Ethernet Communication over longer cable length

[dhara_pillai]

Hi NXP Team,

We are designing a product on i.MX8M Mini processor and Ethernet signals are used for VoIP calling and we have implemented Microchip PHY- KSZ9031.

We are trying to check the communication/ data transfer from our product to network using 40 meter CAT5 cable length. But some how this is not getting possible.

We checked with 20 meter and 10 meter cable length and data transfer is possbile. Is there any way to increase the drive strength of differential lines- TXRXP/TXRXM pins..? We have worked out on driving strength for RGMII lines.

But just want to check if through any register setting we can control the drive strenght of the differential lines or any suggestion from NXP to test the communiction with more than 40 mtr cable length..?

Pls advice.

-Dhara

[nija_mankodi]

Hi Mrudang and Gusarambula,

Thanks for your prompt support.

I am working with Dhara on the same project. I have attached our Ethernet design for your review.

Let us know if you need any further information.

[mrudangshelat1]

Hi Nija,

Have you tried the as per Gusarambula comment on driving strength?

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drive strength property you may change it on the DSE (Drive Strength Field) property on the IOMUXC_SW_PAD_CTL_PAD_ENET register of the ENET signals, albeit my recommendation would be using the Pins Tool for i.MX if you aren’t using it already as it makes much easier to setup these registers.

https://www.nxp.com/design/designs/pins-tool-for-i-mx-application-processors:PINS-TOOL-IMX

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Comments on the schematic: Follow the EVK design of Ethernet Phy.

• ESD protection diodes U6 and U1718 should be near to the RJ-45 connector not after Phy Chip. And it is not required.
• Is there any particular reason to use ESD diode and Common mode choke? You have a magnetic transformer before the RJ-45 connector.

gusarambula‌

Kindly provide your view.

Mrudang

[dhara_pillai]

Hi Mrudang,

Thanks for reviewing the schematic.

Due to mechanical constraints in our design, we had to plan RJ45 on seperate board (Board1) and then signals are routed to another board (Board2) through flex cable.

So it was recommended to implement seperate Magnetics rather than choosing RJ45 with integrated magnetics. So magnetics is implemented on Board2 with signals passing through with ESD and CMC and other side of magnetics routed to PHY.

gusarambula we have configured drive strength property on the DSE (Drive Strength Field) property on the IOMUXC_SW_PAD_CTL_PAD_ENET register of the ENET signals and have changed drive strength to maximum and minimum value and checked. But still the data transfer is not possible with 40 meter cable.

Are there any drive strength values that you have tested for these ENET signals for data transfer over 100meter cable..?? Please share ..

Thanks,

Dhara

[gusarambula]

Hello Dhara,

The 100m is part of the Ethernet standard under ideal conditions, but there may be several factors that could degrade the signal so that this maximum length cannot be met like outside interference or temperature.

I understand the logic behind your design decisions but having the magnetics on a different board and them going through a flex cable before the PHY already exposes the signals to unwanted interference for example (as twisted pair cables help to even out electromagnetic interference) which may not allow for the 100m maximum length of the ethernet standard.

I couldn’t find data on the 100m being tested for the different drive strength values. My apologies.

Regards,

[dhara_pillai]

Hi Gusarambula,

We will try to iterate interface with set up to connect RJ45 connector near to Board2, closer to magnetics-PHY interface, through some rework and test it.

Meanwhile, we tested data transfer over Ethernet interface in i.MX8M Mini EVM using 100 meter long cable, but its not working even on EVM.. Are such test conducted with different CAT cable length to evaluate ethernet capabilities on i.MX8M Mini EVM..??

[gusarambula]

Hello Dhara,

I couldn't find any details on the EVK being tested for 100m ethernet compliance or details on the maximum ethernet cable lengths tied to different drive strengths. This is probably because the EVK is similar to a smart device which most often do not require overly long ethernet cables.

My apologies for the inconvenience.

Regards,

[mrudangshelat1]

Hi Dhara,

I believe that because of multiple board connectivity for Ethernet TxRx lane cause degrade the performance. The interface should be on the same board between Phy and RJ45 to maintain the performance. 

Mrudang

[dhara_pillai]

Hi mrudangshelat‌ and gusarambula‌

We tried with a newly purchased CAT6 cable with 40 meter, 60 meter and 100 meter cable length..

And Good News is.. Our board is able to establish connection and transfer data successfully over all the three cables.

We didn't modified our interface set-up for Board1 and Board2. As explained previously, the Ethernet cable is attached to RJ45 connector on Board1 and Board1 is connected to Board2  over FFC cable. Ethernet PHY and Processor is implemented on Board2.

So may be the issue would be with the earlier cable that we were using and it quality. But again, it was working perfectly when we were connecting it over to Laptop or PC, but didn't worked with our set-up.. Its strange behavior..

Many thanks for both or yours advice and suggestions..

-Dhara

[gusarambula]

Hello Dhara,

I'm glad to hear that your board is working as expected now. Thanks for letting us know!

Regards,

[mrudangshelat1]

Hi Dhara,

Please sare your schematic design for review and verification between CPU and Ethernet Phy.

Mrudang

[gusarambula]

Hello Dhara,

As Mrudang mentioned, sharing your ENET design may help to provide recommendations, just make sure not to share any part of your design you wish to keep private, as this is a public forum.

As for the drive strength property you may change it on the DSE (Drive Strength Field) property on the IOMUXC_SW_PAD_CTL_PAD_ENET register of the ENET signals, albeit my recommendation would be using the Pins Tool for i.MX if you aren’t using it already as it makes much easier to setup these registers.

https://www.nxp.com/design/designs/pins-tool-for-i-mx-application-processors:PINS-TOOL-IMX

I hope this helps!

Regards,