AN 958: Board Design Guidelines

ID 683073
Date 6/26/2023
Document Table of Contents

4.7. Unused I/O Pins Guidelines

To allow flexibility in board design, you can specify the state of unused pins as one of the following five states in the Quartus® Prime or Quartus II software: as inputs that are tri-stated, as outputs that drive ground, as outputs that drive an unspecified signal, as input tri-stated with bus-hold, or as input tri-stated with weak pull-up resistors. To improve signal integrity, set unused pins as outputs that drive ground and tie them directly to the ground plane on the board. Doing so reduces inductance by creating a shorter return path and reduces noise on the neighboring I/O. To reduce power dissipation, set clock pins to drive ground and set other unused I/O pins as inputs that are tri-stated. To make the setting that is appropriate for your design, choose one of the five allowable states for Reserve all unused pins on theUnused Pins tab in the Device & Pin Options dialog box, or apply the Reserve Pin assignment to specific pins in the Pin Planner.

When you compile your design, the software generates the pin report file (.pin) to specify how you should connect the device pins. Unused I/O pins are marked in the report file according to the unused pins option you set in the software. All I/O pins specified as GND* can either be connected to ground to improve the device's immunity to noise, or left unconnected. Leave all RESERVED I/O pins unconnected on your board because these I/O pins drive out unspecified signals. Tying a RESERVED I/O pin to VCC, ground, or another signal source can create contention that can damage the device output driver. You can connect RESERVED_INPUT I/O pins to a high or low signal on the board, while RESERVED_INPUT_WITH_WEAK_PULLUP and RESERVED_INPUT_WITH_BUS_HOLD pins can be left unconnected.