Intel® Stratix® 10 Hard Processor System Technical Reference Manual

ID 683222
Date 11/28/2022
Public

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17.6.2.6.2. Receive Frame Processing

The MAC transfers the received frames to the Host memory only when the frame passes the address filter and frame size is greater than or equal to the configurable threshold bytes set for the receive FIFO buffer of MTL, or when the complete frame is written to the FIFO buffer in store-and-forward mode. †

If the frame fails the address filtering, it is dropped in the MAC block itself (unless Bit 31 (Receive All) of Register 1 (MAC Frame Filter) is set). Frames that are shorter than 64 bytes, because of collision or premature termination, can be removed from the MTL receive FIFO buffer. †

After 64 (configurable threshold) bytes have been received, the MTL block requests the DMA block to begin transferring the frame data to the receive buffer pointed by the current descriptor. The DMA sets the First Descriptor (RDES0[9]) after the DMA Host interface becomes ready to receive a data transfer (if the DMA is not fetching transmit data from the host), to delimit the frame. The descriptors are released when the Own (RDES[31]) bit is clear, either as the Data buffer fills up or as the last segment of the frame is transferred to the receive buffer. If the frame is contained in a single descriptor, both Last Descriptor (RDES0[8]) and First Descriptor (RDES0[9]) are set.

The DMA fetches the next descriptor, sets the Last Descriptor (RDES[8]) bit, and releases the RDES0 status bits in the previous frame descriptor. Then the DMA sets bit 6 (Receive Interrupt) of Register 5 (Status Register). The same process repeats unless the DMA encounters a descriptor flagged as being owned by the host. If this occurs,Bit 7 (Receive Buffer Unavailable) of Register 5 (Status Register) is set and the receive process enters the Suspend state. The position in the receive list is retained. †