Forum Discussion

Altera_Forum's avatar
Altera_Forum
Icon for Honored Contributor rankHonored Contributor
9 years ago

Cyclone III 3.3V LVTTL/LVCMOS input impedance

I have a customer that has an unusual driver arrangement for a couple Cyclone III 3.3V LVTTL/LVCMOS inputs and they want to know what the Cyclone III input impedance will be. Digging through the Altera info pile I found numbers specified for an output driver and an447 discusses PCB trace impedances and series resistor values, but have not come up with a specified value (range) for the input impedance. Cyclone III part number: EP3C25U256I6N.

Thanks in advance for the help.

3 Replies

  • Altera_Forum's avatar
    Altera_Forum
    Icon for Honored Contributor rankHonored Contributor
    9 years ago

    LVTTL/LVCMOS means high input impedance, the input pin impedance can be best modeled by a small capacitance of 5 - 6 pF, see datasheet table "Cyclone III Devices Pin Capacitance".

  • Altera_Forum's avatar
    Altera_Forum
    Icon for Honored Contributor rankHonored Contributor
    9 years ago

    --- Quote Start ---

    LVTTL/LVCMOS means high input impedance, the input pin impedance can be best modeled by a small capacitance of 5 - 6 pF, see datasheet table "Cyclone III Devices Pin Capacitance".

    --- Quote End ---

    Thanks FvM.

    I am looking for a quantitative numeric value/range rather than a generic word description that can be interpreted multiple ways. While I do appreciate it can be modeled, the results are only as good as the input to the model and I would think that Altera can supply the needed details.

    Regards.
  • Altera_Forum's avatar
    Altera_Forum
    Icon for Honored Contributor rankHonored Contributor
    9 years ago

    Your comment is rather unspecific und doesn't actually tell which kind of model parameters you are looking for and in which frequency and input voltage range you want to model the input pin behavior. There are different quantitative data available from Altera. In particularly you have:

    - datasheet specifications

    - IBIS files

    - HSPICE models (encrypted)