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.
- 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.
- 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.
- Run the graphical view mode.
- Navigate to the 1c_ttl33_io_d8 model data from tree-view pane on the left column in the editor.
- 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.
- Select the pull-up I/V curve.
- Select the Pullup tab in the dialog window.
- 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.
- 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.
- 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.