Device Specific Transceiver Receive Elastic Buffers - 16.2 English

1G/2.5G Ethernet PCS/PMA or SGMII LogiCORE IP Product Guide (PG047)

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16.2 English

The following figure shows the transceiver receive elastic buffer depths and thresholds in AMD UltraScale+™ , AMD UltraScale™ architecture, AMD Zynq™ 7000, and 7 series families. Each FIFO word corresponds to a single character of data (equivalent to a single byte of data following 8B/10B decoding).

Figure 1. Elastic Buffer Sizes for All Transceiver Families Ethernet 1000BASE-X PCS/PMA or SGMII Page-1 Sheet.1 Sheet.2 Sheet.3 Sheet.4 Sheet.5 Sheet.6 64 64 Sheet.7 Sheet.8 Sheet.9 Sheet.10 Sheet.11 36 36 Sheet.13 Sheet.14 33 33 Sheet.15 61 – Overflow Mark 61 – Overflow Mark Sheet.16 8 – Underflow Mark 8 – Underflow Mark Sheet.17 Sheet.18 X12839 X12839 Sheet.19 RX Elastic Buffer RX Elastic Buffer

Consider the example, where the shaded area represents the usable buffer availability for the duration of frame reception.

  • If the buffer is filling during frame reception, there are 61 - 36 = 25 FIFO locations available before the buffer reaches the overflow mark.
  • If the buffer is emptying during reception, then there are 33-8 = 25 FIFO locations available before the buffer reaches the underflow mark.

This analysis assumes that the buffer is approximately at the half-full level at the start of the frame reception. As shown, there are two locations of uncertainty, above and below the exact half-full mark of 32, resulting from the clock correction decision, and is based across an asynchronous boundary.

Because there is a worst-case scenario of one clock edge difference every 5000 clock periods, the maximum number of clock cycles (bytes) that can exist in a single frame passing through the buffer before an error occurs is:

5000 x 25=125000 bytes

The following figure translates this into maximum frame size at different Ethernet speeds. At SGMII speeds lower than 1 Gbps, performance is diminished because bytes are repeated multiple times (see Using the Client-Side GMII for the SGMII Standard).

Table 1. Maximum Frame Sizes: Transceiver Receive Elastic Buffers (100 ppm Clock Tolerance)
Standard/Speed Maximum Frame Size (Bytes)
1000BASE-X (1 Gbps only) 125000
SGMII (1 Gbps) 125000
SGMII (100 Mbps) 12500
SGMII (10 Mbps) 1250