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TINA/Spice/SN74HC244: SN74HC244 Ibis Model Tpd Timing Issue.

HyungJe Woo
Prodigy 40 points
Part Number: SN74HC244
Other Parts Discussed in Thread: TINA-TI,

Tool/software: TINA-TI or Spice Models

Hello.

I`m simulating with a SN74HC244 ibis Model on ADS.

I found some issue unexpected.

here is issue.

if Input signal rate is fast(now simulating 25Mhz), the tR/tF of output become about 1ns with no cap& no load.

and input rate is 10MHz, the tR/tF of  output become about 5ns.

is it normal or do i have some fault with simulation?

I red there is nessasory time for Tpd(Tplh & Tphl) on datasheet.

and i know it is severe condition for sn74hc244 buffer. 

but i think It is abnormal to change tR/tF at buffer.  

here is my simulation  schemetic.

please let me know any solution or idea for understanding.

  • 0
    TI__Genius 16680 points

    Hi HyungJe,

    Looking at the model, I would expect around 2ns Tr/Tf. The ramp data for OUT_50 has 1.53ns Tr and 2.29ns Tf into 1000 Ohms.

    Since IBIS is modeling only the output characteristics, it shouldn't be dependent upon the input data rate at all, and there's no way to capture prop delay with IBIS either.

    Your schematic didn't get attached for some reason.

    Let me see if I can recreate what you've seen and get back to you.

    Regards,

    David Larkin

  • 0 in reply to David Larkin
    Prodigy 40 points

    hi David

    thank you for your reply.

    I attached schemetic again.

    as you said. i also thought there is no relation between input rate and output tr/tf.

    but result was not.

    please look at and let me know if there is any fault.

    Regards.

    Woo HyungJe

     

  • 0 in reply to HyungJe Woo
    TI__Genius 16680 points

    Hi HyungJe,

    I'm starting to build up my ADS test bench, but noticed something immediately in your setup. You're driving the switching signal into the IBIS input, and then taking the output of the input buffer, and driving that into another IBIS instantiation for that's configured for output.

    This seems backwards to me.

    I think you should be using IBIS_O for the output (where the VtPulse will connect to the pin T (I think). You can then take the output, and connect that to IBIS_I.

    I'll continue working on my test bench but please review your setup.

    David

  • 0 in reply to David Larkin
    Prodigy 40 points

    Hi David.

    Thank you for reply.

    as you can see i am using SN74HC244 IBIS Model for IBIS_I and IBIS_O.

    would you recommand i should use IBIS_O only?

    i also exemined using only IBIS_O only( Vt pulse connect Pin_T and get output)

    but the result didnt change.

    is it the problem with IBIS_Model or my fault for using?

    if you find any idea please let me know.

    regard

    HyungJe

  • 0 in reply to HyungJe Woo
    TI__Genius 16680 points

    Hi HyungJe,

    I've found the issue. There's some additional 'dead' time at the beginning of the VT data. It's flat for about 20 ns before switching. It should start to switch pretty much at time 0.

    That's why at slower speeds it works correctly. It has enough time to get past the initial state to get all of the switching information in the VT. At higher speeds, the simulation is only capturing part of the transition.

    A couple of options are available.

    A) You can manually subtract off the roughly 20ns.

    B) If you have HyperLynx, there is an option to have it remove the time offset.

    C) I can manually subtract off the offset and if you provide me an email, send you the corrected IBIS file. (I'll later upload it to the web to correct the model as well, but that'll take some time).

    Let me know how you want to proceed,

    David

  • 0 in reply to David Larkin
    Prodigy 40 points

    Hi David.

    thank you for finding issue.

    i want to receive a new IBIS File.

    here is my E-mail Address.

    woohyungje@jem-net.co.jp

    Regard.

    Woo HyungJe