SWCU192 November 2021 CC1312R7 , CC1352P7 , CC2652P7 , CC2652R7
Entropy is defined as a result of:
The more FROs are enabled and the longer they run (that is, how many samples have been stored in the LSFR), the higher the entropy becomes.
The TRNG module must be running at maximum frequency when creating random values.
Creation time for a random value is defined by the values set in the TRNG:CTL.STARTUP_CYCLES register, the TRNG:CFG0.MIN_REFILL_CYCLES or the TRNG:CFG0.MAX_REFILL_CYCLES register and the TRNG:CFG0.SMPL_DIV register; modifications of all these registers can only be done when the TRNG:CTL.TRNG_EN register is 0.
The TRNG:CFG0.SMPL_DIV register defines how often a sample is collected from the FRO, default value 0 indicates that samples are taken every clock cycle, maximum value 0xF takes one sample every 16 clock cycles. All values of SAMPLE_DIV can be used on this device and it must be set as small as possible.
To have the same amount of entropy in each created seed, the startup and minimum refill times must be identical. By using minimum startup and minimum refill time, the entropy per bit is very low. When all FROs are enabled, a start-up time of 5 ms generates a word with 64-bit entropy.
Low values in the TRNG:CTL.STARTUP_CYCLES register and the TRNG:CFG0.MIN_REFILL_CYCLES or TRNG:CFG0.MAX_REFILL_CYCLES registers must only be used to generate random values for nonsecure use like synchronization words, CRC initialization, and so forth. For more secure usages the minimum of 64-bit entropy and beyond must be defined.