I have a problem with a device that I’ve built: It contains a LPC4300 microcontroller and it's supplied with 24V DC.
I’ve noticed a strange problem when I connect it to a USB port. The device is used as a Slave.
First I have to say that this device has no USB protection (my fault) and all the USB pins of the microcontroller are directly connected to the USB connector.
The problem is the following:
- When I connect the device to my desktop PC it works correctly.
- If I turn off the device and I connect it to my Laptop, the device also works correctly.
- But if I connect it first to my PC desktop, then I disconnect the USB cable and I connect the device to the laptop, the USB port results broken (I measure a short circuit between the VBUS and GND line).
- This happens just when I don’t turn off the device.
What could be the cause of this problem?
The laptop supply should be isolated (I measure more that 1MOhm between the Earth and the Output pin).