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

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ID 683295
Date 3/28/2022
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Interfacing Intel® FPGA Devices with 3.3/3.0/2.5 V LVTTL/LVCMOS I/O Systems
Interfacing Intel® FPGA Devices with 3.3/3.0/2.5 V LVTTL/LVCMOS I/O Systems x
Receiver Level Requirements Guideline: Use Internal PCI Clamp Diode on the Pin Guideline: Use Series Termination Resistor Guideline: Select Appropriate Driver Interface Current between Supported Intel® Devices Document Revision History for AN 447: Interfacing Intel FPGA Devices with 3.3/3.0/2.5 V LVTTL/LVCMOS I/O Systems
Guideline: Use Internal PCI Clamp Diode on the Pin x
Measuring DC Current with PCI Clamp Diode
Guideline: Use Series Termination Resistor x
Selecting Appropriate Series Termination Resistor Value Example of Determining Series Termination Resistor Value
Guideline: Select Appropriate Driver x
Driver Selection Reference Current Limits Measurement Examples Evaluating Interface Using Driver Selection Method
Interfacing Intel® FPGA Devices with 3.3/3.0/2.5 V LVTTL/LVCMOS I/O Systems

Measuring DC Current with PCI Clamp Diode

The PCI clamp diode is forward-biased when the driver voltage level exceeds the VCCIO plus diode forward voltage.

DC current exists when the diode is forward-biased. The amount of DC current depends on the driver output impedance, driver and receiver supply voltage, diode forward voltage, and a small resistance intrinsic to the transmission line.

Figure 3. Simulation Setup and Result to Determine DC Current that Flows into the PCI Clamp Diode


  1. Set up the driver to drive static logic-high signal into the receiver of the supported Intel® device with the PCI clamp diode enabled.
  2. Apply the maximum supply voltage at the driver and the minimum VCCIO at the receiver of the supported Intel® device for the highest DC current.
  3. Take the current measurement from the die pad of the supported Intel® device—denoted by the red pointer in the preceding figure.
    You can obtain current measurements using a small sense resistor (in mili-Ω) placed in series to the transmission line.
As shown in the preceding figure, a 20.88 mA DC current sink in the PCI clamp diode is measured. The result significantly exceeds the 10 mA maximum current supported by the diode. You must disable the diode in the Intel® Quartus® Prime software for this input pin.
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