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16.7.1.7 CFG0 Register (Offset = 18h) [reset = X]
CFG0 is shown in Figure 16-10 and described in Table 16-12.
Configuration 0
Figure 16-10. CFG0 Register
31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16
MAX_REFILL_CYCLES
R/W-X
15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
RESERVED SMPL_DIV MIN_REFILL_CYCLES
R-X R/W-X R/W-X
Table 16-12. CFG0 Register Field Descriptions
Bit Field Type Reset Description
31-16 MAX_REFILL_CYCLES R/W X
This field determines the maximum number of samples (between 2^8
and 2^24) taken to re-generate entropy from the FROs after reading
out a 64 bits random number. If the written value of this field is zero,
the number of samples is 2^24, otherwise the number of samples
equals the written value times 2^8. 0x0000: 2^24 samples 0x0001:
1*2^8 samples 0x0002: 2*2^8 samples 0x0003: 3*2^8 samples ...
0x8000: 32768*2^8 samples 0xC000: 49152*2^8 samples ...
0xFFFF: 65535*2^8 samples This field can only be modified while
CTL.TRNG_EN is 0.
15-12 RESERVED R X
Software should not rely on the value of a reserved. Writing any
other value than the reset value may result in undefined behavior.
11-8 SMPL_DIV R/W X
This field directly controls the number of clock cycles between
samples taken from the FROs. Default value 0 indicates that
samples are taken every clock cycle, maximum value 0xF takes one
sample every 16 clock cycles. This field must be set to a value such
that the slowest FRO (even under worst-case conditions) has a cycle
time less than twice the sample period. This field can only be
modified while CTL.TRNG_EN is '0'.
7-0 MIN_REFILL_CYCLES R/W X
This field determines the minimum number of samples (between 2^6
and 2^14) taken to re-generate entropy from the FROs after reading
out a 64 bits random number. If the value of this field is zero, the
number of samples is fixed to the value determined by the
MAX_REFILL_CYCLES field, otherwise the minimum number of
samples equals the written value times 64 (which can be up to
2^14). To ensure same entropy in all generated random numbers the
value 0 should be used. Then MAX_REFILL_CYCLES controls the
minimum refill interval. The number of samples defined here cannot
be higher than the number defined by the 'max_refill_cycles' field
(i.e. that field takes precedence). No random value will be created if
min refill > max refill. This field can only be modified while
CTL.TRNG_EN = 0. 0x00: Minimum samples =
MAX_REFILL_CYCLES (all numbers have same entropy) 0x01:
1*2^6 samples 0x02: 2*2^6 samples ... 0xFF: 255*2^ 6 samples
1170
Random Number Generator SWCU117A–February 2015–Revised March 2015
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