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// SPDX-License-Identifier: Apache-2.0
package chisel3.util.random
import chisel3._
/** Fibonacci Linear Feedback Shift Register (LFSR) generator.
*
* A Fibonacci LFSR can be generated by defining a width and a set of tap points (corresponding to a polynomial). An
* optional initial seed and a reduction operation ([[XOR]], the default, or [[XNOR]]) can be used to augment the
* generated hardware. The resulting hardware has support for a run-time programmable seed (via [[PRNGIO.seed]]) and
* conditional increment (via [[PRNGIO.increment]]).
*
* $seedExplanation
*
* In the example below, a 4-bit Fibonacci LFSR is constructed. Tap points are defined as four and three (using LFSR
* convention of indexing from one). This results in the hardware configuration shown in the diagram.
*
* {{{
* val lfsr4 = Module(new FibonacciLFSR(4, Set(4, 3))
* // +---+
* // +-------------->|XOR|-------------------------------------------------------+
* // | +---+ |
* // | +-------+ ^ +-------+ +-------+ +-------+ |
* // | | | | | | | | | | |
* // +---+ x^4 |<----+-----| x^3 |<----------| x^2 |<----------| x^1 |<--+
* // | | | | | | | |
* // +-------+ +-------+ +-------+ +-------+
* }}}
*
* If you require a maximal period Fibonacci LFSR of a specific width, you can use [[MaxPeriodFibonacciLFSR]]. If you
* only require a pseudorandom [[UInt]] you can use the [[FibonacciLFSR$ FibonacciLFSR companion
* object]].
* @see [[https://en.wikipedia.org/wiki/Linear-feedback_shift_register#Fibonacci_LFSRs]]
* $paramWidth
* $paramTaps
* $paramSeed
* $paramReduction
* $paramStep
* $paramUpdateSeed
*/
class FibonacciLFSR(
width: Int,
taps: Set[Int],
seed: Option[BigInt] = Some(1),
val reduction: LFSRReduce = XOR,
step: Int = 1,
updateSeed: Boolean = false) extends PRNG(width, seed, step, updateSeed) with LFSR {
def delta(s: Seq[Bool]): Seq[Bool] = taps.map{ case i => s(i - 1) }.reduce(reduction) +: s.dropRight(1)
}
/** A maximal period Fibonacci Linear Feedback Shift Register (LFSR) generator. The maximal period taps are sourced from
* [[LFSR.tapsMaxPeriod LFSR.tapsMaxPeriod]].
* {{{
* val lfsr8 = Module(new MaxPeriodFibonacciLFSR(8))
* }}}
* $paramWidth
* $paramSeed
* $paramReduction
*/
class MaxPeriodFibonacciLFSR(width: Int, seed: Option[BigInt] = Some(1), reduction: LFSRReduce = XOR)
extends FibonacciLFSR(width, LFSR.tapsMaxPeriod.getOrElse(width, LFSR.badWidth(width)).head, seed, reduction)
/** Utility for generating a pseudorandom [[UInt]] from a [[FibonacciLFSR]].
*
* For example, to generate a pseudorandom 8-bit [[UInt]] that changes every cycle, you can use:
* {{{
* val pseudoRandomNumber = FibonacciLFSR.maxPeriod(8)
* }}}
*
* @define paramWidth @param width of pseudorandom output
* @define paramTaps @param taps a set of tap points to use when constructing the LFSR
* @define paramIncrement @param increment when asserted, a new random value will be generated
* @define paramSeed @param seed an initial value for internal LFSR state
* @define paramReduction @param reduction the reduction operation (either [[XOR]] or
* [[XNOR]])
*/
object FibonacciLFSR {
/** Return a pseudorandom [[UInt]] generated from a [[FibonacciLFSR]].
* $paramWidth
* $paramTaps
* $paramIncrement
* $paramSeed
* $paramReduction
*/
def apply(
width: Int,
taps: Set[Int],
increment: Bool = true.B,
seed: Option[BigInt] = Some(1),
reduction: LFSRReduce = XOR): UInt = PRNG(new FibonacciLFSR(width, taps, seed, reduction), increment)
/** Return a pseudorandom [[UInt]] generated using a maximal period [[FibonacciLFSR]]
* $paramWidth
* $paramIncrement
* $paramSeed
* $paramReduction
*/
def maxPeriod(
width: Int,
increment: Bool = true.B,
seed: Option[BigInt] = Some(1),
reduction: LFSRReduce = XOR): UInt = PRNG(new MaxPeriodFibonacciLFSR(width, seed, reduction), increment)
}
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