SPRUJB6B November 2024 – May 2025 AM2612
The slowest FRO determines the sampling rate of the TRNG. The sampling frequency should be significantly lower than the frequency of the slowest ring (To prevent false FRO locking alarms, the slowest FRO frequency must be more than half the sampling rate; to increase the entropy rate, it is recommended the slowest FRO to run much faster than that limit). The [11-8] SAMPLE_DIV register field sets the sampling rate to one sample per N module clocks, with N in the range 1–16. Although the goal is to implement the FROs so that a sampling rate of one sample per module clock can be used even at the highest module operating frequency, the actual setting of this control register cannot be determined before the absolute worst-case frequency of the slowest FRO is known (either by timing prediction after synthesis and layout or measuring on a finished device). When adjusting the sampling rate to achieve proper sampling and varying the frequency of the module clock, the setting of the control register may be varied to keep the sampling rate as high as possible. This does, however, require restarting the TRNG module every time the sampling rate must be changed (all timing parameters including the sampling rate setting are locked after start-up).