5.2.4.2 Transmit Cut-Through

The MAC-PHY accepts packets from the host controller and transmits them to the network in store-and-forward mode by default. To enable transmit cut-through operation, set the Transmit Cut-Through Enable (TXCTE) bit in the OPEN Alliance Configuration 0 (OA_CONFIG0) register.

When transmit cut-through is enabled, special care must be taken not to allow the MAC-PHY transmit buffer to underflow. Once the host controller begins to send an Ethernet frame, the MAC-PHY may begin transmitting the frame onto the network as soon as a threshold of data has arrived. At this point the host controller must continue to send the remainder of the Ethernet frame over SPI faster than the MAC-PHY is transmitting the frame to the network. If the host controller fails to keep up with the rate at which the frame is transmitted to the network a MAC-PHY transmit buffer underflow condition occurs. When this condition occurs, the MAC-PHY will terminate transmitting the frame and set the Transmit Buffer Underflow Error (TXBUE) bit in the OPEN Alliance Status 0 (OA_STATUS0) register. The remainder of the Ethernet packet from the host controller will be ignored by the MAC-PHY and the packet will be dropped. The Ethernet frame will not be delivered to its destination and must be retransmitted by the host controller.

The risk of under-running the MAC-PHY transmit buffer and dropping frames may be reduced by increasing the threshold of Ethernet data the MAC-PHY must receive from the host controller before it begins to transmit. By default, the threshold of Ethernet packet data the MAC-PHY must receive from the SPI host before it begins transmitting is two blocks. This threshold may be increased by writing to the Cut-Through Threshold (CTTHR) field of the Queue Transmit Configuration (QTXCFG) register prior to enabling cut-through operation.

Note: Increasing the transmit Cut-Through Threshold will also increase transmit latency.

The cut-through latency improvement over store-and-forward is limited as the MAC-PHY may still need to wait for other devices on the network to complete transmitting on the shared media.

Transmit cut-through operation must be configured and enabled prior to the setting of the Configuration Synchronization (SYNC) bit in the OA_CONFIG0 register.