pyb.rng() why 30 bits?

[manitou]

why does pyb.rng() provide only 30 bits of the 32-bit hardware random number generator?

hal rng.c shifts off lower 2 bits HAL_RNG_GetRandomNumber(&RNGHandle) >> 2

This app note on NIST tests of STM32Fxx random number generator doesn't report any problems?
http://www.st.com/web/en/resource/techn ... 073853.pdf

inquiring minds want to know ...

[dhylands]

If I had to guess I would say that it because at the time the rng code was written, MicroPython probably only supported small ints, which are 31-bit signed, or 30-bit unsigned numbers.

Now that there is multi-precision integer support, rng can probably be reworked to support the full 32-bits.

[Damien]

Yes it's for efficiency: 30 bits gives you a positive integer that doesn't require allocating memory.

You can also use the standard os.urandom which returns a bytes object made using the hardware rng.

[pythoncoder]

Perhaps it should stay at 30 bits to ensure it can be called from within an interrupt handler.