Intel® Agilex™ Hard Processor System Technical Reference Manual

ID 683567
Date 2/14/2023
Public

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17.6.1. Transmit and Receive Data FIFO Buffers

Each EMAC component has associated transmit and receive data FIFO buffers to regulate the frames between the application system memory and the EMAC. The RX and TX FIFO buffers are each 16KB dual ported memories. Both buffers are designed to support jumbo frames. A FIFO buffer word consists of:

  • Data: 32 bits
  • Sideband:
    • Byte enables: 2 bits
    • End of frame (EOF): 1 bit
    • Error correction code (ECC): 7 bits

The FIFO RAMs are each supported by an ECC controller that performs single-bit error detection and correction and double-bit error detection. The ECC Controllers have a dedicated hardware block for memory data initialization and can log error events and generate interrupts on single and double-error events. See the Error Checking and Correction (ECC) Controller for more information regarding the function of the ECC RAMs.

TX FIFO

The time at which data is sent from the TX FIFO to the EMAC is dependent on the transfer mode selected:
  • Cut-through mode: Data is popped from the TX FIFO when the number of bytes in the TX FIFO crosses the configured threshold level (or when the end of the frame is written before the threshold is crossed). The threshold level is configured using the TTC bit of Register 0 (Bus Mode Register).
    Note: After more than 96 bytes (or 548 bytes in 1000 Mbps mode) are popped to the EMAC, the TX FIFO controller frees that space and makes it available to the DMA and a retry is not possible.
  • Store-and-Forward mode: Data is popped from the TX FIFO when one or more of the following conditions are true:
    • A complete frame is stored in the FIFO
    • The TX FIFO becomes almost full

The application can flush the TX FIFO of all contents by setting bit 20 (FTF) of Register 6 (Operation Mode Register). This bit is self-clearing and initializes the FIFO pointers to the default state. If the FTF bit is set during a frame transfer to the EMAC, further transfers are stopped because the FIFO is considered empty. This cessation causes an underflow event and a runt frame to be transmitted and the corresponding status word is forwarded to the DMA.

If a collision occurs in half-duplex mode operation before an end of the frame, a retry attempt is sent before the end of the frame is transferred. When notified of the retransmission, the MAC pops the frame from the FIFO again.

Note: Only packets of 3800 bytes or less can be supported when the checksum offload feature is enabled by software.

RX FIFO

Frames received by the EMAC are pushed into the RX FIFO. The fill level of the RX FIFO is indicated to the DMA when it crosses the configured receive threshold which is programmed by the RTC field of Register 6 (Operation Mode Register). The time at which data is sent from the RX FIFO to the DMA is dependent on the configuration of the RX FIFO:
  • Cut-through (default) mode: When 64 bytes or a full packet of data is received into the FIFO, data is popped out of the FIFO and sent to the DMA until a complete packet has been transferred. Upon completion of the end-of-frame transfer, the status word is popped and sent to the DMA.
  • Store and forward mode: A frame is read out only after being written completely in the RX FIFO. This mode is configured by setting the RSF bit of Register 6 (Operation Mode Register).
If the RX FIFO is full before it receives the EOF data from the EMAC, an overflow is declared and the whole frame (including the status word) is dropped and the overflow counter in the DMA, (Register 8) Missed Frame and Buffer Overflow Counter Register, is incremented. This outcome is true even if the Forward Error Frame (FEF) bit of Register 6 (Operation Mode Register) is set. If the start address of such a frame has already been transferred, the rest of the frame is dropped and a dummy EOF is written to the FIFO along with the status word. The status indicates a partial frame because of overflow. In such frames, the Frame Length field is invalid. If the RX FIFO is configured to operate in the store-and-forward mode and if the length of the received frame is more than the FIFO size, overflow occurs and all such frames are dropped.
Note: In store-and-forward mode, only received frames with length 3800 bytes or less prevent overflow errors and frames from being dropped.