18.104.22.168.3. 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 50MHz 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: 4-bits @ 25 MHz/2.5 MHz to/from 2-bits@ 50 MHz, 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, and 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 block to select between counter outputs glitch-free.
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