[PIC] PIC + USB hub problem with using non-standard connectors

[kkolehma]

Hello,

First I must say that I am pretty noob with electronics design, but I try to understand things best I can.

I am having a hard time with one embedded system I trying to get to work. Problem relates to USB which is used as communication infrastructure. There is a host device that has several PIC based devices connected via USB hubs (max two in between the host and device). All the wiring is internal and space is an issue so I am trying to use JST SR series connectors like these: https://www.jst-mfg.com/product/pdf/eng/eSH.pdf with ribbon cables (I have tried to twist the data wire pair) which is unshielded. Maximum wire length is about 5 cm (or two inch) with ribbon cables.
USB Hub is based on TUSB2077A chip and made also by me which adds to the uncertainty...

The problem I am facing is that devices don't enumerate to the host. If I connect device directly, it does enumerate without problem. Also if I connect real device (pic kit 3) the USB hub I have made, it works just fine. When I connect the device that I have made to the USB hub I have made, it goes to hell. Device power up alright, but that's it.

There is one known issue with the USB Hub however. USB hub power manager (TPS2044) trigger over current issue in power up. There aren't RC filter currently between TPS2044 and TUS2077 which probably results that when capasitors are charging, the over current is drawn. I have tested the configuration also self-powered so that over current condition is not an issue but no change to the underlying problem.

Right now I haven't got faintest idea what could be the real problem (apart from both PIC device and USB HUB being borderline sketchy design)... Alone they work, but not together... Only thing that changes is the cables and connectors. The cables I have used to test HUB and device with PC, have same JST connectors in one end although they use shielded type cable (apart from 1cm part before the JST connector)

Any suggestion of what could be the problem or where to look next would be greatly appreciated.

--- Kari

 

[FvM]

There are unlikely any problems with short unshielded cables (e.g. 10 or 20 cm) for full speed USB. The problem is most likely about your hub design and hasn't to do with the cabling.

You didn't report circuit details, so no further comments possible.

 

[kkolehma]

Thanks for the input. I also thought it would be unlikely that short unshielded cables could be the problem unless there was some massive EMI from some device (Pic device has 24Mhz crystal). Here is the schema of USB hub. I have added RC filter between TPS2044 and TUSB2077A (ganged power management) and ferrite beads to the downstream port power lines. Those are not on the real device right now.

I have followed the TUSB2077A data sheet best I can. What puzzles me is the fact that with pickit connected to HUB with custom cable, it works OK...