How to choose a random number within a given time?

Question

I want that an LED should turn on at any random time within 15 seconds. For example after pressing a button it should turn on after 4 seconds or 7 seconds, that is randomly.

I came up with the code to produce a 15 second delay, but I cant figure out a way to select a random time between this.

Below is my code for the 15 second delay:

always @ (posedge clock or posedge reset)
begin
 if(reset)

  ticker <= 0;

 else if(ticker == 750000000) //if it reaches the desired max value that equates 15 second reset it
  ticker <= 0;
 else if(start) //only start if the input is set high
  ticker <= ticker + 1;
end

assign click = ((ticker == 750000000)?1'b1:1'b0); //click to be assigned high every 0.1 second

Also I want a synthesizable solution please.

http://en.wikipedia.org/wiki/Linear_feedback_shift_register – e19293001 Jan 21 '13 at 23:59 — e19293001, Jan 21 '13 at 23:59

Morgan · Accepted Answer · 2013-01-22T09:05:01.760

2

You need a hardware way to create a random number. You have $random for simulation but not hardware.

I would suggest creating an lfsr which runs and when a button is pressed, you capture the current value which you count up to, or preset a counter and count down to 0.

edited Jan 22 '13 at 09:05

answered Jan 21 '13 at 16:59

Morgan

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Thank you. Ive been looking into LFSR as advised and am greatly confused by the selection of taps. What determines which feedback bits must be used? Also I can find a consistent example on it. Thanks – ipunished Jan 23 '13 at 08:15
1

This might be best done by starting another question. You need to decide how many random numbers you need (feedback terms), Or maybe just the (bit) width of a random number. An example [Verilog lfsr](https://github.com/rcastle/verilog_lfsr/blob/master/fibonacci_lfsr.v) For [choosing taps](http://www.ece.cmu.edu/~koopman/lfsr/index.html) make sure you have a maximal LFSR. – Morgan Jan 23 '13 at 08:42
Thanks. I just need a 30 bit wide pseudo random number from this, I care not for the number of random terms before it starts repeating. I was trying to understand the link you posted, I understand that ``"iterations per value"`` show the random numbers before it starts repeating. Am I right? But what is the start and end value? As it does not show the tap arrangement. Can I just use any tap arrangement and get away with it? Thank you – ipunished Jan 24 '13 at 07:17
1

@ipunished, I have added this Q&A http://stackoverflow.com/questions/14497877/how-to-implement-a-sudo-hardware-random-number-generator/ Please add comments there if I have missed anything. – Morgan Jan 24 '13 at 09:29
Dude, you are a legend. A clear detailed explanation was greatly needed regarding LFSR. Thanks a lot :) – ipunished Jan 25 '13 at 06:32

score 2 · Answer 2 · answered Jan 21 '13 at 16:59

2

If you need a random number in hardware, you can use a Linear feedback shift register (LFSR) circuit to generates a pseudorandom number.

LFSRs are easy to implement and you'll find many examples online.

When you get a button press, you can capture the current value of the LFSR to a register (you may need to scale in some way for your application) and use that value to count seconds until you turn on the LED.

answered Jan 21 '13 at 16:59

dwikle

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Thank you. Ive been looking into LFSR as advised and am greatly confused by the selection of taps. What determines which feedback bits must be used? Thanks – ipunished Jan 23 '13 at 08:14

How to choose a random number within a given time?

2 Answers2