ETHERMET PHY pins MDIO/MDC, through resistor or direct connection?

[triedgetech]

Okay, so there's a PICO-IMX8M-MINI dev board, and what bothered me is that the pins of PHY AR8031_AL1A chip has its pin MDIO connect to the CPU BALL N19 through a 100K ohm resistor.

(X1_35 N19)

See p. 18 of the dev kit datasheet. Scroll down for schematic under "Carrier Board".

schematic

But why? I looked at the R310 and R310 both are 0402 100K resistors. Was this a mistake? Shouldn't MDIO be directly connected to the MAC portion of the i.MX CPU?

I wanted to use a different PHY (make my own board), and now am wondering if I need the 100k ohm resistors for mdio/mdc pins between MAC(inside CPU chip) and PHY.

[igorpadykov]

Hi Ed

yes they can be connected directly if voltage levels on both sides are equal, also

one can look at NXP i.MX8M Mini EVK design  i.MX 8M Mini Evaluation Kit LPDDR4 Design Files

Questions for PICO-IMX8M-MINI board can be posted on its vendor support : 

https://developer.technexion.com/v1/en

Best regards
igor

[triedgetech]

I see thanks! Would you happen to know what voltage is on the side of CPU i.MX8M for mdio line? Is it 2.5V? The Ethernet physical-layer transceiver I'm using is KSZ8091, its pull up for mdio pin is 3.3V.

So if there's mismatch, I should use 100k resistor for MDIO between PHY (KZS8091) and MAC?