MDIO bus: MDC signal inverted on iMX8XL

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MDIO bus: MDC signal inverted on iMX8XL

Contributor II



I'm working with iMX8DXL BSP "imx-5.10.72-2.2.0".

I'm trying to get the ethernet up, but my phytter doesn't answer.

Using an oscilloscope I see the clock signal of MDIO bus (MDC) is inverted as expexcted.

From datasheet:


As you can see here, output data must change at the latest 20ns after MDC falling edge; it must be stable when rising edge occurs.




Reality is upside down. It seems to be synchronized with falling edge instead of rising edge.

Does anyone know a way to invert (by software) this signal? Or maybe using another mdio driver?

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2 Replies

NXP Employee
NXP Employee

Because you osc is not fine enough to get the wave clearly.

The same is at "tail". It respects to the spec.  It is about 15ns.  

It respects to the spec   10ns, sourced by sta(input). 

It is 20ns for soured by phy(output), respects to the spec  0 ~ 300ns. 


BTW, spec never says failing edge, is somewhere else in spec? 

Marked in yellow in your picture from datasheet.

MDIO (Management Data Input/Output) is a bidirectional signal that can be sourced by the Station Management
Entity (STA) or the PHY. When the STA sources the MDIO signal, the STA shall provide a minimum
of 10 ns of setup time and a minimum of 10 ns of hold time referenced to the rising edge of MDC, as shown
in figure 22-16, measured at the MII connector.
When the MDIO signal is sourced by the PHY, it is sampled by the STA synchronously with respect to the
rising edge of MDC. The clock to output delay from the PHY, as measured at the MII connector, shall be a
minimum of 0 ns, and a maximum of 300 ns, as shown in figure 22-17.








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Contributor II


Setup time is about 12ns.

"BTW, spec never says failing edge, is somewhere else in spec? "

True, there is not written explanation, just "Each bit is transferred on the rising edge of the MDIO clock (MDC signal).".

I am based on the picture I attached from the iMX8DXL datasheet. There you can see that the timing of data change is related to the falling edge of the clock. 

" MDIO" from IMX8DXLAEC_RevB.pdf; " MDIO" from IMX8DXLAEC_EMU_RevB.pdf

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