DP83826E: Trace length on MDI side before\after magnetics

Hi Meir,

On the trace length, we recommend the following

1. MDI routes from PHY to Magnetic to be less than 3 inches with length difference between differential pair not exceeding 20 mils (budget for whole routing from PHY to connector)
2. MDI routes from Magnetic to RJ45 not exceeding 1-2 inches.
3. MII trace length not exceeding 5 inches with skew between them not exceeding 200ps.

Impedance on all these lines should be contained to +/-5% variation from the intended values.

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Regards,
Gokul.

Hi Gokul,

Thank for your answer

We have mechanical constrain on the PCB  - 2 boards of ~23cm  Rigid-Flex.  Total  Trace length of ~46cm

Trace length between MAC to PHY (DP83826E)  :  ~37cm

Trace length between PHY (DP83826E) to  Magnetic :  ~7mm

Trace length between Magnetic to Connector  :  ~8cm 

The placement of the MAC And the Connector on PCB are Rigid  - Cannot be move due to enclosure box

only the PHY and Magnetics have flexibility to change placement

What is the best solution to those constrains ?

Thanks,

Meir

Hi Meir,

Total length of 46 cm ( 18 inches) is too much for the distance between MAC and connector. I don't recommend a distance of greater than 6 inches between MAC and PHY, Distance between magnetic and the connector should be made as short of possible (8cm should be okay to be used). Reset of the distance should be between PHY and magnetic, if there is no other choice.

This will affect the standard compliance of the PHY on MDI side. The performance can't be guaranteed as this is not validated. I suggest running good amount of system tests before qualifying your board.

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Regards,
Gokul.

HI Gokul,

We will try this Diagram

If we will use the lower rate  10Mbps - Is it going to improve performance despite the problem of  trace lengths?

Another approach we consider to use it that solution

Using DS64BR111 as reDriver 

Can we use DS64BR11 to improve performance?

What other Solutions\alternatives are there that can overcome on long trace length problem?

Repeater mode ?

Regards,

Meir

Hi Meir,

Going to 10M speed will help with having a longer MDI routing. But, is your application okay with 10M speed?

DS64BR111 is not compatible with ethernet PHYs and hence not recommended for use.
It is hard to do a repeater configuration as we will end up needing 3 ethernet PHYs and it will not be a effective way of using the board space and cost constraints.

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Regards,
Gokul.

Hi Gokul,

Can  we use this Passive Power-Saver Equalizer DS38EP100 ?

Is there any IC that work at rate of 10\100Mbps ?

Compatible to PHY ethernet DP83826E ?

Regards,

Meir

Hi Meir,

DP83849I can be used in 'Extender' mode and can be used. DP83849I can be used as repeater.

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Regards,
Gokul.