We're currently designing a custom board based on SABRE SDP. We chose to use KSZ9021RN as ethernet PHY.
We're having some difficulties about the voltage levels.
In our design:
NVCC_RGMII = 2.5V and NVCC_ENET = 3.3V as in SABRE SDP.
AVDDH = 3.3V, DVDDH = 2.5V (similar SABRE SDP's configuration with AR8031).
Since the voltage domain of the signals RGMII_MDC, RGMII_MDIO, ENET_REF_CLK, RGMII_INT is NVCC_ENET,
it seems to me that a voltage level translation is needed for these signals. However, in SABRE SDP, a level translator
exists only for ENET_REF_CLK.
In datasheets of both AR8031 and KSZ9021RN, it says that an external pull-up resistor is required to 3.3V/2.5V digital VDD
for MDIO and INT. Since RGMII 2.5V is used in SABRE SDP, I believe it should be the latter (2.5V), yet they are tied to 3.3V.
What am I missing here? This issue confuses me a lot and I cannot be certain of the connections in our design.
Can anyone clarify this issue?
Thanks in advance.