LAN7800 to KSZ9477 Gigabit link all nets cross! - Page 1
Forum section: electronics / projects

author: Northy
author_rank: Regular Contributor
timestamp: February 05, 2021, 10:42:56 am
content: Hi,I'm connecting a LAN7800 USB to Ethernet chip to a KSZ9477 switch using capacitive coupling:https://microchip.my.salesforce.com/sfc/p/#o0000000KAkK/a/3l000000OPR7/A5nl.qK2M99AhwtowdAMdISxnr0AJND5NAFAK73lLhsThe trouble is, all the pairs cross themselves (+ve and -ve) and all the actual pairs cross (A, B, C & D).Is there some nice tricks to routing this that doesn't violate the rules of routing gigabit ethernet too much?Ta,G

author: Scrts
author_rank: Frequent Contributor
timestamp: February 05, 2021, 01:55:04 pm
content: Quote from: Northy on February 05, 2021, 10:42:56 amSee KSZ9477 datasheet section 4.1.5You can swap A with B and C with D pairs, but not between A/B with C/D. You can swap all polarities.

author: Northy
author_rank: Regular Contributor
timestamp: February 05, 2021, 10:09:54 pm
content: thanks, I just saw that too this afternoon.Is there a good way to cross highspeed signals if there isn't any other option?Thanks,G

author: ajb
author_rank: Super Contributor
timestamp: February 05, 2021, 10:45:15 pm
content: Due to multi-level encoding the bandwidth required for Gigabit Ethernet is only 125MHz, so you should be careful with layout and routing but it really isn't that big of a deal compared to other high speed interfaces. Ethernet is pretty robust, after all it's designed to work up to 100m through buildings wired by the lowest bidder. Since you're looking at capacitively coupling you're presumably not even leaving the PCB, so you'll have plenty of margin for error. You'll be fine with just dropping the diff pair through a pair of close together vias to change layers.That said, if one of the KSZ9477's MII ports is available you might consider using a LAN7801 and connecting to the switch via RGMII instead of capacitive coupling PHY ports and just skip the PHYs entirely. (Generally switches with MII ports allow you to configure those to operate in MAC mode (for connecting to an external PHY) or in PHY mode (for connecting to an external MAC), so you easily cascade switches or connect processors or whatever without consuming a PHY. On unmanaged switches this may be a pin strapping option, otherwise you might need to poke at the management interface.)If you go with an MII connection you will need to be a bit more careful of layout, including length matching, and it's not a bad idea to throw some footprints for termination on those signals that you can adjust (or even replace with 0R jumpers) after testing.

author: Northy
author_rank: Regular Contributor
timestamp: February 05, 2021, 11:01:11 pm
content: Hi, thanks for the reply.I'm using RMII on port 6 to connect to my on board micro.Port 7 is an SGMII port, so I can't use that one can I?Ports 1 - 5 are ethernet PHYs.So that means I can't use the LAN7801?I'm a bit nervous of connecting them with capacitive coupling as it's something I've never done, but I've read about it in a few places now so it should work?Thanks,G

author: ajb
author_rank: Super Contributor
timestamp: February 05, 2021, 11:12:26 pm
content: Ahh, the product page for the 9477 makes it sound like both MII ports can do RGMII. In that case, maybe the KSZ9897R (note the R suffix!) would work for you. I haven't compared the two in depth, so not sure if it would be suitable but they appear to be similar otherwise, and the 9897 is cheaper as a bonus. Or maybe there's a USB<->SGMII part you could use. Or, of course, you can just go with capacitively coupling the PHYs like you originally planned and ignore me entirely

author: Northy
author_rank: Regular Contributor
timestamp: February 09, 2021, 09:08:22 am
content: Quote from: ajb on February 05, 2021, 11:12:26 pmThanks for your reply.I'm interested in the AVB stuff in the KSZ9477, but thanks for your feedback. At least you didn't say it's a bad idea!G