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SX-Key failing with added subcircuit load? — Parallax Forums

SX-Key failing with added subcircuit load?

BebopALotBebopALot Posts: 79
edited 2006-02-08 18:53 in General Discussion
(New Subject)

Hey folks, here is my setup: I have the SX28 with two ports connected in output mode to (2) mosfet push-pull driver subcircuits toggling 18 volts from ABOVE my LM2394 voltage regulator fed by 18 volts DC (1 amp). When I had one push-pull subcircuit, the set up worked fine. When I added # 2, the SX Key programs the chip then just before or at the beginning of running I get an error "Cannot find SX Key on com 3". I assume I am running out of power - is the SX "browing out"? What is happening and how to reconcile.

MR. BBAL

Comments

  • BeanBean Posts: 8,129
    edited 2006-02-06 03:11
    You could try adding a cap right at the Vss and Vdd pins at the SX-Key connections.
    Something like 10uF to 100uF.
    Bean.

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  • NateNate Posts: 154
    edited 2006-02-06 17:57
    I have gotten this error message before, and it usually was caused by the SX-Key needing more power (amperage) than the SX circuit could supply (from my experence, the SX-Key will crash before the SX itself browns-out).· Since your problem happens when your second sub-circuit is attached, it sounds like the sub-circuit is draining so much power that there is not enough left for the SX-Key.· A short or mis-wiring perhaps?

    Try switching the two sub-circuits, does the error now appear when #1 is attached?

    Nate
  • BebopALotBebopALot Posts: 79
    edited 2006-02-07 05:53
    Hi Nate and Bean,

    Adding the capacitor didn't fix the problem. It appears that when 1 sub circuit is attached, sometimes the SX Key fails but when both subcircuits are attached the key fails all the time. What do I do? Tryt he system without the SX Key (just crystal).


    BBAL
  • Jon WilliamsJon Williams Posts: 6,491
    edited 2006-02-07 06:09
    If you can't disconnect the offending circuits for programming, you best bet -- as Nate points out -- is to get a beefier supply.

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    Jon Williams
    Applications Engineer, Parallax
  • BebopALotBebopALot Posts: 79
    edited 2006-02-08 17:23
    Say it "ain't so" Jon (cue dramatic music, horrible background weeping, wailing, falling to the knees clutching temples). I must be doing something wrong. I can't believe I need more than 1 amp..but what we beleive is often reconciled by REALITY.

    Seriously, what do you think about increasing the inductor value on the LM? I've got the typical connection at 330uh on the datasheet...www.national.com/ds/LM/LM2574.pdf. Or would u - increase the wallwart voltage or amperage feeding the LM?
  • SteveWSteveW Posts: 246
    edited 2006-02-08 17:32
    Could it be that the floating pins on the SX during programming cause the pushpull modules to go half-on, causing them to sink current from the 18V?
    4K7 pulldown resistors to ground on the appropriate SX pins would discourage this behaviour.
    (Post a schematic, or specs on the drivers, for better advice)

    Steve
  • gibbmangibbman Posts: 98
    edited 2006-02-08 18:53
    BBAL,
    Do you have a handy spot that you could insert an ammeter and see what the real deal is? With the data on the current draw, you can really see what's happening with the supply. Just a thought,
    Jim

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    In the end, it seems that it's all about getting the LEDs to blink....
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