SLAZ333AC October   2012  – September 2021 MSP430F6635

 

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6.49 USB12

USB Module

Category

Functional

Function

The 2nd byte of a slave-to-host transmission is sent twice.

Description

In extremely rare cases, when the USB module's PLL is disabled (by clearing the UPLLEN bit), the USB module can be placed into an undetermined state, resulting in an extra byte being sent to the host over the bus. The PLL is usually disabled by software when the USB module detects that the USB device has been suspended by the host. Suspend events can occur at any time, but are typically invoked during periods of inactivity.

Workaround

Once this error occurs, the USB module needs to be reset (by clearing the USBEN bit), and then the module can be re-initialized.  For example, software can call the MSP430 USB API USB_disable() followed by USB_enable(). These actions are taken by the USB APIs when the user unplugs and replugs the USB cable, which is likely to happen when the user realizes the bus is no longer working.  

If automatic detection of the error is required, then software on the host and device could implement a CRC check on the data payload (above the USB API) to detect the extra byte.  If detected, software could then disable/re-enable the USB module.  (The CRC inherent in the USB protocol calculates over the data packet, and thus cannot detect the erroneously added byte.)