I have a USB device that I designed several years ago, using the Microchip PIC18F2550, and it has worked mostly OK on various laptops. It has four DC-DC converters driven by the USB 5V power and it is configured for the maximum 500 mA current draw. The actual current is 350 mA. The 5V USB supply connects to a 47 uF 16V aluminum electrolytic and a 1 uF 16V MLC capacitor. The 5V logic supply for the PIC and other components is fed through a Schottky diode and then multiple 1 uF bypass capacitors. There is a P-channel MOSFET which is turned on once the USB has been enumerated, and it provides power to the four DC-DC converters bypassed by a 47 uF capacitor.
The laptop, running Win 8.0, accepts the USB device with no complaints, and everything works OK. But when I plug into the Winbook, the status LEDs indicate that the device starts to initialize, but it seems to hang up when the MOSFET is turned on, and it then turns off with the LEDs showing an incomplete initialization. The odd thing is that the Device Manager reports the device to be working normally as a serial port. I tried changing the values of the large capacitors but the problem persists.
I would like to be able to operate this device from the WinBook (which is fully charged and plugged into the charger). The WinBook is running Win 8.1, which shouldn't make any difference to the USB subsystem. If I had encountered this when designing it, I could have incorporated some current limiting circuitry, but this is a mature design and not easy to modify. Any ideas on how to troubleshoot?
Thanks,
Paul