Re: how to control LED array?

"Lord Garth" <LGarth@xxxxxxxxxxxx> wrote in
news:lKWae.354$m85.305@xxxxxxxxxxxxxxxxxxxxxxxxxx:

>
> "Michael Noone" <mnoone.uiuc.edu@xxxxxxxxx> wrote in message
> news:Xns9642BF7FE98BEmnooneuiucedu127001@xxxxxxxxxxxxxxxxx
>> John Fields <jfields@xxxxxxxxxxxxxxxxxxxxx> wrote in
>> news:07bo615abq9oqkfn99no1gap9m1sai0fb7@xxxxxxx:
>>
>> > On Sun, 24 Apr 2005 22:52:26 GMT, Michael Noone
>> > <mnoone.uiuc.edu@xxxxxxxxx> wrote:
>> >
>> >>Hi - I'm working on a project to make an 8x8 LED array. The idea is
>> >>that the anodes in each row are tied together, and the cathodes in
>> >>each column are attached together, giving me 16 pins which I can
>> >>control 64 LEDs with. But now the only problem is making sure they
>> >>get the right current/voltage? My plan is to only have one column
>> >>turned on at any given time, but then any number of the 8 LEDs in
>> >>that column could be on at that time, so things get a little
>> >>difficult. I could apply 0 and 5V then just have a current limiting
>> >>resistor on each LED, but that would be cumbersome and awkward. My
>> >>end goal is to be able to drive this with 16 pins from a
>> >>microcontroller. Hopefully I'm being somewhat clear about what I'm
>> >>asking. Does anybody have any suggestions as to how I could do
>> >>this? Thanks so much,
>> >
>> > ---
>> > Do you want to control only one LED at a time or any number of LEDs
>> > up to and including all 64?
>> >
>>
>> Hi Mr. Fields - I'd like to control as many at a time as possible. My
>> plan was to control one column at a time, and cycle through each
>> column about 100 times per second. Does this seem like a good path to
>> take, or is there a better way to accomplish this? I thought that
>> this method would allow
> the
>> board to be fairly simple (anode traces on the top of the board,
>> cathode traces on the bottom), and would be fairly easy to control
>> (just 16
> control
>> lines). Thanks,
>>
>> -Michael. J Noone
>
> As you cycle through each column, only one LED per row is on at any
> given time.
> You would put your limit resistors in the row line after an
> appropriate driver.
I can't believe I never realized that. Now it seems so simple and
obvious - but before I was struggling trying to figure out how to limit
current.

> You are multiplexing the array. I suggest you use a 1 of 8 demux chip
> such as the
> 74138. The chip is fed by a binary counter. You clock the counter
> with 1 bit from
> your port.

Correct me if I'm wrong though, but wouldn't that require an extra two
chips? I had been thinking that if I wanted to decrease control lines I
could use an 8b shift register with parallel output to drive them.

> This saves IO lines. You can create a clock on the Data
> Strobe* pin 1 of
> the parallel port. Any clock rate over 16 or so pulses per second is
> no necessary as the
> eye will blend the image. Faster is not better as it gives you a
> shorter time slot in which
> to write the active LEDs. You can read the counter overflow on one of
> the input bits
> such as paper empty.

I should mention that I don't plan on using a computer to drive this
array - my plan was for it to be entirely microcontroller driven.

> There are also specialty chips for doing this very thing to sets of
> such displays. Maxim
> comes to mind, I think their chip, in no decode mode, will handle 128
> lamp columns.
> That is 16) 8x8 arrays.

Do you have any idea of a part number for such a chip? I looked through
their LED display driver chip section, but most all of them looked more
like they were designed for controlling digits, not arrays. Also - any
idea if any would be able to do PWM? Does this chip perhaps look right?
http://pdfserv.maxim-ic.com/en/ds/MAX6964.pdf

Thanks,

-Michael Noone
.

Relevant Pages

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