AN 886: Intel® Agilex™ Device Design Guidelines

ID 683634
Date 8/26/2022
Public
Document Table of Contents

5.1.7.1.3. PHY Interfaces Connected Through FPGA I/O

Using FPGA I/O for an HPS EMAC PHY interface can be helpful when there are not enough left to accommodate the PHY interface or when you want to adapt to a PHY interface not natively supported by the HPS EMAC.

GUIDELINE: Specify the PHY interface transmit clock frequency when configuring the HPS component in Platform Designer.

For either GMII or MII, including adapting to other PHY interfaces, specify the maximum transmit path clock frequency for the HPS EMAC PHY interface: 125 MHz for GMII, 25 MHz for MII. This configuration results in the proper clock timing constraints being applied to the PHY interface transmit clock upon Platform Designer system generation.

GMII/MII

GMII and MII are only available in the Intel® Agilex™ device 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.

Adapting to RGMII

The Intel® Agilex™ SoC device does not support adapting the HPS EMAC signals to RGMII using FPGA I/O pins.

Adapting to RMII

It is possible to adapt the MII HPS EMAC PHY signals to an RMII PHY interface at the FPGA I/O pins using logic in the FPGA.

GUIDELINE: Provide a 50 MHz REF_CLK source.

An RMII PHY uses a single 50 MHz reference clock (REF_CLK) for both transmit and receive data and control. Provide the 50 MHz REF_CLK either with a board-level clock source, a generated clock from the FPGA fabric, or from a PHY capable of generating the REF_CLK.

GUIDELINE: Adapt the transmit and receive data and control paths.

The HPS EMAC PHY interface exposed in the FPGA fabric is MII, which requires separate transmit and receive clock inputs of 2.5 MHz and 25 MHz for 10 Mbps and 100 Mbps modes of operation, respectively. Both transmit and receive datapaths are 4-bits wide. The RMII PHY uses the 50 MHz REF_CLK for both its transmit and receive datapaths and at both 10 Mbps and 100 Mbps modes of operation. The RMII transmit and receive datapaths are 2-bits wide. At 10 Mbps, transmit and receive data and control are held stable for 10 clock cycles of the 50 MHz REF_CLK. You must provide adaptation logic in the FPGA fabric to adapt between the HPS EMAC MII and external RMII PHY interfaces: four bits at 25MHz and 2.5 MHz, to and from two bits at 50 MHz, and 10x oversampled in 10 Mbps mode.

GUIDELINE: Provide a glitch-free clock source on the HPS EMAC MII tx_clk_in clock input.

The HPS component’s MII interface requires a 2.5/25 MHz transmit clock on its emac[0,1,2]_tx_clk_in input port. The switch between 2.5 MHz and 25 MHz must be done glitch free as required by the HPS EMAC. An FPGA PLL can be used to provide the 2.5 MHz and 25 MHz transmit clock along with an ALTCLKCTRL IP block to select between counter outputs glitch-free.

Adapting to SGMII

You can use the GMII-to-SGMII Adapter core to adapt the GMII HPS EMAC PHY signals to a Serial Gigabit Media Independent Interface (SGMII) PHY interface at the FPGA transceiver I/O pins using logic in the FPGA and the multi gigabit transceiver I/O. While it is possible to design custom logic for this adaptation, this section describes using Platform Designer adapter IP.

GUIDELINE: Use the GMII to SGMII Adapter IP available in Platform Designer.

Configure the HPS component in Platform Designer for an EMAC “To FPGA” I/O instance and choose GMII as the PHY interface type along with a management interface. Do not export the resulting HPS component GMII signals in Platform Designer. Instead, add the Intel GMII to SGMII Adapter IP to the Platform Designer subsystem and connect to the HPS component’s GMII signals. The GMII to SGMII Adapter IP makes use of the Intel HPS EMAC Interface Splitter IP in Platform Designer to split out the “emac” conduit from the HPS component for use by the GMII to SGMII Adapter. The adapter IP instantiates the Intel Triple Speed Ethernet (TSE) MAC IP, configured in 1000BASE-X/SGMII PCS PHY-only mode (that is, no soft MAC component). For more information about how to use the Intel GMII to SGMII Adapter IP, refer to the Embedded Peripherals User Guide.

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