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?
After reviewing this case for the afternoon, I can understand the misunderdtanding in the documentation. In short, the documentation is not very explicit regarding "self powered" in terms of USB communicaion.The interrupt you are getting when you connect VBUS to 3.3V or GND is expected. In order for there to be USB communication between a host and the target device, VBUS must always be 5V. This is a USB standard. To answer your final sentence, " How do I configure a self powered USB system without using +5V on the VBUS pin?" - If you want to maintain USB communication between the host and target device, you cannot.
In order to progress with development of your system I need to understand more about your end goal. This will help us configure your power management to determine what should be receiving power from where.
I hope this helps
Hi! I was dealing with a similar problem. I had an external regulator powering the VUSB line (through a schottky diode) and also the VBUS connected (also through a schottky diode).
In that situation, I intended to disable the internal 3.3 LDO (since I have 3.3 externally).
I have the MSP configured as MSC device, and when I connect it the PC didn''t recognized the device.
The solution in that case, was to enable the LDO and all was OK.
So, the conclusion was that If you want to work with the USB connection to a host, I MUST enable the LDO.
The extrenal source to VUSB seems to be usefull only when you need the D+ and D- lines configures as I/Os (withoutVBUS available)
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