0
\$\begingroup\$

I'm using a TUSB2046B hub connected to a raspberry pi zero. The hub circuit looks like this:

enter image description here

There appears to be a 12MHz sine wave coming from the crystal (as counted by my scope) and 3.3V on the VCC, /EXTMEM and EEDATA pins. If I connect a device to the Pi using a USB OTG micro-to-micro cable, lsusb shows the device. But if I connect the hub shown in the diagram, it shows nothing. And I don't see any signalling on the USBDM / USBDP lines when I inspect them with the scope.

Have I got something seriously wrong? Is there some good reason it's not talking back to me?

\$\endgroup\$
5
  • \$\begingroup\$ What is the DC voltage on the DP/DM lines? A connected (full/high speed) device should pull DP to about 3.3V when the bus is idle. \$\endgroup\$
    – Turbo J
    Jun 20, 2018 at 13:52
  • \$\begingroup\$ Both lines are at approx 0V when idle. The data sheet for the TUSB2046B shows the DP line pulled to 3.3V through 1.5k-ohm, but other users of the same device (eg openhardwarecoza.wordpress.com/2015/12/07/…) don't even bother with the isolation resistors - they just wire DP and DM directly to the Pi's USB port. \$\endgroup\$
    – Tom
    Jun 20, 2018 at 14:43
  • \$\begingroup\$ Just try to connect a 1k5 resistor from USBDP to the 3.3V source for testing. It is possible that the chip behaves differently in self powered vs. bus powered mode. \$\endgroup\$
    – Turbo J
    Jun 20, 2018 at 15:28
  • \$\begingroup\$ I'll have a go and report back. \$\endgroup\$
    – Tom
    Jun 20, 2018 at 15:45
  • \$\begingroup\$ Do you mean TUSB2046B full-speed hub from Texas Instruments, right? \$\endgroup\$ Jun 21, 2018 at 0:26

1 Answer 1

2
\$\begingroup\$

You have no speed setting selected for the uplink. In the datasheet Figure 11 it shows the complete schematic.

enter image description here

\$\endgroup\$
4
  • \$\begingroup\$ +1 This is a full-speed extinct IC. It needs also "smoothing" caps on D+/D- lines, something like 22pF, otherwise the signals might be ringing and confuse normal high-speed USB 2.0 hosts. \$\endgroup\$ Jun 21, 2018 at 0:30
  • \$\begingroup\$ @AliChen. You can't service High Speed USB on a Full speed hub....so the devices won't be 'confused' ...they simply won't and can't work. \$\endgroup\$ Jun 21, 2018 at 20:51
  • \$\begingroup\$ I am not sure what you are objecting to. Did I say "confusing devices"? No. Pi Zero is a HS host. FS hubs always should work under HS hosts. FS hub must exercise edge control, so it needs the caps, to avoid possible non-monotonic edges and cause confusion during CDR - clock-data-recovery. Shall I continue, or stop here? \$\endgroup\$ Jun 21, 2018 at 21:21
  • \$\begingroup\$ @AliChen You are right, my bad ...you did say the host might be confused. But the datasheet does show the caps ...so one would expect no problems with this FS plugged into a HS host. \$\endgroup\$ Jun 21, 2018 at 21:49

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.