One key feature of this EVM is that level shifters are used to allow the PHYs to communicate with the ASPEED AST2600 processor. The AST2600 contains four RGMII interfaces, however, two of them are forced to use VDDIO = 1.8V. However, the MDC/MDIO lines from the AST2600 all have VDDIO = 3.3V. So, to have a consistent VDDIO level, level shifters are used to convert the MDC/MDIO voltage level from 3.3V to 1.8V.

One important design consideration is that the level shifters used contain built-in pullup resistors, so users must include these resistor values in their calculations when deciding what pull-up resistor to use for the MDIO lines. The schematic below shows which level shifters were used.

Figure 3-5 Schematic : MDC/MDIO Level Shifters