Tracking of USB signals on 2 layer board is OK? - Page 1
Forum section: electronics / projects

author: makebit
author_rank: Unknown
timestamp: October 12, 2025, 01:19:22 am
content: Hi,Obviously I am giving away no secrets here as it just shows chip footprints with questionable layout.We have here (as attached) some high speed layout on a 2 layer board with a badly cut-up ground plane and decoupling capacitorsnot at all near to Vcc pins of chips.Do you think we can get away with this?So the USB signal comes in on the USB-C connector....then the USB signals get tracked to the shown USBLC6-2SC6ESD protector chip.Then from there the USB signals go to the TS3USB221ARSER USB switch chip.Then from there the USB signals go to the CH340x (USB to serial port chip)The blue (bottom layer) copper pouring is GND net.The Red (top layer) copper pouring is ground net also.One of the images shows the four 100nF/0402 capacitors on this bit of the board...these are the only capacitors that exist on this shown bit of the board.So do you think this layout is adequate?USB-C connectorhttps://www.lcsc.com/product-detail/C165948.htmlTS3USB221ARSER (high speed USB switch)https://www.ti.com/lit/ds/symlink/ts3usb221a.pdf?ts=1760207924065&ref_url=https%253A%252F%252Fwww.google.com%252FUSBLC6-2SC6 (ESD protection chip for USB)https://www.st.com/resource/en/datasheet/usblc6-2.pdfCH340x (USB to serial port chip)https://www.lcsc.com/product-detail/C3035748.html

author: wraper
author_rank: Unknown
timestamp: October 12, 2025, 01:32:23 am
content: If there is just CH340, then TS3USB221ARSER is there for what? CH340 itself is a USB full speed (12mbps) device, so 2 layer board is not an issue for that.

author: sleemanj
author_rank: Super Contributor
timestamp: October 12, 2025, 02:03:12 am
content: Quote from: wraper on October 12, 2025, 01:32:23 amThe PCB shows traces for a second USB end point from the TS3 (the two pins above where the CH3 connects to the TS3, running off vertically).

author: wraper
author_rank: Unknown
timestamp: October 12, 2025, 02:15:15 am
content: Quote from: sleemanj on October 12, 2025, 02:03:12 amOP did not mention that. It's important what that something else is, that's why I asked

author: makebit
author_rank: Unknown
timestamp: October 12, 2025, 10:28:00 am
content: Thanks for that, yes the other track pair of the TS3USB221ARSER goes off to a BQ25895 battery charger IC as shown attached.(i am not giving any secrets away by attaching this as its just a load of footprints).Is the routing of those USB traces OK?.....shouldnt they have ground plane beneath them everywhere?The shown USB track pair is approx 50mm long.BQ25895https://www.ti.com/product/BQ25895

author: wraper
author_rank: Unknown
timestamp: October 12, 2025, 11:29:49 am
content: Then it should be fine.

author: makebit
author_rank: Unknown
timestamp: October 13, 2025, 09:36:26 am
content: Thanks, i didnt realise USB was exempted from "high speed digital" layout requirements. Eg stripline transmission line type thing etc etc.

author: tggzzz
author_rank: Super Contributor
timestamp: October 13, 2025, 10:31:08 am
content: Caution.It is believed that makebit==Treez/Faringdon/ocset/zenerbjt etc etc