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74HCxx output behavior from long-term bus contention?

Adam Dixon
Prodigy 30 points

What would be the long term performance of +5V 74HCxx logic if two outputs in a single package have a bus contention condition?  I was looking at a lab PCB with an incorrect resistor population resulting in 100 ohms between adjacent outputs in a single HC package + one downstream HC input.  It has been running for months w/o issue, but this got me to wondering. 

Normal output drive current is +/-6mA from looking at several 74HCxx datasheets.  Scoping the downstream input shows +1.4V amplitude regions at 30% duty cycle when the two outputs drive opposing logic levels.  At +1.4V, there are no errant VIH crossings for the downstream input, so functional performance has been fine.  Should I expect to see a test failure if I let it run for several more months?  A curiosity for me.  Thanks!

  • 0
    TI__Guru* 97046 points

    Adam,

    You would be surprised at how much punishment our parts (in general) are subjected to and survive.  I'm afraid I don't have any particular numbers for this, but I can say that the datasheet values are usually band-guarded quite heavily.  If this part isn't overheating under your normal operating conditions, I wouldn't expect it to fail any sooner than usual. 

    If you could pull scope shots of each output and provide a partial schematic it would be helpful for me to reproduce these conditions.  I can't promise any miracles, but I can get into the lab and reproduce your situation with some of our parts and see how much torture they will take.