3.4 Hardware Issue - MII Connectivity

If the management interface is confirmed to function, but there are still communication issues, confirm that the MAC-to-PHY connection is functioning. Focus on the following items:

  1. Check Transmit and Receive Directions: Some datasheets present the MII data lines from different perspectives. Make sure to pay close attention to the direction of the pins when connecting the signals. For example, the RXD pins are listed as Outputs on the KSZ8081MNX, so they need to be connected to pins that are Inputs on the MAC.
  2. Check Voltage Levels: As with the Management Interface, confirm that both parts operate at the same voltage levels. Using an oscilloscope to monitor the actual voltage levels can be helpful. Sometimes, the voltage measured at a VDDIO test point may differ from the actual voltage entering the KSZ8081MNX.
  3. Execute the Internal Loopback Test: The KSZ8081MNX has a loopback feature that can be utilized to test just the MII connections. Refer to section 3.8 of the KSZ8081MNX Datasheet for more details on how to implement the Local (Digital) Loopback test.
  4. Execute an external traffic test: If a link is able to be established, implement a full duplex data transfer where both link partners attempt to send packets. If the traffic flows in one direction but not the other, that will help narrow down the source of the problem.
  5. Measure Clock to Data Timing: Another potential error is the variation in data propagation time between different data lines and the clock lines. Measure the clock and various data lines on a high speed oscilloscope, and look for the edges where the data and clock pins change states. If the edges look misaligned, changing the series resistors or adjusting the trace lengths on the PCB may be required.