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In the DP83TC811S-Q1 SGMII's datasheet it says

Because the DP83TC811S-Q1 operates at 100 Mbps, the 1.25 Gbps rate of the SGMII is excessive. The SGMII specification allows for 100 Mbps operation by replicating each byte within a frame 10 times.

So the MAC can only use a speed of 100 Mb/s over SGMII to be able to TX/RX to this PHY? What if the MAC uses 1 Gb/s, will it not work at all?

ocrdu
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qand
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    no, a 100Base PHY cannot operate at 1 Gb/s. So, that paragraph already tells you what your MAC needs to do in order to talk to the PHY. I think you already got your answer? – Marcus Müller Oct 05 '22 at 10:46
  • If they just say "SGMII must operate at 100Mbps to work with this phy" then I have no problem. Their roundabout way of saying it made me thinking that 100Mbps is optional, and it can also work at 1Gbps – qand Oct 06 '22 at 06:58
  • that's really not what they're saying here. – Marcus Müller Oct 06 '22 at 07:00

1 Answers1

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SGMII is independent of the ethernet speed, while it can support 1GBPS speeds, this does not mean that the PHY can. The DP83TC811S-Q1's ethernet speed is clearly listed as 100Mbps. If you need 1Gbps, you'll need a different PHY.

What they are saying is if you do choose to use SGMII with the DP83TC811S-Q1, the SGMII frames are replicated 10 times to down-convert the SGMII speed to 100Mbps of the DP83TC811S-Q1

Voltage Spike
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  • "the SGMII frames are replicated 10 times" must this be done by MAC, or is it done by PHY automatically ? – qand Oct 06 '22 at 06:56
  • Yes, I also believe on the processor side of the SGMII you must "hold" the same packet for ten frames – Voltage Spike Oct 06 '22 at 14:05