psm@manta.NOSC.MIL (Scot Mcintosh) (10/05/90)
I need a circuit to produce a random or pseudorandom bit stream that doesn't repeat until at least 2^32 bits have gone by. A software method can't be used because there won't be a cpu available. Can anyone supply such a circuit or refer me to a source? Thanks muchly.
ed@odi.com (Ed Schwalenberg) (10/05/90)
To produce truly random bits, you need to rely on truly random events, such as radioactive decay. Sampling the state of a high-frequency digital oscillator every time you detect a radioactive particle will give you truly random bits. Producing pseudo-random bits is easier. The canonical method is to use a shift register, the input to which is the XOR of, say, the 19th, 22nd and 35th bits in the shift register. The length of the register and the number of inputs to the XOR gate can be varied at will. I don't know how to prove, for a given configuration, when the pattern repeats; certainly if you need 2^32 non-repeating bits one of the inputs to the XOR function must come from bit 32 or higher of the shift register.
kdq@demott.COM (Kevin D. Quitt) (10/06/90)
In article <1234@manta.NOSC.MIL> psm@manta.NOSC.MIL (Scot Mcintosh) writes: >I need a circuit to produce a random or pseudorandom >bit stream that doesn't repeat until at least 2^32 >bits have gone by. If you need bytes, use a schmidt buffer with its output tied to its input (makes a pretty good very-high speed unstable oscillator), and run the output to the clock of a counter chip. Since the counter will be wrapping around so many times more often than you sample, and your sampling is asynchronous to the oscillator, the output will effectively random. If you need a bit stream, you can feed the output into a flip-flop's D, and issue a CLK whenever you want a bit. Also pretty good. Another possibility is to use a cheap (i.e. noisy) transistor that's forward biased, and let it amplify its own noise to act as the clock for either of the above. -- _ Kevin D. Quitt demott!kdq kdq@demott.com DeMott Electronics Co. 14707 Keswick St. Van Nuys, CA 91405-1266 VOICE (818) 988-4975 FAX (818) 997-1190 MODEM (818) 997-4496 PEP last 96.37% of all statistics are made up.