AN 796: Cyclone® V and Arria® V SoC Device Design Guidelines

ID 683360
Date 3/30/2022
Public
Document Table of Contents

4.5.1.2.5. MDIO

The MDIO PHY management bus has two signals per MAC: MDC and MDIO. MDC is the clock output, which is not free running. At 2.5 MHz, it has a 400-ns minimum period. MDIO is a bi-directional data signal with a High-Z bus turnaround period.

When the MAC writes to the PHY, the data is launched on the falling edge, meaning there is 200 ns -10 ns = 190 ns for flight time, signal settling, and setup at the receiver. Because data is not switched until the following negative edge, there is also 200 ns of hold time. These requirements are very easy to meet with almost any board topology. When the MAC reads from the PHY, the PHY is responsible for outputting the read data from 0 to 300 ns back to the MAC, leaving 100 ns less 10 ns setup time, or 90 ns for flight time, signal settling, and setup at the receiver. This requirement is also very easy to meet.

GUIDELINE: Implement pull-up resistor on board for MDC/MDIO.

Both signals require an external pull-up resistor, typically 1K but PHY data-sheets may vary.

GUIDELINE: Ensure interface timing is met.

There is a 10ns setup and hold requirement for MDIO for data with respect to MDC.