/**************************************************************************************
* Copyright (c) 2020 Institute of Computing Technology, CAS
* Copyright (c) 2020 University of Chinese Academy of Sciences
* 
* NutShell is licensed under Mulan PSL v2.
* You can use this software according to the terms and conditions of the Mulan PSL v2. 
* You may obtain a copy of Mulan PSL v2 at:
*             http://license.coscl.org.cn/MulanPSL2 
* 
* THIS SOFTWARE IS PROVIDED ON AN "AS IS" BASIS, WITHOUT WARRANTIES OF ANY KIND, EITHER 
* EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO NON-INFRINGEMENT, MERCHANTABILITY OR 
* FIT FOR A PARTICULAR PURPOSE.  
*
* See the Mulan PSL v2 for more details.  
***************************************************************************************/

package utils

import chisel3._
import chisel3.util._

object LFSR64 { 
  def apply(increment: Bool = true.B): UInt = { 
    val wide = 64
    val lfsr = RegInit(0x1234567887654321L.U(wide.W)) // random initial value based on simulation seed
    val xor = lfsr(0) ^ lfsr(1) ^ lfsr(3) ^ lfsr(4)
    when (increment) {
      lfsr := Mux(lfsr === 0.U, 1.U, Cat(xor, lfsr(wide-1,1)))
    }
    lfsr
  }
}
