20.7.2 Transition Error

QEI mode: Decoder compares QEA and QEB states (after polarity inversion/ swap/filtering) against previous state. If more than one bit is different, it is an invalid transition that should never occur for properly working hardware, and a transition error flag, TERRIRQ, will be set in the QEI Status register (QEIxSTAT[17]). If TERRIEN bit in QEI Status register (QEIxSTAT[16]) is set, an interrupt will be generated.

Hall mode: Decoder compares QEA, QEB and QEC signals (after polarity/swap/filtering) against previous Hall state. If more than one bit changes, it is an invalid transition that should never occur for properly working hardware, and a Transition Error Flag, TERRIRQ, will be set in QEI Status register (QEIxSTAT[17]). If the TERRIEN bit in the QEI Status register (QEIxSTAT[16]) is set, an interrupt will be generated.