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

Receiver Level Requirements

You must address signal integrity issues if you use the supported Intel® devices in 3.3 V, 3.0 V, 2.5 V LVTTL or LVCMOS interfaces. Otherwise, you may not meet the devices' absolute maximum DC input voltage and maximum allowed overshoot/undershoot voltage requirements.

The supported Intel® devices have one VCCIO voltage level per I/O bank. Additionally, the devices can also have different driver input voltage levels for input signaling. Not all combinations of VCCIO and driver input voltage require attention with regards to the maximum input voltage.

Follow the guidelines in this document to manage the voltage overshoot and input requirements.

Figure 1. Simulation Waveform of 3.3 V LVTTL Output Interfacing 3.3 V LVTTL InputThis figure shows an exampe of a supported Intel® device with 3.3 V LVTTL interface at the highest drive current setting and without termination. The simulation shows that an excessively large overshoot is present at the receiver when the I/O is driven from a high current driver through an unterminated transmission line.


The conditions and actions in the following table apply only if you assign the supported Intel® device's I/O pin as input, bidirectional, or tristated output using the 3.3/3.0/2.5 V LVTTL/LVCMOS I/O standards. No attention is required when the device's I/O pin is used as output only. The Intel® Quartus® Prime software enables the PCI-clamp diode on this pin for each of these conditions by default. Other I/O standards, such as 1.8 V, 1.5 V, or 1.2 V LVTTL or LVCMOS, 3.0 V PCI or PCI-X, voltage-referenced, and differential I/O standards, do not require attention on the maximum input voltage.

Table 1.  Receiver Level Requirements for 3.3/3.0/2.5 V LVTTL/LVCMOS This table lists the recommended actions for the I/O interface voltage combinations that require attention. For more details about these recommendations, refer to the relevant sections.
Supported Intel® Device Receiver Bank VCCIO LVTTL/LVCMOS Driver Voltage Level
2.5 V 3.0 V 3.3 V
2.5 V No action required

The I/O pin of the device is overdriven by a higher external voltage. If you enable the diode, you must meet the DC current specification of the diode.

If the current exceeds the DC diode specification, disable the diode. For further details, refer to the following sections:

Alternatively, you can apply series termination to manage voltage overshoot. For further details, refer to Guideline: Use Series Termination Resistor.

3.0 V No action required. The I/O pin is not overdriven beyond the diode forward voltage. You can leave the diode enabled without concern for the DC current.
3.3 V

The I/O pin is not overdriven. You can leave the diode enabled without concern for the DC current.

Diode clamped voltage can still exceed the maximum DC and AC specifications because of the high VCCIO voltage level of the bank in which the I/O pin resides. You must manage the voltage overshoot. Apply series termination or use driver selection table. For further details, refer to the following sections:

For more information about the absolute maximum DC input voltage and maximum allowed overshoot/undershoot voltage for the device families covered in this document, refer to the related information.

Related Information
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