Small form factor USB 3+2 hub - two alternate designs - Page 1
Forum section: electronics / projects

author: timgiles
author_rank: Regular Contributor
timestamp: October 17, 2018, 07:21:02 pm
content: History of the project.I own a telescope mount that has a built in, unpowered USB 2 4 port hub. For those not in the know, a telescope hub is a tripod with a powered, GPS / computer controlled mechanism that can move a 'plate' to track stars, planets etc..The usb 2 hub PCB and a second PCB with telescope mount related ports on it, are built on identical 2 layer PCBs, 87mm x 23mm. These sit one above the other, under the plate that holds the telescope, in the part of the telescope mount that moves and tracks the sky, with cables for the connections board and USB 2 board being fed through the mount to the main connection point in the part of the mount that does not move. To power the telescope mount, 12V (12-14.4) is provided that feeds through to both parts. There is an identical space at the other end of the 'plate' section that is empty but can hold two other identical 87x23mm boards. The USB 2 cable will be replaced with a USB 3 if I can get the new hub working.I thought it would be a great idea to design a replacement hub that was USB 3 and powered. I could use the incoming 12V supply to power the new board(s). To this end, I designed a 12-5V (decided on 6A) and 5V-3.3 and 1.1V DC-DC boards. These I designed no problem, tested and working a treat - voltage, current and ripple all good. At the moment they sit on two seperate boards (filling the other end of the plate mechanism).Then I moved on to designing a replacement USB 3 hub board. I made up 3 revisions - based on TIs TUSB 8041 hub ic. The second and third revision worked for USB 2 transfers, but the USB 3 transfers suffered from instability. At some point during 2017 I decided that enough time and money had been spent, so I then designed a replacement 4 port powered USB 2 hub board (so at least I had a powered USB 2 4 port hub).Three photos attached: first showing the two dc to dc boards (top) and the USB 2 powered board (bottom left) and the third revision of the USB 3 hub board. (bottom right). Second photo shows the USB 2 powered board and the third, the semi working USB 3 hub board from 2017 I designed.However, like with any project started but not quite perfected/finished, I have had a nagging feeling that just one more revision was needed to 'make it work'. Here I find myself. I use Eagle PCB which supports trace length matching and do not have a problem with impedance calculations for the stack up. A few weeks ago I mentioned in a post my upcoming rework of the board and someone suggested that since there is a lack of space for all the diff pairs, did I really need 4 ports and 4 ports USB 3. The first answer is yes - most people using the mount will have a camera, a guide camera - and then possibly a filter wheel. So at least 3 ports - but - only 1 or possibly 2 needed to be USB 3. So - with this suggestion in mind, I went about designing two new revisions.In both revisions, every diff pair is length matched to well within the 0.1mm requirement. Some meanders are Eagles, some are my own. Both revisions are designed as 6 layer PCBS with 90 ohm impedance. Layer 1 = Signal (Diff pairs, almost nothing else), Layer 2 = Ground plane, 3 = 1.1V plane, 4 = 3.3V plane, 5 = 5V plane and 6 = Signal / everything else. My previous revisions were 4 layer PCBs but I thought since the costs have come down so much and this is my final attempt - I might as well give it every chance.Revision 4 (attached USB 3 Hub Rev 4 - 2 2 3 3.zip)I replaced two of the USB 3 ports with USB 2. The USB 2 ports are Wurth Electronics new 'built in filter' designs. In this design I have placed these on one side of the hub IC, allowing me to have room to place the Upstream USB 3 port on this same side and have space to route the USB 3 differential pairs (U3DPs) with plenty of room between them. I am still not 100% happy with needing to route some of the U3DPs pairs through the downstream port footprints - but I do not see any way around this.Revision 5 (attached USB 3 Hub Rev 4 - 3 2 3 2.zip)For this revision, I have switched to a design having a USB 3 and 2 port on both sides of the PCB, with the upstream port at an angle. This angle is extremely important as the incoming cable is coming in from the right hand side of the cutout. Without the angle, there is not enough space to get the cable plugged in. Not overly happy with the routing on port 4 but its just the routing of the slower DP/DN pair that I am not totally happy with. Also, the need for this weird angle of the upstream USB port means that I have needed to do the angles of the traces by eye....So - I was hoping that a few of you who have experience with USB 3 might give the two revisions a look over and feedback. Also - if you think neither will work and / or its just not possible to squeeze everything in on such a cramped PCB. I really only want to spend more money on this project, if there is a fair chance of it working.Thanks in advanceTimothy

author: timgiles
author_rank: Regular Contributor
timestamp: October 17, 2018, 07:26:26 pm
content: Images of top layer or both new designs

author: wasyoungonce
author_rank: Frequent Contributor
timestamp: October 17, 2018, 10:14:32 pm
content: Quote from: timgiles on October 17, 2018, 07:26:26 pmHi TimWell done indeed. You can never have enough USB ports for Astro work.I made or shall I say used a cheap powered USB 2 hub built in with my own power internal. Also made it a 12v and 8v camera pwr distribution.I wanted low profile to fit between my scope and dovetail plate. It’s only USB2.Still it runs main cameras, USB/RS232 converter, focus control, guide camera, etc. yet I’m still running out of ports!Oh what case did you use? Keep up the good workClear skiesBrendanSent from my iPhone using Tapatalk

author: timgiles
author_rank: Regular Contributor
timestamp: October 18, 2018, 06:10:37 am
content: The hub is built in. Also its built in to a large chunk of steel so its not possible to make the PCB any larger or open up the hole.