Cora Z7 dev kit pull ups on the RGMII RxD lines

[Sean Kelly]

Can anyone explain why the dev kit has 4k7 pull ups on the RxD 0-3 lines?  I can't find anything in the PHY datasheet to indicate the need and I have never seen this done before.  Thanks.

[JColvin]

Hi @Sean Kelly,

I'll need double check this, but I believe this how some of the hardware configuration is done based on Section 7.7 of the datasheet (the one I'm looking at is here: https://datasheet.lcsc.com/szlcsc/Realtek-Semicon-RTL8211EG-VB-CG_C69264.pdf). This enables all communication speeds, enables the 2 ns TXC delay required for RGMII, and allows the module to work with 1.8V.

Thanks,
JColvin

[Sean Kelly]

Thank you so much.  That makes sense, but what does not make sense is that the datasheet states to pullup 2.5V signal lines to 3V3.  The Cora on the other hand did what I would have thought and that is tie the PUs to the bank voltage being used.  Thoughts?  I am guessing it was an oversight on the datasheet writer.

 

[JColvin]

Hi @Sean Kelly,

I asked one of our design engineers about this and they are thinking it was something similar where the RTL8211E-VL is consistently called out in other sections that mention voltage to be limited to 1.8V rather than the 3.3V (which was for other chip variants of this PHY) and it wasn't mentioned in this particular section of the RTL datasheet for whatever reason (probably because 3.3V is the more common setting for the other two chip variants), so the Digilent engineers opted to use the bank voltage as the reference pull-up resistors since that makes the most sense. Beyond that, I'm not certain on the "why".

Thanks,
JColvin