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Help Ram connections !!!!

Discussion in 'Electronic Design' started by BSL, Sep 18, 2007.

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  1. BSL

    BSL Guest

    Hi Guys , i have to use a 8 bits SRam to fix a 4 bits design , on the
    new ram i will have 4 data lines not use ,so Is it safe to leave
    these pins floating ??? can the Ram be unstable if they're not pulled
    up or down ???

    Thanks
     
  2. Eeyore

    Eeyore Guest

    Pull them one way or the other (it doesn't matter which).

    Inputs left open/floating may cause various problems.

    Graham
     
  3. BSL

    BSL Guest

    Would it be a good idea to tie the not use ones with adjacent use
    ones ??? let's say d1 tie to d2 !!!
     
  4. Eeyore

    Eeyore Guest

    You'll double the input capacitance that way which might be problematic.

    Graham
     
  5. BSL

    BSL Guest

    The problem i have is that i must do a small adapter boards and don't
    want a put many resistors on it !!!
     
  6. John Fields

    John Fields Guest

     
  7. John Fields

    John Fields Guest

     
  8. Eeyore

    Eeyore Guest

    If you're not driving it very fast, then that may be fine.

    Why the problem with resistors though ? Couldn't they be small surface mount
    types ?

    Graham
     
  9. BSL

    BSL Guest

    :) yes smt are small !!, just wanted to put less parts as possible ,
    btw the max speed of the cpu is merely 1 MHz , it is an old slow
    system , the ram is a lo power used to keep status while power off , a
    memory cap is the power keeper !!!!
     
  10. Guest

    Just ground the unused bits. No resistors needed.
     
  11. krw

    krw Guest

    Why does it matter? Are the pins Bi-Di (then read would be a
    problem)? CMOS inputs shouldn't matter where they're tied to as long
    as it's inside the rails and not in the threshold region. I wouldn't
    short inputs together because it would increase fan-in and power
    consumption (slightly) for no advantage.
     
  12. BSL

    BSL Guest

    So Keith , do i leave the 4 unused data bits floating ???
     
  13. Rich Grise

    Rich Grise Guest

    No, you never ever leave any chip input floating. In this case, since
    it's RAM, strapping the pins to either Vee or Vcc would be OK, as long
    as you perform a write to each byte before trying to read it - that way,
    you won't blow out a pin trying to drive it against a short. You might
    say that these pins are "don't care", but who knows what happens to the
    rest of the chip when one of the output drivers gets blown?

    But in most cases, I recommend pullup resistors - in this case, probably
    something in the range of 22K ~ 100K, since it's CMOS and doesn't draw
    any significant current.

    Hope This Helps!
    Rich
     
  14. krw

    krw Guest

    Never! A floating pin often wants to go towards the middle and it
    may draw lots of current. Tie them one way or the other.
     
  15. John Fields

    John Fields Guest

    ---
    Because if the chip powers up in WRITE and you don't know what'll be
    in the cell array, You could short the I/O pin to either Vcc or GND
    with the I/O pin in the opposite state.

    Also read Rich Grise's post for more detail.
    ---
    ---
    No, WRITE would. See above.
    ---
     
  16. John Fields

    John Fields Guest

     
  17. krw

    krw Guest

    Now I see, assuming Bidirectional I/O. I got the impression that the
    RAM in question had separate I/Os; ground the gazintas and float the
    gazoutas.
     
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