SLAZ333AC October   2012  – September 2021 MSP430F6635

 

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6.46 USB9

USB Module

Category

Functional

Function

VBUS detection may fail after powerup

Description

In rare cases, some USB-equipped MSP430 devices may experience a failure in the bandgap that aids in detecting the presence of 5V on the VBUS pin. Two primary effects of this are:
- The USBBGVBV bit fails to show the presence of a valid voltage on the VBUS pin.

and

- The USB LDOs fail to start.

Workaround

This error state can be "reset" by clearing all the bits in the USBPWRCTL register, which disables the USB LDOs, among other actions. The bits can then be set again normally, and the device functions properly.

This has been added to the USB_Init() function in v3.11 and later of the MSP430 USB API. Therefore, this problem is automatically addressed in applications using the API.

However, if the integrated 3.3V USB LDO (the output of the VUSB pin) is used to power the devices's DVCC pin, as in many bus-powered applications, and if the rare bandgap error occurs, the CPU will fail to power up, because the USB LDO fails to operate. The problem might be resolved by cycling power to the VBUS pin; for example, if the end user responds to the failure by unplugging and re-plugging the USB cable. The bandgap failure is also known to occur more often with slow DVCC ramps >200ms; for example, when there is excessive capacitance on the DVCC pin, in excess of what the USB specification allows. However, the only sure way to prevent the problem from occuring in the first place is to avoid making DVCC power reliant on VUSB.