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Problems using Dallas High Speed Micro (89C420) with 74F373 address latch

Discussion in 'Embedded' started by bob, Jan 16, 2005.

  1. bob

    bob Guest

    Hi,

    I'm building an embedded controller that uses the Dallas 89C420 ultra
    high speed micro with an external 32K SRAM. There's no external
    program memory - I'm using the internal flash for that.

    Right now it works just fine at 15Mhz with a 74HC373 address latch
    and a 70ns SRAM, however I want to run in clock doubled mode at 30Mhz.
    For this I figure I'll need to use a 74F373 latch instead of the HC
    part.

    The trouble is that it doesn't work, even at 15Mhz 1X (i.e. not
    doubled) with the 74F part. Running a little memory diagnostic that I
    wrote, I get random SRAM errors that _don't_ occur with the 74HC
    address latch.

    Are there any circuit changes that need to be made when using the
    74F373 instead of the HC part? Am I going to need series damping
    resistors or pull up resistors on the F373s outputs?

    There are lots of Dallas appnotes for the 320, 550 and 420 family
    that talk about replacing the 74HC latch with a 74F part, but I
    couldn't find one actual schematic where they did it!
    Thanks much,
    Bob Armstrong
     
    bob, Jan 16, 2005
    #1
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  2. bob

    Eric Smith Guest

    You don't need pullups unless you're tristating it. You need series
    termination resistors if it's driving a transmission line. If it's a
    short point-to-point trace from the 373 to the RAM, you should't need
    them.

    But you need *really* good bypassing on the supply lines to 74F-series
    parts, just like on 74S.

    I'd suggest using 74AC, 74FC, or 74ABT. Still need good bypassing,
    as is true for any high-speed parts, but not as finicky as 74F in
    my experince.

    Eric
     
    Eric Smith, Jan 16, 2005
    #2
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  3. The F373 does have TTL levels, whilst the HC373 has CMOS thresholds ?

    You could try the newer Dallas 89C430/440/450, and also try a
    CMOS version of the F373 (as that is really a dinosaur), like the
    P74LVC573 from Philips (etc) ?
    -jg
     
    Jim Granville, Jan 16, 2005
    #3
  4. bob

    bob Guest

    Thanks for the quick answers, guys.
    Absolutely. Correct me if I'm wrong, but this shouldn't be a concern
    for CMOS outputs driving a TTL input (i.e. the 89C420 driving the
    74F373) because the CMOS voltage swings are much wider than TTL
    requires. It _is_ an issue for TTL outputs driving CMOS inputs (i.e.
    the 74F373 driving the 62256 SRAM) because the TTL logic 1 level is
    pretty low by CMOS standards. BTW, Eric - that's the logic for adding
    pullups to the F373 outputs - to drag the TTL high levels up a little
    higher. I tried adding 4.7K pullups to the outputs of F373, and it did
    improve the output levels but it didn't fix the problem.
    Sorry - I must have missed something - how would that help? I'd
    still need a faster address latch, right?

    Thanks again,
    Bob Armstrong
     
    bob, Jan 16, 2005
    #4
  5. bob

    bob Guest

    Thanks for the quick answers, guys.
    Absolutely. Correct me if I'm wrong, but this shouldn't be a concern
    for CMOS outputs driving a TTL input (i.e. the 89C420 driving the
    74F373) because the CMOS voltage swings are much wider than TTL
    requires. It _is_ an issue for TTL outputs driving CMOS inputs (i.e.
    the 74F373 driving the 62256 SRAM) because the TTL logic 1 level is
    pretty low by CMOS standards. BTW, Eric - that's the logic for adding
    pullups to the F373 outputs - to drag the TTL high levels up a little
    higher. I tried adding 4.7K pullups to the outputs of F373, and it did
    improve the output levels but it didn't fix the problem.
    Sorry - I must have missed something - how would that help? I'd
    still need a faster address latch, right?

    Thanks again,
    Bob Armstrong
     
    bob, Jan 16, 2005
    #5
  6. What do you see with a 'scope: does the part meet setup/hold
    at the specified voltage levels?

    Early 74F parts were notoriously noisy: you may need .01 || 0.1
    uF decoupling caps just for that IC.

    You don't mention what ram you are trying to drive with the F373.
     
    Nicholas O. Lindan, Jan 16, 2005
    #6
  7. It matters for noise margin.
    They may have different noise characteristics. Newer devices and all...

    Most 80C51 variants have a cut/paste warning from older devices, that
    ALE may have noise during multiple address bit transistions.
    I have never seen this in a real design, but on loaded devices it
    could happen.

    You could try a series resistor PAD in the ALE to the F373, to slow the
    edge slightly, and shift the threshold - but I would avoid the F373....

    -jg
     
    Jim Granville, Jan 16, 2005
    #7
  8. I would suspect ground bounce as a potential problem. I haven't
    experienced it personally, but have had reports of such problems
    from another developer who was using similar latches on an IDE
    interface. In his case I think the solution involved some changes
    in PCB layout.
    Mark Borgerson
     
    Mark Borgerson, Jan 16, 2005
    #8
  9. bob

    bob Guest

    I would suspect ground bounce as a potential problem.

    Can't see any ground bounce on the scope, but that doesn't prove that
    it isn't there :) I did try tacking a 1uF tantalum directly across
    the F373's power pins and that didn't help.

    To answer some other questions people have posed - it is a PC board
    (not wire wrap) although only a two layer one so there is some
    inductance in the power supply leads depending on how good the routing
    is.

    The SRAM chip in question is a 62256 (more exactly, a SEC
    KM62256CLP-7L).

    It works fine with the HC373 latch, but not with a F373 or ACT373
    part. I made no other changes except swapping the latch - the clock
    speed is still 15Mhz. I would have liked to try an AC373, but I don't
    have one handy.

    Thanks again,
    Bob
     
    bob, Jan 16, 2005
    #9
  10. bob

    Steve Taylor Guest

    It wouldn't - the minimum impedance will have been at around 1Mhz or
    lower. You need to bypass the 1uF with a 10n ceramic - minimum impedance
    ~100Mhz.

    Steve
     
    Steve Taylor, Jan 17, 2005
    #10
  11. bob

    Paul Probert Guest

    If you look at the outputs of the 373 on a high speed scope (with a very short scope
    probe ground lead, please), I bet you see ringing.
    If so, you need to use some form of line flattening. The easiest is probably series 100
    ohm resistors right at the outputs. This only works if all your loads are close together
    on the far end of the line.

    Paul Probert
     
    Paul Probert, Jan 18, 2005
    #11
  12. bob

    bob Guest

    Thanks to all the people who offered suggestions. In the end I
    never did figure out the cause of the problem - it proved to be more
    expedient to just use a fast cache SRAM (access time around 35ns)
    together with a slow latch (74HC373 - tpd around 35ns!) to get a 70ns
    memory system.

    7C199 35ns 32Kx8 SRAMs are available from Jameco, qty 1, for less
    than $2 each. That's the same or less than what a 62256 costs. The
    7C199 is a power hog compared to the 62256 but that's not an issue for
    this application.

    Anyway, thanks again,
    Bob Armstrong
     
    bob, Jan 19, 2005
    #12
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