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not_A_quitter's avatar
not_A_quitter
Icon for New Contributor rankNew Contributor
3 years ago

Not getting the 3.3-V LVTTL and LVCOMS cyclone IV full range?

Hi,

I’m using a cyclone IV FPGA (EP4CE55F23C8N) and I’m outputting a 48Mhz clock signal (generated by the internal PLLs) that feeds a microcontroller. The problem is I’m not getting the required 3.3 voltage level (using the 3.3-V LVTTL and LVCOMS), I get 1 vpp (peak to peak) on my oscilloscope.

I figured the problem is related to the FPGA current limiting circuitry. So I dabbled with lower voltage values like 3v,2.5v which offer higher current limits. But still I wasn’t able to squeeze out sufficiently high logic levels. The best I got was around 1.4vpp. Also I activated the internal weak pull up resistor with no significant improvement from the voltage level standpoint.

Any thoughts on how I can generate the 3.3v full range.

4 Replies

  • ak6dn's avatar
    ak6dn
    Icon for Regular Contributor rankRegular Contributor
    3 years ago

    What are the characteristics of your scope probe and input?

    Is it a high impedance or 50ohm termination that you have on your probe?

    What is the input bandwidth of your scope channel?

    A CycloneIV with LVTTL or LVTTL 3.3V output should be able to drive a 4ma to 8ma load (depending on drive selected) 48MHz (20ns period) with a 2ns to 3ns rise/fall time pretty much rail to rail.

    • not_A_quitter's avatar
      not_A_quitter
      Icon for New Contributor rankNew Contributor
      3 years ago

      You are right. I think the issue had to do with the probe bandwidth as a function of the probe multiplier. I had my probe set to x1 multiplier (no amplification no attenuation) which reduces the BW to 5Mhz according to probe the datasheet. By setting the probe to x10 i get 100Mhz BW. I did my measurements (pic attached) i got 400~500mVpp which translates to 5vpp after accounting for the probe multiplier.

  • FakhrulA_altera's avatar
    FakhrulA_altera
    Icon for Regular Contributor rankRegular Contributor
    3 years ago

    Hi not_A_quitter,


    I hope the above ak6dn's suggestion may have addressed your question.

    Do you have any further questions on this matter?

    Else, I would like to have your permission to close this forum ticket so this thread will be transitioned to community support.


    If you have a new question, feel free to open a new thread to get support from Intel experts. Otherwise, the community users will continue to help you on this thread. Thank you


    Regards,

    Fakhrul



    • not_A_quitter's avatar
      not_A_quitter
      Icon for New Contributor rankNew Contributor
      3 years ago

      No I have no further question. My problem is solved. Thank you for your support.