Ethernet EMI voltage compatibility

[ammasikaniv]

Hi All,  I am using T2080 processor.As per processor ethernet spec, EMI1 will support 2.5V/1.8V. EMI2 will support 1.2V. I am interfacing two marvell 88E1512 (1.8V) phy tranceiver (RGMII) to both of the EMI separately. Please let me know this voltage level mismatch will make any hamper to the ethernet interface. Thanks in advance.

[ufedor]

Both 88E1512 PHYs have to be connected to the EMI1 (which supports Clause 22).
Useful reference:

http://www.ieee802.org/3/efm/public/nov02/oam/pannell_oam_1_1102.pdf 

[ammasikaniv]

Hi Fedor,

       Thanks for your reply. Now i am here to explain my existing board design (about T2080 & Ethernet PHY interface) clearly than my previous one. T2080 datasheets recommends, "Selecting RGMII limits to LVDD = 2.5 V" But I am working with RGMII by giving LVDD=1.8V. I am not able to come to conclusion that how T2080 & PHY is able to communicate with RGMII (with LVDD=1.8V) but processor wants LVDD=2.5V for RGMII?. I have placed the design below in image format.  Pleas help me to find out/clarify the problem.

[ufedor]

In the described case the following has to be done:

T2080 LVDD=2.5V

88E1512 VDDO=2.5V

EMI1 (2.5V) connected to both PHYs

[ammasikaniv]

Hi Fedor,

    Thanks again. I will consider your points for my next design. In my existing T2080 board (as per diagram placed in my previous reply), RGMII is working with EMI1 LVDD=1.8V & PHY VDDO=1.8V . Is there any suggestions/points/hints i can get from your side about how RGMII can work so with that wrong combination?. 

[ufedor]

T2080 was not characterized to operate RGMII at 1.8V.

This means that no timing characteristics are provided and guaranteed.

Nevertheless, it is higly probable that there will be no issues with the RGMII operation in this case.