Total Rng Shutdown: Reset Rng Module - Texas Instruments OMAP5912 Reference Manual

Power Management User Services
Figure 23. Partial RNG Shut Down
Clock and
reset module
OCP clock
External
clocks
ULPD
ARM926EJS

Total RNG Shutdown: Reset RNG Module

82 Power Management
Input system clock
(OCP)
cutted (2)
OCP Clock disable (1)
If an application no longer needs the RNG functions and needs to go into deep
sleep mode without waiting for a transition to the IDLERNG state, the
application can write 0 in the field RESET_COUNT of the RNG_CONFIG and
the input system clock can be switched off directly.
If this scenario is completed, and if a random number is needed afterwards,
it is then best to perform a soft reset, because randomness cannot be ensured.
The penalty is that again 2
is ready. This method must be done as a last resort when RNG is not used
anymore.
Enter secure mode through normal entry sequence, and then activate the
RNG test mode by writing 1 into the RNG_CTRL (0x08) bit number 1.
This unlocks access to the RESET_COUNT field (bits 11−6) of RNG_CONFIG
register (0x28).
At this point you have broken the RNG functionality and the RNG cannot be
used again for security functions unless a proper soft reset is performed with
bit 11 set to 1.
RINGS
oscillators
Output
registers
Input
registers
25
cycles are needed before the first random number
RNG
generator
State
machine
RNG module
OMAP 5912
SPRU753A