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Minimums for Logic families' rise and fall times on the clock pin

Gary Dowdy
Prodigy 50 points

What logic families are sensitive to slow rise and slow fall times?  I find that HC/HCT needs at least a 500ns rise/fall time in an old Signetics user's guide that I have.  Does TTL, 4000 series CMOS, and most newer CMOS technologies have specs for minimum rise/fall times?  The follow-up question is, is the slow rise or slow fall time on the clock destructive physically, stressing the part (overheating due to latch-up, metastable, linear region operation, etc), or is the spec there to maintain data integrity and predictability.  The system level question is, can I use a sinusoidal shaped clock signal for testing circuits prior to shipping?

  • 0
    TI__Guru 62240 points

    Hi Gary ,

    Most of the logic families have the recommended spec on the transition rate but schmitt trigger devices are built to handle slow signals like sinusoid on its input .

    There are issues with slow signals which are better explained in the app note attached . Usually , output oscillations , higher power consumption which can be damaging to the device in extreme cases .

    7510.slownfloatingCMOS_scba004c.pdf

  • 0
    Prodigy 50 points
    Thank you for this response. You are answering the question at the technical level that I need. I am seeing the destructive mechanisms in the shoot-through current, through current, when the stacked transistors are both on (partially on), creating a direct path, power to ground. A follow-up question --- The SCBA004C App Note/White Paper is specific to advanced CMOS (AC/ACT, AHC/AHCT, ALVC, CBT, CBTLV, HC/HCT, LVC, LV/LV-A) and BiCMOS (ABT, ALVT, BCT, FB, GTL, and LVT). What about 4000 series CMOS (CD4000)? If CMOS devices, in general, have a rise/fall time minimum in the datasheet, then I am covered for CMOS. The question I would have for 4000 series CMOS and TTL (54/74, LS, S) would be, what are the rise/fall times that one would design-in a Schmitt-trigger input. What rise and fall transitions, would I design in a 54/74LS14, 54/74S14, or CD40106? Another way to ask the question is --- is there a shoot-through scenario in the bipolar TTL architecture, and if so, what is the slew rate that I would be ok not being destructive to the device?
  • 0 in reply to Gary Dowdy
    TI__Guru 62240 points
    Hi Gary ,

    I am unsure of the transition rate for bipolar or CD4K parts unless mentioned in the datasheets .
    I would see similar devices in the same family if I can find one . Also , there are no requirements for transition rate for Schmitt trigger parts since they are required to handle slow , noisy signals .