Does the CP220x Ethernet Controller support MDI/MDIX Auto Cross?
An Ethernet cable has 8 wires. Only 4 wires are used Ethernet signaling and the 4 remaining wires are unused or carry power to the embedded system. There can only be two devices connected using a single cable. These devices are called link partners.
The 4 signals of interest are TX+/TX- (Transmit differential pair) and RX+/RX- (Receive differential pair). An Ethernet controller's Transmit pins must be connected to its link partner's receive pins to establish communication. This makes the need for two types of devices:
1. MDI devices (Media Dependent Interface) such as the CP220x, PCs and notebooks.
2. MDIX devices (Media Dependent Interface with Crossover) such as hubs, switches, and routers.
If two MDI devices need to communicate without going through an MDIX device, or if two MDIX devices need to communicate, then a Crossover cable is required. A crossover cable switches the Transmit and Receive lines to line up each device's transmit pins with its link partner's receive pins.
To overcome the requirement of using a Crossover cable, most hubs, switches, and routers support MDI/MDIX Auto Cross. This capability allows the network equipment to automatically switch its transmit and receive pins in case an incorrect cable is used. This functionality is found in a very large percentage of network equipment and is not commonly implemented on MDI devices. The CP220x is an MDI device and does not support MDI/MDIX Auto Cross.