| Electronics > Projects, Designs, and Technical Stuff |
| need help in troubleshooting problem in my layout design with lan8742a |
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| bson:
--- Quote from: mrpackethead on October 16, 2018, 06:02:17 am ---Also, why did you try to path lengh match the TX/RX pairs? really no need to do that at all, its only 125Mhz.. --- End quote --- Agreed, but no good reason not to if there's plenty of space - it does improve common-mode rejection. And the 100M frequency is only 31.25MHz since it uses MLT-3... |
| bson:
How is this chip different from the LAN8740A? Anything other than WoL support? I do note you don't seem to have any management interrupt support, but I guess the PHY can be polled for things like link status or speed changes. Have you made sure it doesn't overheat? PHYs can be pretty power hungry... Also, QFN and HASL doesn't go well together in my experience... you should at least clean excess tinning off the pads before reflowing. It could be a combination of the chip "floating" in solder along with thermal stress that causes cracks in solder joints. Maybe also the PCB flexing from thermal stress. I'd make sure it gets a bit of a top/bottom zone to dissipate heat, then pay the extra for ENIG... Oh, and if you have access to a scope with an active FET probe, check the xtal to make sure the XO is running and the levels look reasonable. Just a quick and easy check, just to make sure the problem is with the IC and not the crystal or load caps. Speaking of load caps... I hope they're C0G with a reasonable TC? |
| Siwastaja:
--- Quote from: me.rishabh12 on October 16, 2018, 10:29:46 am ---and is it any drawback of length matching the pairs? --- End quote --- Yes, the drawback you get is you need to read whining on the EEVblog forum even when your problem is totally unrelated. TLMJW's, or Trace Length Matching Justice Warriors, looking to comment on "unnecessary" trace length matching, is the new trend now after the DGND/AGND dead horse has become too stinky. I understand the irony in length-matching a 10kHz I2C or a blinking LED IOs, and I tend to have a laugh as well. But really, this is a fairly high-speed impedance controlled design which, while working just fine without it, is not too far from the design territory where the length matching starts to really matter. But TLMJW's are pissed because now every beginner can use the simple length-matching tool in their PCB packages so it's not their high-speed black art anymore. |
| mrpackethead:
I had this exact same issue, when i was developing my first prototypes and did a hand assembly. When VCCDR is over 1.26V the PHY becomes unreliable. and its related to how well that central pad is soldered down. |
| Apollyon25_:
Another gotcha with these Microchip LAN parts can be the high crystal drive level from the chip, and the crystal's rating. Found this one the hard way. Agree however that the thermal pad looks insufficient. |
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