UbiSwitch carrier board design : routing constraints clarifications

BotBlox Hardware
1

Hi,
I have a few questions regarding the routing constraints when designing our own carrier board for the UbiSwitch module.

There is some documentation on the design guide (https://botblox.io/content/BB-AN-0004%20-%20Designing%20a%20custom%20board%20for%20UbiSwitch.pdf, page 10), but some of it is not totally clear to me :

  1. you say that on your boards, you use 95 ohms differential impedance, but then seem to say the target is 100 ohms. Should I aim for 95 ohms or for 100 ohms?
  2. for the 1Gbps PHYs, you recommend <1mm intra-pair skew but <0.5mm inter-pair skew : shouldn’t the intra pair skew be smaller than the inter-pair skew?
  3. for the skew recommendations, is it from Ubiswitch module to transformer, or from transformer to connector, or the total skew from module to connector? Or should we even include the cable?

Thanks a lot in advance
Best regards
Felix

2

you say that on your boards, you use 95 ohms differential impedance, but then seem to say the target is 100 ohms. Should I aim for 95 ohms or for 100 ohms?

The target is 100Ohms. Our board is 95Ohms which is within 10%. So aim for 100Ohms but ±10% is fine based on what is practically possible with your manufacturing process and stackup.

for the 1Gbps PHYs, you recommend <1mm intra-pair skew but <0.5mm inter-pair skew : shouldn’t the intra pair skew be smaller than the inter-pair skew?

This is a typo, it should be the other way round. Intra-pair skew <0.5mm, inter-pair skew <1mm.

for the skew recommendations, is it from Ubiswitch module to transformer, or from transformer to connector, or the total skew from module to connector? Or should we even include the cable?

The total skew on the board, from module to connector, including the transformer. The cable ideally should have similar controlled lengths but that’s not always possible; so the PCB should keep to these skew requirements as a minimum.

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Thanks a lot.

For routing to SFP+ cages (similar on your board), do you confirm that the USXGMII guidelines apply (even if, as far as I understand, SFP+ modules are using SFI instead of USXGMII)?

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USXGMII and SFI have very similar guidelines, in fact we use the same guidelines for both. You’re correct that SFP uses SFI.

By the way, we offer a design review service, so we can do a full check on your PCB and schematic to ensure it will work first time. Reach out to info@botblox.org if you would like that.

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