CS8900 Link Light/LED

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I've designed a little board with a Cirrus Logic CS8900A-CQ3 ethernet
controller on it, in 16-bit mode. When I connect my board to a hub, I
only get a link light on my board, but not on the hub. However, I am
able to read 0x630E from the PacketPage 0x0C register as Cirrus says I
should be able to do in the AN 205 application note.

Furthermore, using an external loopback connection (TX+ tied to RX+,
TX- tied to RX-), I do not get a link light on my board and the LinkOK
bit in the LineST register is clear (it was set when I was connecting
to the hub). Note: I set the FDX bit in the TestCTL register to 1 for
the loopback test.

I have checked that the 20MHz crystal I am using is producing an

It seems that my TX+/- connection is bad, but I've double checked my
schematic against those in the AN 83-3 application note and I have
checked for bad solder joints. Cirrus says in their datasheet that I
should get a link light on both ends with only power applied, so I
don't think I need to set any registers to transmit data to get a link
light on the hub. Has anyone discovered otherwise?

Any ideas what could be wrong? Any ideas for tests I should run to
debug this problem? I have posted the schematic on
http://www.its.caltech.edu/~hiszpans/ethphone/schematic.pdf . The
ethernet section is on the last page. Please note that I have made one
revision since this version: TEST (pin 76 is now tied high. I had it in
boundary scan mode :P).

On a related note: how do I probe with an analog two channel scope a
twisted pair ethernet line? I connected one channel to Tx- the other to
Tx+, set channel 2 to invert, set the channels to add, set both to
1V/div, and ran at 10ms/div. I also connected the common clips on each
lead together. With only one test lead connected I see a 2.8Vpp 16ms
period link pulse, but I see this on all 8 wires. With both test leads
connected I get nothing. What am I doing wrong?

I would appreciate any help.


Re: CS8900 Link Light/LED
Fyi, I fixed the problem. My SMT transformer had a bad solder joint on
the Tx+ pad that was not visible with the eye, but was with the
continuity checker.


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