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8 to 256 decoder

Discussion in 'Electronic Design' started by Roger Bourne, Feb 23, 2006.

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  1. Roger Bourne

    Roger Bourne Guest

    Good day to all,

    Nowadays, in 2006, what would be the best approach to make a 8 to 256
    decoder?

    -Regards
    Roger
     
  2. 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.
     
  3. Tim Shoppa

    Tim Shoppa Guest

    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.
     
  4. 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
     
  5. Guest

    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).
     
  6. Bob Monsen

    Bob Monsen Guest

    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
     
  7. Deefoo

    Deefoo Guest

    That's why they invented JTAG.

    --DF
     
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