters are monospaced, 5x7
sweep is top to bottom (which may consist with a 3-to-8 decoder)
when the text is sliding, it looks kind of slanted (i guess that
happens because of the sweep)

to reduce flickering, we can steal some ideas from the TV: many of you
know about interlaced scanning. first you draw odd lines, and then
even lines. this would help in reducing flicker and keep brightness.

So what do you people think? is this a good idea?

Hernán Freschi
<25a1a0990501171537b424497@mail.gmail.com> quoted-printable

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