Cyclone® V Device Handbook: Volume 1: Device Interfaces and Integration

ID 683375
Date 10/18/2023
Document Table of Contents

5.8.7. Open-Drain Output

The optional open-drain output for each I/O pin is equivalent to an open collector output. If it is configured as an open drain, the logic value of the output is either high-Z or logic low.

You can attach several open-drain output to a wire. This connection type is like a logical OR function and is commonly called an active-low wired-OR circuit. If at least one of the outputs is in logic 0 state (active), the circuit sinks the current and brings the line to low voltage.

You can use open-drain output if you are connecting multiple devices to a bus. For example, you can use the open-drain output for system-level control signals that can be asserted by any device or as an interrupt.

You can enable the open-drain output assignment using one these methods:
  • Design the tristate buffer using OPNDRN primitive.
  • Turn on the Auto Open-Drain Pins option in the Intel® Quartus® Prime software.
Although you can design open-drain output without enabling the option assignment, you will not be using the open-drain output feature of the I/O buffer. The open-drain output feature in the I/O buffer provides you the best propagation delay from OE to output.