The PHY drives the Carrier Sense signal ( phy_crs ) active to indicate that either the transmit or receive is non-idle when operating in half-duplex mode. The signal phy_crs is deasserted when both the transmit and receive are idle. The PHY asserts phy_crs for the duration of a collision condition. The signal phy_crs is not synchronous to either the phy_tx_clk or the phy_rx_clk . The phy_crs signal is not used in full duplex mode. The phy_crs signal is used by both the AXI Ethernet Lite MAC transmit and receive circuitry and is double-synchronized to the processor clock as it enters the AXI Ethernet Lite MAC core.