5.2.4.1 Receive Cut-Through

By default, the MAC-PHY will receive packets and send them to the host controller in store-and-forward mode. Receive cut-through operation is enabled by setting the Receive Cut-Through Enable (RXCTE) bit in the OPEN Alliance Configuration 0 (OA_CONFIG0) register.

When receive cut-through is enabled, the MAC-PHY will begin to send receive packet data to the host controller once it has received enough data from the network to fill a data block. The SPI host cannot read packet data from the MAC-PHY fast enough to overrun the MAC-PHY receive packet buffer. If the MAC-PHY does not have a full block of data to send to the host over SPI (and the end of the frame has not been received), the MAC-PHY will simply respond with a receive data block containing no data, Data Valid will be ‘0’ in the block’s receive data footer.

In store-and-forward operation, packets received from the network are checked for validity before being forwarded to the host controller. For example, packets with an incorrect frame check are normally not forwarded to the host controller. When operating in cut-through mode, the packet is already partially received by the host controller before the MAC can identify receive errors. As it would be a burden on the host controller to verify the receive packet integrity, when the MAC detects an invalid receive packet it will signal to the host controller that the packet should be ignored. Packets that should be ignored by the host are identified by the Frame Drop bit being set in the receive data footer in the final block of the packet when End Valid is also set.

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