Re: Controlling hundreds of LEDs

From: Si Ballenger (shb*NO*SPAM*_at_comporium.net)
Date: 12/21/04 

Date: Tue, 21 Dec 2004 23:52:05 GMT

On Tue, 21 Dec 2004 21:13:40 +0000, Danny T <danny@nospam.oops>
wrote:

>Si Ballenger wrote:
>
>>74HCT259
>
>Was just looking at the data*** for this - I think it'll do exactly
>what I want! Will need a lot of them hooked together though, and with
>the 3 input pins of some being attached to output pins of others, it's
>going to be a bit fiddly on the software side!
>
>Can I just make sure I understand this correct...
>
>Using the 3 input pins, I choose which output I'm modifying (a binary
>value of 0 to 8), and send the input on the input pin. Then I clock(?)
>the "latch" pin, which copies the input state to the output pin I
>addressed, and then holds it at that until I change again?

In the latching mode (used to control the individual LEDs), you
use the A0, A1, and A2 input pins to select the chip output pin
you want to set high or low. You set the D data pin on the chip
at 0v for the chosen output pin to output 0v, or set the D pin to
+5v for the output pin to output +5v. Then you clock the LE pin
with +5v to latch the output pin in the desired state (0v or
+5v). The same A0, A1, A2, and D control lines can be connected
to all the 259 chips. You clock the LE input on the individual
chip you want to control. The As and the D can be done using four
of the output pins on the parallel port. The other parallel port
pins can be used for clocking theindividual chips You can use two
of the 259 chips in the demultiplex mode to clock the individual
259 chips used in the latching mode. Using the eight parallel
port data pins and 17 259 chips (2 in demultiplex and 15 in latch
mode), you could individually control 120 LEDs. The output page
below shows the latching mode schematic, and the ppswitcher3 page
shows the chips in the demultiplex mode. If you used the parallel
port control and data pins, this number could be multiplied many
times.

http://www.geocities.com/zoomkat/output.htm
http://www.geocities.com/zoomkat/ppswitcher3.htm

Using parallel port data pins 0, 1, and 2 to control the As, pin
3 for D, and pin 4 for the LE clock on a chip, here are the byte
sequences to turn Q7 on and off:

00001111 sets all As high (Q7) and the D high for +5v output
00011111 clocks the chip LE pin high
00001111 drops the LE pin low to latch Q7 at +5v output

00000111 sets all As high (Q7) and the D low for 0v output
00010111 clocks the chip LE pin high
00000111 drops the LE pin low to latch Q7 at 0v output

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