FEC-Only and FlexO Modes - 2.1 English

Versal ACAP 600G Channelized Multirate Ethernet Subsystem (DCMAC) LogiCORE IP Product Guide (PG369)

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2.1 English

The FEC-only and FlexO modes run at half the effective clock rate of the SerDes (tx_serdes_clk[5:0]/rx_serdes_clk[5:0]) clock (the actual flexif clock rate might be higher, but enabled cycles are half the SerDes clock rate). In the transmit direction, you must indicate the start of the codeword and then continue to provide data until the end of the codeword while observing tx_flex_stall_<N>. No gaps are allowed other than those required by the stall within codewords. Gaps are allowed between codewords, giving you the ability to control the effective data rate.

For 50G FEC-only mode, each port supports two independent streams (the tx_flex_stall_<N> and tx_flex_ena_<N> are shared between them). The first stream uses tx_flex_data_<N>[159:0] and tx_flex_start_<N>. The second stream uses tx_flex_data_<N>[319:160] and tx_flex_startb_<N>. For 100G RS(528,514) FEC-only mode, because the codeword size is not an integer number of cycles, tx_flex_startb_<N> is used to indicate when the codeword starts in the second half of the tx_flex_data_<N> signal (this cycle contains both the end of the first codeword, and the start of the second).
Note: For FEC-only modes tx_flex_ena_<N> must continue to be asserted (unless indicated by tx_flex_stall_<N>) beyond the end of the codeword. The data is ignored until the next tx_flex_start_<N> is asserted. In FEC modes that use parallel codewords, both codewords must be aligned.

The 50G FEC-only mode can be combined with programmable logic to create a 64GFC solution in faster speed grades. Likewise, the 100G FEC-only mode can be combined with programmable logic to create a 64GFC solution (at half-density) in a wider range of speed grades.

For FEC-only modes, the latency through DCMAC depends on clock frequencies and on start-up conditions. Resetting a given port of DCMAC may result in a new latency through that port of DCMAC even if the clock frequencies remain fixed. Users are encouraged to utilize the information provided by the start signals on the FLXIF and transceiver interface.

FlexO modes are similar to the raw FEC-only RS(544,514) modes, but there must be no gaps between codewords on TX, and alignment markers are indicated.

In the receive direction, valid data is indicated by the per-client rx_flex_ena_<N>. When rx_flex_ena_<N> is deasserted, the data is invalid. The rx_flex_ena_<N> might be deasserted at any time, including part way through a codeword.

Note: The rx_flex_fec_rx_dout_flags_<N> (and rx_flex_fec_rx_dout_flagsb_<N> for 50G modes) signals indicate the AM status flags. These arrive for every codeword. The rx_flex_ena_<N> signal must not be used to qualify the flags.

In the following example, client 0 is 100G. Client 2 is two streams of 50G. Client 4 is 200G FlexO, where tx_flex_amflag_4 indicates the position of the alignment marker.

Figure 1. TX FEC-Only/FlexO Flex Interface

In the following example, client 0 is 100G RS(528,514), with two back-to-back codewords shown. The rx_flex_start_0 signal indicates when the codeword starts in the first half of the rx_flex_data_0 bus while the rx_flex_startb_0 signal indicates when the codeword starts in the second half of the rx_flex_data_0 bus. Client 2 is 200G. The rx_flex_data_2 signal contains codeword A. The rx_flex_data_3 contains codeword B. Data after the end of the codeword can be ignored. Client 4 is 200G FlexO and includes rx_flex_amflag_4 to indicate the location of the alignment marker. Because the SerDes interface runs at line rate, there must be no ignored data other than non-enabled cycles.

Figure 2. RX FEC-Only/FlexO Flex Interface