Accessing the USR pins in the FPGA sandbox

Most imperix controllers feature 36 bidirectional 3.3V I/O lines, commonly known as USR pins. Driven directly from the FPGA, these custom-application I/Os are ideal for various uses, including communicating with digital encoders, interfacing with additional ADCs, and connecting to other peripheral devices.

Physical location of the USR pins

The following table details the physical locations of USR pins on imperix controllers.

(*) Accessible via a breakout board such as the VHDCI breakout board.

Using the USR pins

There are two specific cases where the USR pins are reserved by the imperix system and remain unavailable for custom applications:

• The target is a TPI8032.
• A motor interface is connected to the VHDCI connector.

If neither of these conditions applies, the default USR interface port can be safely deleted to free up the pins.

Delete the existing USR interface port

• Right-click on the existing USR interface.
• Click Delete.

Create new interface ports

• Right-click, then Create Port…
• Give it a name, select a direction, and click OK.

Edit the constraints

The constraints\sandbox_pins.xdc must be edited accordingly. As shown below, we recommend commenting (#) the unused pins to avoid generated unnecessary warning the Vivado. For more information on constraints in Xilinx FPGA please refer to the using constraints in Vivado Design Suite user guide.

Update the block design wrapper

Whenever top level ports are modified, it is required to updated the block design wrapper to reflect the changes. The recommended procedure is the following:

• Remove the existing wrapper by right-clicking on top_wrapper and select Remove File from Project.
• Check the Also delete the project file from disk option, then OK.
• In the Design sources of the Sources panel on the left, right-click on top (top.bd) and select Create HDL wrapper.
• Check Copy generated wrapper to allow user edits. Press OK.

To go further

The following application notes illustrate practical uses of the 3.3V USR pins for communicating with external peripherals:

• TN130: SPI communication with an LTC2314 ADC chip
• TN149: decoding Delta-Sigma modulation from an AMC1035 ADC