I have my own designed board. It has USB device interface circuit. The MCU is LPC1769 Cortex M3.

Basically when I use USB bus power for the board, its USB device feature is working. It means when I connect to PC, it is identified.

But when I supply the board with external power, then the usb device feature isn't working.

The external power is driven by 24V SMPS. On the board, 5V and 3.3V are regulated by regulator ICs.

Is there anyone experienced similar like this ?

The interface circuit is very simple. Vbus is float on board. GND, D+, D- are connected. For D+, D-, 33ohms resistors are connected in series.

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    \$\begingroup\$ I suggest showing a circuit diagram, especially to show how board VCC, USB bus VCC and regulator VCC are connected. \$\endgroup\$ – Reinderien Aug 11 '17 at 16:54
  • \$\begingroup\$ There is actually no extra things in schematic. According to @Alie Chen's comment, I added the missed connection. D+ is pulled up with Vdd ( of board 5V not USB 5V ). D+, D- are connected to MCU D+, D- in series of 33ohms resistor. The GND pin of USB connector is just connect to board +5V GND. That's it all. \$\endgroup\$ – Paul Jay H.K Aug 12 '17 at 10:16

VBUS shouldn't be "float". If powered from USB cable, the VBUS should go to pin P1[30], configured as VBUS, with proper level translation.

If on internal power, this pin still should still be related to USB VBUS on the cable. The VBUS wire is an important side-band signal in USB protocol, it should be logical HIGH for the USB device to start operating.

You need to check your "own designed board" against manufacturer's demo platform.

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    \$\begingroup\$ I just put an extended answer to a related question (4 years old), here: electronics.stackexchange.com/a/323551/117785 \$\endgroup\$ – Ale..chenski Aug 11 '17 at 19:33
  • \$\begingroup\$ Yes. You are right. I made a mistake to not connect Vbus pin. Thanks a lot. But now I have different error. My Windows XP displays ' USB Hub Power exceeded' error. So the USB device detection isn't successful. Sometimes it succeeds one a tenth. I have unplugged all the USB connection except only my board USB connection. But still the USB Hub Power exceed error message shows. \$\endgroup\$ – Paul Jay H.K Aug 12 '17 at 10:15
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    \$\begingroup\$ @PaulJayH.K, it means that you overloaded your host port. What do you mean "not connect Vbus pin"? Where? In self-powered case the VBUS should be used as LOGICAL HIGH signal, not taking any power from host port. You need to separate power and VBUS. \$\endgroup\$ – Ale..chenski Aug 12 '17 at 15:36

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