SWCU192 November 2021 CC1312R7 , CC1352P7 , CC2652P7 , CC2652R7
TRNG alarms happen and are most likely caused by FRO clock to sampling clock frequency locking. The sampling clock is the same as the system clock for the TRNG, and when the FRO oscillating frequency gets too close to a multiple of this clock, frequency lock might result in sampling the FRO clock at the same phase too many times so a repeated pattern is detected. When such a repeated pattern is detected it is counted, and when this count exceeds the limit set by the TRNG:ALARMCNT.ALARM_THR register and the alarm event is triggered. Keeping this value high limits the number of alarms, and default is 255 alarm indications before an alarm event is enabled.
When an alarm event is triggered, the associated FRO is automatically shut down and not allowed to contribute to entropy accumulation. The user must then decide what to do with this event. Two options exist:
For the first option, a bit in the TRNG:FRODETUNE.FRO_MASK register set to 1 allows the associated FRO run approximately 5 percent faster. The value of one of these bits may only be changed while the corresponding FRO is turned off (by temporary writing 0 in the corresponding bits of the TRNG:FROEN register—in case of an alarm this bit is already set to 0). When the value is updated, the corresponding FRO must be enabled again.
For the second option, the detune probably had no effect, or the FRO is not oscillating. This state must be stored so the corresponding bit in the TRNG:FROEN register is kept in off state to eliminate new alarm triggers caused by the particular FRO.