Hard Processor System Technical Reference Manual: Agilex™ 5 SoCs

ID 814346
Date 4/01/2024
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5.1.9.3. RGMII Clock Skew Guidelines

RGMII is the most common interface because it supports 10 Mbps, 100 Mbps, and 1000 Mbps connection speeds at the PHY layer.

RGMII uses four-bit wide transmit and receive data paths, each with its own source synchronous clock. All transmit data and control signals are source synchronous to TX_CLK, and all receive data and control signals are source synchronous to RX_CLK. For all speed modes, TX_CLK is sourced by the MAC, and RX_CLK is sourced by the PHY. In 1000 Mbps mode, TX_CLK and RX_CLK are 125 MHz, and Dual Data Rate (DDR) signaling is used. In 10 Mbps and 100 Mbps modes, TX_CLK and RX_CLK are 2.5 MHz and 25 MHz, respectively, and rising edge Single Data Rate (SDR) signaling is used.

Figure 87. RGMII MAC/PHY Interface

This section addresses RGMII interface timing from the perspective of meeting requirements in the 1000 Mbps mode. The interface timing margins are most demanding in 1000 Mbps mode, thus it is the only scenario you consider here.

At 125 MHz, the period is 8 ns, but because both edges are used, the effective period is only 4 ns. The TX and RX buses are separate and source synchronous, simplifying timing. The RGMII specification calls for CLK to be delayed from DATA at the receiver in either direction by a minimum 1.0 ns and a maximum 2.6 ns.

In other words, the TX_CLK must be delayed from the MAC output to the PHY input and the RX_CLK from the PHY output to the MAC input. The signals are transmitted source synchronously within the +/- 500 ps RGMII skew specification in each direction as measured at the output pins. The minimum delay needed in each direction is 1 ns but Intel recommends to target a delay of 1.5 ns to 2.0 ns to ensure significant timing margin.

You can use the methods described below to meet desired RGMII clock skew relationship.