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Altera_Forum's avatar
Altera_Forum
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16 years ago

Need "repeater/buffer" for AS config of multiple Cyclone iii slaves

As it states on page 10-14 of the Cyclone III device handbook: "When cascading Cyclone III devices in multi-device configuration, you must connect repeater buffers between the Cyclone III master and slave device(s) for DATA and DCLK. The output resistance of the repeater buffers has to fit the maximum overshoot equation given by: .8Zo <= Re <= 1.8Zo.

Well, if I am shooting for a Zo of around 80 Ohms does anyone have a suggestion for some common type of buffer that I could use, be it CMOS or other? I suppose that I could configure a general purpose op amp with a series build out resistor equal to my Zo, but there is probably an easier way to do this.

What kind of "off the shelf" buffer has been used by people on this forum to configure multiple Cyclone III slave devices?

Thank You

Tom

1 Reply

  • Altera_Forum's avatar
    Altera_Forum
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    16 years ago

    The output resistance recommendation is misleading in so far, as you won't most likely find no fast buffer with a that high output impedance. Provide a series termination resistance and adjust it for sufficient low overshoot. You should use the lowest acceptable resistance value to minimize delay. I think, fast single gate LVCMOS buffers are a good option, e.g. LVC1G34, they have an internal output impedance of about 25 ohms.