3 Replies Latest reply on Sep 4, 2018 11:52 AM by Carlos_A

    i210 AT with oscillator and fan out buffer

    swankmue Community Member

      Hi,

       

       

      I’m working on a design with three i210 AT network controllers, all connected to the same CPU. To avoid interference, I would prefer to connect all three network controller to the same clock source. My plan ist o use an oscillator as clock source and a fan out buffer to distribute the clock to the three controllers.

      FanOutBuffer.jpg

      Do you have any experience with such a design and can you propose any components? Also some questions stay not answered after working my way through the various documents.

       

       

      In the Intel® Ethernet Controller i210 Datasheet in section 11.6.5.2 it is suggested to use a capacitive voltage divider with a capacitance of C1 = 51 pF when using a 3.3 V clock signal. Also in section 12.4 it is mentioned that the input capacitance is about 20 pF. So the circuit would look like this.

      i210_osci.jpg

      If I calculate equivalent capacitance of all three capacitors, I get a value of 29 pF. Most fan out buffers are only able to drive a load up to 15 pF. Is there a way to use smaller capacitors or do you know any buffers that have a high enough performance?

       

       

      Even if I can reduce the equivalent capacitance, the fan out buffers I found have only 8 to 12 mA output current. So the rise or fall time is even in best scenario about 6 ns, which seems suspiciously long for a period of 40 ns (25 MHz!). Can you confirm, that such a long rise time would be no problem?

       

      Thank you very much for helping me with this problem.

       

       

      Best regards

      Sebastian