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I'm working on an embedded system project, utilizing an STM32F103 microcontroller, and an FE1.1S USB hub.

The STM32F103 fails connection to the FE1.1S USB hub (wired to port 4).

Ports 1-3 work on the FE1.1S hub, even when using a "blue pill" STM32 development board.

I've made 3 PCB revisions so far. The STM32 on 2 of the 3 revisions basically never connects, while the microcontroller on the other revision will connect to the hub if I cycle the power just right during the attempted connection with the USB hub.

I know the FE1.1S is far from a premium USB hub, but the blue pill works reliably, so I'm convinced it's up to the task. I'm not doing much data transfer, and am aware it's a single TT device. Expected additional devices are: Wifi N dongle, Bluetooth adapter, and keyboard / other HID device.

The STM32 firmware is working properly, as there's a status LED that blinks when it's running. Regardless of USB hub connection status, the status LED blinks as expected.

The D+ and D- lines between the STM32 and USB hub have 22R series resistors, and the D+ line has a 1K5 resistor pulling it up to 3.3V, per spec.

Troubleshooting already performed:

  • Source 3.3V from bench power supply instead of pcb power supply
  • Source 3.3V from LDO connected to 5V on same PCB
  • Pulled D+ and D- out of circuit, into different USB port on FE1.1S hub
  • Added 5 and 10 second delays in the firmware before enabling D+/D-
  • Wired a switch in-line with the D+ 1K5 pull-up resistor to manually pull-up on demand
  • Monitored the D+/D- lines on an oscilloscope. Observed 1ms spaced pull down pulses in two pulse trains during handshake before flatline on D+ line.

Schematic PCB Layout

Schematic Notes:

  1. DFU is pulled high using a dip switch located elsewhere on the PCB
  2. Selectable voltage jumper (J27) is a recent addition to troubleshoot the connection
  3. Zener diodes on some inputs are intended to act as crude protection devices against 5.1V and higher
  4. VCC (5V) > LDO > VDDA (3V3) > DIP Switch > FB > VDD (3V3) - complicated, I know, but I want to be able to disable the microcontroller from a switch
  5. Decoupling capacitors are as close to VDD pins as possible
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  • \$\begingroup\$ "The STM32 on 2 of the 3 revisions basically never connects" this sounds like a more general design problem, are you really manipulating the pullup correct? Or does that only happen on the problem port of the problem hub? \$\endgroup\$
    – Chris Stratton
    Oct 20, 2020 at 17:24
  • \$\begingroup\$ 1st revision: the STM32 connected maybe 3 times across hundreds of different attempts and things I tried. 2nd revision: I can get it to connect if I toggle the power DIP switch just right. It stays connected. 3rd revision: can't get it to connect at all. Diff between rev2 and rev3: VBUS > LDO > 3.3V. \$\endgroup\$
    – thrsaherx
    Oct 20, 2020 at 17:39
  • \$\begingroup\$ That sounds like your design for enumeration is seriously wrong in a way that has nothing to do with this particular hub... \$\endgroup\$
    – Chris Stratton
    Oct 20, 2020 at 18:07
  • \$\begingroup\$ Care to elaborate? I've checked voltage time and time again and it's solid 3.3V to the STM32 \$\endgroup\$
    – thrsaherx
    Oct 20, 2020 at 18:08
  • \$\begingroup\$ Take out those series elements, put a USB cable on it and plug it into various hosts. If that doesn't reliably work you don't have a proper enumeration scheme. Realize a self-powered USB device is supposed to monitor VBus and apply the pull-up responsively. Your all-on-a-board scheme makes it harder still. But get it working in a traditional case first. \$\endgroup\$
    – Chris Stratton
    Oct 20, 2020 at 18:53

1 Answer 1

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The oscillator load capacitance was incorrect. I had C22 and C23 as 20 pF, where the blue pill has 12 pF capacitors. The behavior followed the capcitors after swapping them between my board and the blue pill.

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