SLAZ497H January 2013 – March 2021 MSP430G2744
USCI Module
Functional
Timing of USCI I2C interrupts may cause device reset due to automatic clear of an IFG.
When certain USCI I2C interrupt flags (IFG) are set and an automatic flag-clearing event on the I2C bus occurs, the program counter may become corrupted. This will only happen when the IFG is cleared within a critical time window (~6 CPU clock cycles) after a USCI interrupt request occurs and before the interrupt servicing is initiated. The affected interrupts are UCBxTXIFG, UCSTPIFG, UCSTTIFG and UCNACKIFG.
The automatic flag-clearing scenarios are described in the following situations:
(1) A pending UCBxTXIFG interrupt request is cleared on the falling SCL clock edge following a NACK.
(2) A pending UCSTPIFG, UCSTTIFG, or UCNACKIFG interrupt request is cleared by a following Start condition.
(1) Polling the affected flags instead of enabling the interrupts.
or
(2) Ensuring the above mentioned flag-clearing events occur after a time delay of 6 CPU clock cycles has elapsed since the interrupt request occurred and was accepted.