Category

Functional

Function

USB interface may begin to endlessly transmit to the USB host when a rare timing event occurs between the USB host and MSP430 software execution

Description

When the host sends a SETUP packet for an IN transaction, the SETUPIFG bit always gets set by hardware, and the USB ISR is triggered.  While SETUPIFG is high, the host's attempts to continue the transaction with IN packets are automatically NAKed.  

When the SETUP packet has been decoded and the IN data prepared, the USB ISR clears the SETUPIFG bit. But if it happens to do so within the 2nd CRC bit of an IN packet from the host, the USB module enters an errant state and can begin to endlessly transmit to the host, irrespective of the protocol. The errant state can be cleared by resetting the module with the USB_EN bit; but there's no way for software to reliably detect the condition.

Since the 2nd CRC bit is only an 83ns window, the problem is extremely rare. However, since the timing of IN packets relative to their preceding SETUP packets can vary according to the host's timing, there's no way to ensure for certain that it will never happen.

Workaround

If the problem behavior occurs, and if the MSP430 is bus-powered, the user may naturally unplug/re-plug the devices USB connection.  If this occurs, the behavior will be corrected because power to the MSP430 will be cycled.  After this, its unlikely the problem will occur again soon, since the failure is usually rare.  

The behavior can be prevented altogether by clearing the UBME bit immediately before clearing SETUPIFG, and setting it again immediately after:  

        USBIEPCNF_0 &= ~EPCNF_UBME; // Clear ME to gate off SETUPIFG clear event
        USBOEPCNF_0 &= ~EPCNF_UBME; // Clear ME to gate off SETUPIFG clear event
        USBIFG &= ~SETUPIFG; // clear the interrupt bit
        USBIEPCNF_0 |= EPCNF_UBME; // Set ME to continue with normal operation
        USBOEPCNF_0 |= EPCNF_UBME; // Set ME to continue with normal operation

This workaround is reliable and effective.  However, as a side effect, it results in the creation of orphan tokens on the USB interface.  Although the workaround is field-tested, and no problems have been reported with these orphan packets, it is recommended to use the workaround only if the errata behavior is problematic for the application in question.