40- and 100-Gbps Ethernet MAC and PHY MegaCore Function User Guide

ID 683114
Date 6/15/2022
Public
Document Table of Contents

3.2.20. Clocks

You must set the transceiver reference clock (clk_ref) frequency to a value that your IP core variation supports. For most variations, The 40-100GbE IP core supports clk_ref frequencies of 644.53125 MHz ±100 ppm and 322.265625 MHz ± 100 ppm. The ±100ppm value is required for any clock source providing the transceiver reference clock. For CAUI–4 variations, you must set the frequency of clk_ref to 644.53125 MHz ±100 ppm. For 24.24 Gbps variations, you must set the frequency of clk_ref either to 390.625 MHz ±100 ppm or to 195.3125 MHz ±100 ppm.

Sync–E IP core variations are duplex IP core variations for which you turn on Enable SyncE support in the parameter editor. These variations provide separate IP core input reference clock signals for the TX and RX transceiver PLLs, and provide the RX recovered clock as a top-level output signal.

The Synchronous Ethernet standard, described in the ITU-T G.8261, G.8262, and G.8264 recommendations, requires that the TX clock be filtered to maintain synchronization with the RX reference clock through a sequence of nodes. The expected usage is that user logic drives the tx_ref_clk signal with a filtered version of the RX recovered clock signal, to ensure the receive and transmit functions remain synchronized. In this usage model, a design component outside the LL 40-100GbE IP core performs the filtering.

In a Sync–E IP core, the restrictions apply to each of the rx_clk_ref and tx_clk_ref input clocks.

The minimum clock frequency for the IP core is 315 MHz. The only exception is the 40GbE lower rate 24.24 Gbps MAC and PHY IP core, which has a minimum clock frequency of 190.90 MHz.

Table 33.  Clock InputsDescribes the input clocks that you must provide.

Signal Name

Description

clk_status

A clock for reconfiguration, offset cancellation, and housekeeping functions. This clock is also used for clocking the control and status interface. The clock quality and pin chosen are not critical. clk_status is expected to be a 37.5–50 MHz clock on Stratix IV devices and a 100–125 MHz clock on Stratix V devices.

clk_ref

clk_ref is the reference clock for the transceiver TX PLL and the RX CDR PLL. This input signal is not available in Sync–E variations.

The frequency of this input clock must match the value you specify for PHY reference frequency in the IP core parameter editor.

For the regular 40GbE and 100GbE IP core variations, this clock must have a frequency of 322.265625 or 644.53125 MHz ,with a ±100 ppm accuracy per the IEEE 802.3ba-2010 100G Ethernet Standard.

Despite its apparent availability in the 40-100GbE parameter editor, CAUI–4 variations do not support the 322 MHz clock frequency. For these variations, this clock must have a frequency of 644.53125 MHz with a ±100 ppm accuracy.

For 24.24 Gbps IP core variations, this clock must have a frequency of 390.625 or 195.3125 MHz with a ±100 ppm accuracy.

In addition, clk_ref must meet the jitter specification of the IEEE 802.3ba-2010 100G Ethernet Standard.

The PLL and clock generation logic use this reference clock to derive the transceiver and PCS clocks. The input clock should be a high quality signal on the appropriate dedicated clock pin. Refer to the relevant device datasheet for transceiver reference clock phase noise specifications.

The PCS clock frequency is 257.8125 MHz for standard variations, 201.416 MHz for CAUI–4 variations, and 156.25 MHz for 24.24 Gbps variations.

tx_clk_ref

In Sync–E variations (IP core duplex variations with the Sync–E option enabled), this clock replaces clk_ref as the reference clock for the transceiver TX PLL.

The frequency of this input clock must match the value you specify for PHY reference frequency in the IP core parameter editor.

rx_clk_ref

In Sync–E variations (IP core duplex variations with the Sync–E option enabled), this clock replaces clk_ref as the reference clock for the transceiver CDR PLL.

The frequency of this input clock must match the value you specify for PHY reference frequency in the IP core parameter editor.

clk_txmac

The input TX clock for the IP core with or without adapters is clk_txmac. The recommended TX MAC clock frequency is 190.90 MHz for 24.24 Gbps variations, and 315 MHz for all other IP core variations.

clk_rxmac

The input RX clock for the IP core with or without adapters is clk_rxmac. The recommended TX MAC clock frequency is 190.90 MHz for 24.24 Gbps variations, and 315 MHz for all other IP core variations.

Figure 40. Clock Generation Circuitry Provides a high-level view of the clock generation circuitry and clock distribution to the transceiver. In Sync–E variations, distinct clocks drive the TX PLL (tx_clk_ref) and the CDR block (rx_clk_ref), and the output clock from the CDR is brought out to the top level.