2019/12/18 13:58:02
I've prepared embedded system board based on KSZ8081 PHY. I've soldered all stuff and encountered problem with modifying PHY's registers over MDIO. I am able to read all registers. Values make sense. But I am not able to modify registers. I've sniffed MDIO with an external device (TotalPahse Beagle) while read-write-read register #4 (Auto-Negotiation Advertisment)  operation and result you can see on the ksz8081_mdio.jpg. Tried to clear bit15 (Next Page).
Schematic for PHY circuit could be found on ksz8081_schematic.jpg
VDD_3.3 = 3.3V
VDD_1.2 = 1.15V
50MHz clock is available on REF_CLK.
Straps configuration is about enabling auto-negotation but it never finished. It could be separate problem. So now I would like to focus on MDIO problem.
Does someone has any clues, insights what could be wrong?

Attached Image(s)

2020/11/13 14:12:23
I'm having exactly the same issue.   Register writes won't "take".   Did you get to the bottom of this?
2020/11/16 13:04:17
In my case there were two problem with my design.
First was about wrong footprint for PHY. I've designed thermal pad with wrong shape, e.g. too big. I've soldered IC manually and probably put too much solder on thermal pad. That must have resulted with small distance between IC pins and soldered down thermal pad. And all of that must have resulted with electrical parameters degradation.
Second issue was about crystal. I've seen clock on REF_CLK but I think it was not stable.
All of that together resulted system was not stable. I had suspicion IC got reset all the time. That's why PHY registers modification doesn't work - still got defaults. But I have no proof for that.
I've corrected thermal pad followed the specification. I've put there slightly bigger vias on thermal pad to allow solder exceeds to have a way to be removed while soldering. But it makes sense only in case manual soldering I guess.
I've replaced crystal with clock generator as well.
All of that made PHY worked fine.
I've requested to manufacture 30 boards. Only one of them presented problems with PHY but it was because of manufacturer put PHY IC 90degrees rotated while assembly process :).
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