I am using the USB module a MSP430F5521 along with the TI USB library for this device. I need to configure a system that is 'self powered' and does not use the VBUS voltage from the USB port. Everything works perfectly with the following USB power configuration:
VBUS = USB VBUS through a diode (+5 V) with 4.7 uF cap
VUSB = external 3.3V with 220 nF cap
V18 = internal 1.8V with 220 nF cap
PUR = connected to D+ with 1.4K
The internal 3.3V LDO is disabled (VUSBEN = 0)
The internal 1.8V LDO is enabled (SLDOEN = 1)
XT2 is running at 6 MHz
Again, everything works well with the above configuration. The external host sees the TI USB device and communication is successfull.
Now if I connect VBUS to either ground or 3.3V the system stops working. A UNMI interrupt is generated indicating a usb bus error (BUSIFG) and the USB system is shut down. VBUS should not be required since the internal 3.3V LDO is disabled. This follows the description of a self powered USB in the TO user's guide.
I can duplicate the exact same problem with the TI demo board. How do I configure a self powered USB system without using +5V on the VBUS pin?