verilog how to create linear feedback shift register

However, an LFSR with a well-chosen feedback function can produce a sequence of bits which appears random and which has a very long cycle.Īpplications of LFSRs include generating pseudo-random numbers, pseudo-noise sequences, fast digital counters, and whitening sequences. Likewise, because the register has a finite number of possible states, it must eventually enter a repeating cycle. The initial value of the LFSR is called the seed, and because the operation of the register is deterministic, the stream of values produced by the register is completely determined by its current (or previous) state. The only linear function of single bits is xor, thus it is a shift register whose input bit is driven by the exclusive-or (xor) of some bits of the overall shift register value. The maximal sequence consists of every possible state except the '0000' state.Ī linear feedback shift register (LFSR) is a shift register whose input bit is a linear function of its previous state. The XOR gate provides feedback to the register that shifts bits from left to right. Refer to Pseudo Random Number Generation link for more robust LFSRs.Template:Redirect Template:Refimprove Template:No footnotes File:LFSR-F4.GIFĪ 4-bit Fibonacci LFSR with its state diagram. Increasing the number of flipflops, and conditions in case statements shall result in wide range of output values. Kindly go through it.įrom the comment below, since there are three bits for flip flops, a maximum of eight states are possible, hence output can contain maximum of eight states (as e4 f2 da b6 60 66 3e). I have provided a testbench at EDAPlayground here, with the above mentioned changes. Like folows: parameter seed = 3'b000 // Arbitrary seed value I would prefer to use a parameter seed = 3'b000 which can easily be overridden while instantiation. Adding else condition can remove in unwanted latch creation.Īlso, initially, D<=3'b0 will have zero initial seed, and xoring operation shall also result in zero output ( D^D = 0^0 = 0), so the output shall never change. In the always block, the condition if(rst) and else if(count.) can result into unintentional hardware. After count reaches 20'd100000, it must be reset or re-initialized.

Along with initializing D, count must be initialized. You have not initialized count variable here.

Verilog how to create linear feedback shift register