Hi, I have looking for a Pseudo-random number generator with some maximum linit. I know the LFSR approach but I dont know how to limit the maximum number generated by the LFSR. For example if I want to use an 8-bit LFSR with maximum limit of 200. How to limit the LFSR so that number must not exceed 200. Any link, idea or circuit will be highly appreciated Thanks Ali Umair

You can't. If the number is out of range, discard it and request another until you get one in range.

--- Quote Start --- If the number is out of range, discard it and request another until you get one in range. --- Quote End --- This way an undefinite number of cycles is possibly required before you get a valid result. If you can't allow this, because you need the random number immediately, you must implement a different solution. If you don't bother the indipendence between successive pseudo-random numbers, you can simply feed the LFSR output to a modulo 200 adder: you sum the previous number to the LFSR result, divide by 200 and take the remainder, which is actually a pseudo random number uniformly distributed between 0 and 199.

Infinite is an exaggeration :-) Especially if the source is an LFSR.

--- Quote Start --- Infinite is an exaggeration :-) Especially if the source is an LFSR. --- Quote End --- Infact I said undefinite, not infinite. I mean it could require one cycle (in about 80% of cases), two retries (16%), three (3.2%, right?) and so on.

In theory, you need to change your lfsr_out to the range of 0 to 1 (1 not included) and then scale it by (limit + 1). In your 8-bit case: wire [15:0] temp = lfsr_out * 16'd201; wire [7:0] result = temp [15:8]; lfsr_out is the output of your LFSR and result is what you are looking for.

Pseudo-random Number Generator | Altera Community

12 Replies

Altera_Forum
Honored Contributor
13 years ago
--- Quote Start ---

If the number is out of range, discard it and request another until you get one in range.
--- Quote End ---

This way an undefinite number of cycles is possibly required before you get a valid result.
If you can't allow this, because you need the random number immediately, you must implement a different solution.
If you don't bother the indipendence between successive pseudo-random numbers, you can simply feed the LFSR output to a modulo 200 adder: you sum the previous number to the LFSR result, divide by 200 and take the remainder, which is actually a pseudo random number uniformly distributed between 0 and 199.
Altera_Forum
Honored Contributor
13 years ago
You can't.
If the number is out of range, discard it and request another until you get one in range.

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Pseudo-random Number Generator

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