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We always like to reset registers in a synchronous digital circuit just after power up so they are in a known state before device operation begins.
Pseudo random number sequences make use of a seed value. The rest of the sequence generated by the generator then relies on this seed value in a predictable way.
Is it a good idea (say in FPGA or some other device) to use the initial state of group of registers (that are never reset) as the seed value in pseudo random number sequence?
Though the imbalance of transistor conductivity, and the imbalance of metal_metal capacitance and metal_active capacitance, and imbalance of load capacitance, are what determine the powerup "state", these imbalances are very consistent. You will not get much randomness.
That is bad.
If you want to explore randomness, then design a clocked Comparator that initially resides in Metastability, and disrupt the Metastability with random noise from a broadband amplifier, and then CLOCK the Comparator from Track mode into Hold mode.
It depends what you want to use the random numbers for.
If you want numbers that are not certain to be the same each run, then use the power-up state of registers. You will find that many runs will in fact be the same, but without the certainty that a power-on reset gives you.
If you want numbers that are very likely to be different each time, then you need to go another way. Even though registers are designed to be nominally symmetrical, and you can't predict before testing any particular chip which way each of its registers is going to power up, the accidental asymmetries of line size, capacitance to ground, transistor width, resistance etc etc that they are manufactured with, will mean that you are likely to get the same power-up state essentially every time.
There are various other ways to get good randomness.
Yes if you make the leap, register = RAM. As:-
There is a common (now) technique of using indeterminate S/D/RAM start up states or transitions (meta-stability) as a seed source for cryptographically secure pseudo random number generators (CSPRNGs). They're usually associated with a physical unclonable function (PUF) used in security/authentication applications. In essence they're physical circuits that are impossible to replicate exactly due to non deterministic behaviour.
It's pretty deep, but have a look at An Overview of DRAM-Based Security Primitives, specifically §2.2.2. Random Number Generators. You can them trawl the links to TRNGs. Or PUFKEY: A High-Security and High-Throughput Hardware True Random Number Generator for Sensor Networks, which is the source of my image above.Or Design of True Random Numbers Generators with Ternary Physical Unclonable Functions.
You basically reinvented FPGA PUFs. As other answers already mentioned, this is a really bad source of randomness.
How bad?
Bad enough that the consistency of the results you get here is used to uniquely identify FPGAs.
Don't use this for any serious cryptographic key generation. In fact, as with everything cryptography related: if you think you found a new solution to one of the problems related to anything in cryptography, you're probably wrong and should let experts break it for a few years before you actually use it.
If you need randomness to make your post-covid disco-light party that little bit more geeky, this is an ideal solution, though.
