AN 802: Intel® Stratix® 10 SoC Device Design Guidelines

ID 683117
Date 12/14/2020
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2.4.1.2.1. GMII/MII

GMII and MII are only available in Intel® Stratix® 10 by driving the EMAC signals into the FPGA core routing logic, then ultimately to FPGA I/O pins or to internal registers in the FPGA core.

GUIDELINE: Apply timing constraints and verify timing with Timing Analyzer.

Because routing delays can vary widely in the FPGA core and I/O structures, it is important to read the timing reports, and especially for GMII, create timing constraints. GMII has a 125 MHz clock and is single data rate unlike RGMII. GMII does not have the same considerations for CLK-to-DATA skew though; its signals are automatically centered by design by being launched with the negative edge and captured with the rising edge.

GUIDELINE: Register interface I/O at the FPGA I/O boundary.

With core and I/O delays easily exceeding 8 ns, Intel® recommends to register these buses in each direction in I/O Element (IOE) registers, so they remain aligned as they travel across the core FPGA logic fabric. On the transmit data and control, maintain the clock-to-data/control relationship by latching these signals on the falling edge of the emac[0,1,2]_gtx_clk output from the HPS EMAC. Latch the receive data and control at the FPGA I/O inputs on the rising edge of the RX_CLK sourced by the PHY.

GUIDELINE: Consider transmit timing in MII mode.

MII is 25 MHz when the PHY is in 100 Mbps mode and 2.5 MHz when the PHY is in 10 Mbps mode, so the shortest clock period is 40 ns. The PHY sources the clock for both transmit and receive directions. Because the transmit timing is relative to the TX_CLK clock provided by the PHY, the turnaround time may be of concern, but this is usually not an issue due to the long 40 ns clock period.

Since the reference clock is transmitted through the FPGA, then out for the data – the round-trip delay must be less than 25 ns as there is a 15 ns input setup time. Note that the transmit data and control are launched into the FPGA fabric by the HPS EMAC transmit path logic on the negative edge of the PHY-sourced TX_CLK, which removes 20 ns of the 40 ns clock-to-setup timing budget.

With the round-trip clock path delay on the data arrival timing incurring PHY-to-SoC board propagation delay plus the internal path delay from the SoC pin to and through the HPS EMAC transmit clock mux taking away from the remaining 20 ns setup timing budget, it may be necessary to retime the transmit data and control to the rising edge of the phy_txclk_o clock output registers in the FPGA fabric for MII mode transmit data and control.

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