Author Topic: Can't get link on my board, eval board works fine.  (Read 6032 times)

Offline alager

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Can't get link on my board, eval board works fine.
« on: February 10, 2009, 11:28:47 PM »
I'm looking for guidance on trouble shooting the lack of link on my board.  I've run out of ideas.
I can program the 52235eval board, and it establishes link and runs, as long as I force the mode to 10Mb or 100Mb.  But using the same S19 file on my board, I can not get the link to establish. 
I've tried both the utasker and the coldfire lite systems and I get the same result.
I'm pretty sure it's my hardware, but I don't understand this chip enough to trouble shoot it.

What is going on when the chip is trying to establish link?  How can I monitor it?

Thanks,
Aaron

Offline mark

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Re: Can't get link on my board, eval board works fine.
« Reply #1 on: February 10, 2009, 11:43:33 PM »
Hi Aaron

If you monitor the TX lines and compare the two boards you may see a difference in the pulses which will give an idea.

Check the circuits carefully for any mistakes in wiring. Also the current resistor - it should be accurate (12,4k). I have heard of this being forgotten and then it doesn't work.

One problem that I once had was that a discrete transformer was used on the board. I spend a long time trying to find out why it wasn't working. Only after changing the LAN connector - when it then finally started operating properly - it became obviously that someone had soldered in a LAN connector with internal transformer on the board!! Two transformers doesn't work... and you don't actually see whether the connector has one it it or not...

Regards

Mark

Offline alager

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Re: Can't get link on my board, eval board works fine.
« Reply #2 on: February 11, 2009, 12:11:59 AM »
Mark,

The pulses pretty much look the same. print_006 is my board TXp & TXn, print_007 is the eval board.
The Rbias is 1% 12.4k.  I only have one transformer, a magjack SI-50196-F, integrated transformer and jack.
But I even took the H2019 off the eval board, ran little wires to it and a jack, with nearly the same resultant waveforms and still no link.

One odd think I've found is that if I have the bypass caps on the centertap of the magnetics, the Tx pulse becomes a runt.  The attached plots are with no bypass caps on the centertaps, just the series connected 49.9ohm resistors: TXp--R--CT--R--TXn.

Keep the ideas coming please.

Aaron


Offline alager

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Re: Can't get link on my board, eval board works fine.
« Reply #3 on: February 12, 2009, 05:33:15 PM »
Just updating to close this thread.  I forgot to pull up the center tap of the magnetics.  Now I have link and can serve up web pages.

Thanks for the help.
Aaron