AN 447: Interfacing Intel® FPGA Devices with 3.3/3.0/2.5 V LVTTL/LVCMOS I/O Systems

ID 683295
Date 3/28/2022

Evaluating Interface Using Driver Selection Method

An easy and convenient method to determine the current limits is to perform the measurement on the driver pull-up I/V curve in the IBIS model.

  1. Obtain the IBIS model for the driver.
    The model used as the driver is 1c_ttl33_io_d8 from the cyclone.ibs file. You can perform a DC sweep simulation on the HSPICE model and set the buffer to drive logic-high if the IBIS model is not available for the driver.
  2. Open the IBIS file using the HyperLynx* Visual IBIS Editor.
    The editor provides a graphical view of IBIS model data, which provides a measurement of the I/V values in graphical format.
  3. Run the graphical view mode.
    1. Navigate to the 1c_ttl33_io_d8 model data from tree-view pane on the left column in the editor.
    2. Right-click on the model denoted by [Model] 1c_ttl33_io_d8 and select View Data.
      A dialog box appears with multiple tabs for each data characteristic available for the model.
  4. Select the pull-up I/V curve.
    1. Select the Pullup tab in the dialog window.
    2. In the Display Curves list, select Ground relative.
      Figure 12. Current Limit Measurement for IBIS Pull-Up Data Using Graphical Viewer HyperLynx Visual IBIS EditorIn the figure, the measured current is 33.8 mA.

  5. Identify the appropriate VOH level and perform the current measurement.
    Based on the driver selection reference, the VOH for the 3.3 V LVTTL driver is 2.4 V (see related information). Look for the maximum I/V curve and make the visual approximation current measurement at 2.4 V.
  6. Identify allowed current limit.
    Based on the maximum allowed current metrics for the supported Intel® device (see related information), the current limit is 30 mA for a 3.3 V LVTTL driver to a 3.3 V receiver bank of a supported Intel® device. The measured current exceeds the maximum allowed current limit.
From the preceding example, a Cyclone® device operating as the driver at 3.3 V LVTTL with 8 mA setting is not able to drive directly the supported Intel® device's input at 3.3 V VCCIO supply without termination. The Cyclone® device might not meet the DC and AC input voltage specification of the supported Intel® devices.
To solve the problem, you can apply series termination according to the recommended receiver level requirements.