Re: Multiplexing signals

On Sun, 25 Nov 2007 02:34:54 -0500, T wrote:

I don't know if I'm going down the right path here. I'm an electronics
hobbyist with no formal training.

I've got an Arduino board and I want to drive an 8x8 LED matrix. The
thing is there are 24 pins on the matrix.

Maybe each LED is a red/green pair with one end (anode or cathode) common?

Figured out that with six bits I could control all 24 lines if I could
multiplex it somehow.

Nope.

First, you need to be able to select any anode and any cathode. If it is
red/green, you could skip half of them and drive 16 lines (8 anodes and
8 cathodes) with 6 bits (3+3) or all 24 with 9 bits (3+3+3), but that
isn't really practical. You would only be driving one LED (or one red
and one green) at a time, so each would only be at 1/64th duty cycle.

LED matrices normally have one axis (row or column) multiplexed and
the other driven directly. E.g. a normal 8x8 with 16 lines (8 rows, 8
columns) would need 11 bits (8+3), with 3 bits driving a 3->8 line
decoder and the other 8 connected directly, resulting in a 1/8th duty
cycle.

For a red/green matrix, you could either use 19 bits (8+8+3) with 1/8th
duty cycle or 12 bits (8+4) with 1/16th duty cycle.

I believe what I'm looking for is probably in a 74LS type chip but what
I've run across so far takes eight bits and shoots out a serial signal.
Not what I need at all, I need something with a bit of smarts.

For decoding an 3- or 4-bit number to one of 8 or 16 lines:

137 (3->8, latching), 138 (3->8), 154 (4->16), 159 (4->16, open-collector).

.

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