Ethernet MAC is also supported and it interfaces to the external PHY using the MII or RMII protocol. Typically, the MDC frequency should not be more than 2.5MHz. In the LPC3250 implementation, the minimum attainable MDC frequency will be 3.714MHz. To avoid this issue, either limit the HCLK to 52MHz or select an Ethernet PHY that supports MDC frequencies above 3.714MHz. This behavior will most likely not be noticed since most external PHYs support higher frequencies.