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From: "Michael F. Coyle"
Subject: Re: I need an 8 to 256 decoder or equivalent
X-Newsreader: Microsoft Outlook Express 6.00.2800.1106
Date: Fri, 15 Nov 2002 23:32:33 -0500
NNTP-Posting-Date: Fri, 15 Nov 2002 23:33:07 EST
"David Jones" wrote in message
> I'm trying to use an 8 bit binary input to select 1 of 256 different
> LEDS(I'm using leds as an example). I know there exists a 4bit input 16
> output decoder but I don't know how to get 256 options using it with other
> IC's. How do I make the equivalent of an 8 to 256 decoder with the least
> discete components? any suggestions are welcome.
You would need seventeen 74xx154's. First, wire the bottom 4 address bits
(let's call them ADDR3 thru ADDR0) to the '154's address inputs A3-A0.
Then wire the top address lines, ADDR7-ADDR4 to the address lines A3-A0 of
the seventeenth '154. Wire each of its outputs (Y0-Y15) to an enable line
of one of the other sixteen '154's (CS1, let's say). Ground the other
Now when we apply an address to ADDR7 thru ADDR0, the top 4 bits will be
decoded; the decoded output causes one, and only one, of the remaining chips
to be selected. The bottom 4 address bits will be decoded by that chip to
produce a low voltage on one of its outputs. All the other outputs will be
high (unselected) as will be all the outputs of the other (unselected)
You would need room on your board for seventeen 24-pin chips, plus room for
your 256 outputs. If it won't all fit, you could split it up on several
boards, perhaps with a different high-address decoding scheme.
BTW, what actual loads are you trying to drive?
Hope this helps.
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