8 to 256 decoder

You actually want 256 outputs? Where the heck are they all going?

I think it's going to turn out to be pretty situational. If the decoder is very slow, you could use some kind of serial SR / micro thing. If it has to be fast and one chip, you could use programmable BGA-packaged logic. And you could always use a whack (2^4 = 1 whack) of 74HC154s, plus another to decode. At $3.25 USD total that one will be hard to beat price-wise.

As Spehro points out, it's hard to imagine needing all 256 decoded outputs in one place, much less in a lot of diverse places. That's a huge hassle to route around your PCB etc. Typically you would pipe the

8 address lines around and let whatever boards/subcircuits need some of the decoded outputs decode only the combinations it needs. Since the 80's the preferred way of doing that decoding (tied in with enables and clocks and whatever else is relevant) is PAL's or GAL's, today you would probably do it with whatever programmable logic you have there already (FPGA's, CPLD's, etc.)

The early 70's way would be 16 74154's decoding the high 4 lines, each of these decoded outputs enabling one of 16 74154's decoding the low 4 lines.

But even in the 70's the 74154 was a bit ungainly, much more PCB-space efficient was to use more of the smaller-packaged 74138.

Tim.

True, however the wide SOIC-24 isn't quite as gross as the 600-mil wide DIP-24, depending on design rules etc. TSSOP HC138s would be more compact.

Best regards, Spehro Pefhany

--
"it\'s the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

Ah.. the venerable 74138. I remember using them to decode the bus address on ISA cards. Still use them now to decode the address lines on microcontroller boards though it is getting less common since most things have gone serial (either I2C, SPI or UART).

I had to do this on a recent project. I just brute-force coded a 256 case, case statement in Verilog and programmed it into the Xilinx FPGA. It worked like a "champ".

Hope this helps, Tom

Hello,

The reason why I need the 8-256 decoder is the following: I have an IC module (which has ~200 I/Os). I want/need to bypass its regular IO routing in order to test the IC module's IOs for open/short faults. Hence, I need to be able to test IO nb 1, then IO nb 2.... The only way that I can imagine doing this, is to multiplex all IOs with switches. Now, something must control those switches, rigth ? Well, that's were the decoder comes into the picture. I need a [8to256 decoder] in order to provide control signals to all the switches in the giant switch matrix, so I can change the active IO (of the IC) that is being tested.

If anyone has an idea, that involves reducing my IO count, I am all eyes :-).

Regards,

-Roger

I just got a brillant idea (at least I think I did) to reduce my multiplexing logic. Instead of having 256 SPDT switches (2 position swithes) with their respective 256 control signals, I can have 256 *SP3T* switches (3 position swithes) with their respective 512 (each switch requires 2 control signals) control signals. BUT since a SP3T has 2 control signals, instead of 1, I can use 2 sets of 16 signals to control the SP3T switches (16x16=256). This way I can emulate a 256 SPDT switch matrix; I can't take full advantage of all 3 inputs of the SP3Ts, but that is of no consequence. Hence I only require 2 4to16 decoders.

I think my reasonning is sound. (Actually I got inspirred from somebody else's post. Gotta love ngs!)

Regards

-Roger,

Which you'll make out of ten 2to4 decoders? ;-)

I've used the 74HCT259 chips for similar stuff (link below).

Get a CPLD or FPGA withe enough pins and code it.

-- Uwe Bonnes snipped-for-privacy@elektron.ikp.physik.tu-darmstadt.de

Institut fuer Kernphysik Schlossgartenstrasse 9 64289 Darmstadt

--------- Tel. 06151 162516 -------- Fax. 06151 164321 ----------

You can get 16 to 1 multiplexer analog switches, like the CD4067, which can then be hierarchically staged. The bone-head way to go would be 17 of those, arranged in the obvious way. 8 address bits will then give you a connection to one of 256 input/output pins.

Since it is a 2 way connection, you can do whatever you want through it (pretty much, although they are frequency limited, and have some internal resistance).

--
Regards,
  Bob Monsen

Science without religion is lame, religion without science is blind.
    Albert Einstein, "Science, Philosophy and Religion: a Symposium", 1941

That's why they invented JTAG.

--DF