input edge rise and fall time

Discussion in 'Embedded Systems and Microcontrollers' started by nileshkhupase, Jun 16, 2011.

  1. nileshkhupase

    Thread Starter Member

    Mar 22, 2011
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    what does edge rate actually indicates? does it indicate that what should be the the maximum frequency at which its input should change so that ic can properly sample it.if i have a controller with instruction rate of 8MHz i.e. 125nS per instruction and it is connected to another ic with rise time of 25nS/V ,so whatever data i will send to that ic from controller should remain stable for al least 125nS because for ic it will take 125 nS to rise the input from 0 to 5V.but controller may send another data to ic after 125nS(instruction cycle).so in this case will that ic be able to sample previous data? or i have to consider another ic with low edge rate?
     
  2. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    OK, first off this seems to be a continuation of yesterday's post so it should be added into that thread.

    Next, some whitespace would help. Start another paragraph for another thought.

    It would really help us know what you mean if you post links to the data sheet of these mysterious devices. Even a part number is something! I believe you might be stumbling on some point I cant see as I do not know what data sheet you are looking at. I think you are misreading something (hey, we all did that!) so give me a hint what parts these are!

    Not quite true. 125nS is the instruction rate and thus the minimum time between changes. The time it takes to change from 0 to 5V (or 5V to 0) is a portion of the 125nS.

    Next you seem to be speaking about the edge rate of the input of the "another ic" which is not something typically of concern. It is true that for some digital inputs like clock lines there is a minimum rate of change of the input, it needs to be fast enough to reliably trigger the function.
     
  3. nileshkhupase

    Thread Starter Member

    Mar 22, 2011
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    Hi Erniem,
    Thank you for your clarification actually i am a fresher just passed and joined having not much experience in h/w design.

    i have a controller IN87c196kt working on a 16MHz clock frequency.this is the limk for
    datasheet
    http://pdf1.alldatasheet.net/datasheet-pdf/view/118187/INTEL/TN87C196KT.html


    and for latching its 16 bit data/address bus octal latch 74ac573 is used its link for datasheet is
    http://pdf1.alldatasheet.net/datasheet-pdf/view/179234/FAIRCHILD/74AC573SC.html

    in manual for controller i found that clock is internally divided into half, and provided to cpu and other peripherals for power mang. purpose.
    now since my task was to find alternative ic for octal latch, i found one but it has rise time of 25nS/V ,whereas original/obsolete had 8nS/V.
    so i am confused whether it will work with controller properly or not..

    and edge rate of the input of the ic which is not something typically of concern,why did u say like that?
     
  4. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    Hi nileshkhupase!

    Experience begins where you start....

    Inputs generally do not have a speed requirement. The worst thing that can happen is you move the voltage so slowly the gate gets held at between 0 and 1 and turns full on and burns up. That is slow slow, on the order of minutes to change.

    It looks like Fairchild no longer makes this part, I did not find it on their website (you probably already know that which is why you're looking).

    The Fairchild sheet you linked to does indeed spec a Minimum Input Edge Rate of 125mV/ns, or a rise or fall time of 16 nS (AC devices) or 9.6nS (ACT devices). Those are maximum times and about 3 times faster then the part you are replacing, and a fairly fast rate.

    The Intel chip does not spec it's output rise and fall times, not even a typical number that I could find. If you have a working unit you could measure these to see if it is fast enough.

    Sorry I cannot give you a definite answer, you will have to make that measurement (or read the spec better then I did) to see if the number all play out.
     
  5. nileshkhupase

    Thread Starter Member

    Mar 22, 2011
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    Hi Erniem,

    thank you once again,you are good guider.

    just tell me that if controller instruction rate is 125nS.
    does this information is not enough.then why should we look for its output edge rate?


    and as per this i am again stucked at that point that if controller o/p is changing at min. 125nS and my new ic is having edge rate of 25nS/V

    so for transition onlt it will take 125nS, then when it will sample it ? before sampling this data ,there may be new data from controller.
     
  6. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    The instruction rate is the time in between changes. Edge rate is how long each change takes. Example: a pin outputs 1, 0, 1. The time each 1 or 0 is on the pin is the instruction rate. The time it takes to change from 0 to 1 (or 1 to 0) is the edge rate (commonly called slew rate).

    Rate of change is usually expressed with units of volts per time. That is how I am used to seeing it. If we convert new and old part to rates:

    Old: 8nS / V ==> 125*10^6 V/ns (minimum)

    New: 25nS / V ==> 40 * 10^6 V/ns (minimum)

    I think I misread it last time, now it seems the new part has a lower edge rate requirement, so anything that drove the older part would also drive the newer part.

    Hey, can someone else chime in here and check this?

    IT IS FRIDAY MY BRAIN HURTS

    ;)
     
  7. nileshkhupase

    Thread Starter Member

    Mar 22, 2011
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    Hi erniem,
    very glad to have a friend like you for discussion and guidance.....
    Please don't mind but can you tell me the meaning of minimum in case of edge rate?

    if my edge rate is 40mV/ns (minimum) means it is the minimum speed for increase in input voltage...

    then what is maximum limit?
     
  8. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    Generally if any spec sheets gives only a minimum but not a maximum (or the reverse) it means there is no maximum, here you can go as fast as you can.

    Once you change the edge fast enough to make all the internal stages happy with that change rate you can keep going faster without worry.
     
  9. nileshkhupase

    Thread Starter Member

    Mar 22, 2011
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    hi,
    for 87c196kt they have not specified output edge rate, but they have given external clock drive rise time(Txlxh),fall time(Txhxl) 10nS.

    so can we consider this as the output edge rate.....?
     
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