%---------------------------------------------------------------------------

 Description : Linear Feedback Shift Register
 Project     :
 Edit history...
   Date        Engineer  Rev   Changes
   11/5/1997   SBJ       000   Initial stab

 ---------------------------------------------------------------------------%
TITLE "Linear Feedback Shift Register";

%- include files -%
INCLUDE "lpm_shiftreg";

%- parameters -%
PARAMETERS
(
   LPM_WIDTH = 3,       -- Number of bits
   TAP1 = 3,            -- LFSR taps as defined in table below.
   TAP2 = 2,
   TAP3 = 0,            -- Unused taps should be set to zero.
   TAP4 = 0,            -- Taps 1 and 2 should always be non-zero.
   TAP5 = 0,
   TAP6 = 0
);

CONSTANT A = LPM_WIDTH;
%- Convert taps to register indices.  Note that if taps 3 - 6 are zero
   (ie unused) then they are affectively eliminated by converting to
   tap 1 indice.  This becomes obvious by noting that ( a !$ b !$ c )
   becomes ( a !$ b !$ a ) = ( a !$ b ) when c is set to a, and thus
   c is ignored. -%
CONSTANT T1 = TAP1 - 1;
CONSTANT T2 = TAP2 - 1;
CONSTANT T3 = ( TAP3 != 0 ) ? ( TAP3 - 1 ) : ( T1 );
CONSTANT T4 = ( TAP4 != 0 ) ? ( TAP4 - 1 ) : ( T1 );
CONSTANT T5 = ( TAP5 != 0 ) ? ( TAP5 - 1 ) : ( T1 );
CONSTANT T6 = ( TAP6 != 0 ) ? ( TAP6 - 1 ) : ( T1 );

SUBDESIGN lfsr
%---------------------------------------------------------------------------
 /Sys_Rst            Asynchronous reset input.
 Sys_Clk             Clock input.
 Ena                 Enable next LFSR value.
 Q[]                 LFSR value.
 Prev[]              Previous LFSR value.
 Next[]					Next LFSR value (note: LSB is not registered).
---------------------------------------------------------------------------%
(
   /Sys_Rst          : INPUT;
   Sys_Clk           : INPUT;
   Ena               : INPUT;
   Q[A-1..0]         : OUTPUT;
   Prev[A-1..0]      : OUTPUT;
   Next[A-1..0]      : OUTPUT;
)

VARIABLE
   reg               : lpm_shiftreg WITH ( LPM_WIDTH = A, LPM_DIRECTION = "LEFT" );
   msb               : DFFE;
   fb                : NODE;

BEGIN
   Q[]               = reg.q[];
   Prev[]            = ( msb, reg.q[A-1..1] );
   Next[]				= ( reg.q[A-2..0], fb );

   %- Shift (left) register with feedback -%
   reg.clock         = Sys_Clk;
   reg.aclr          = !/Sys_Rst;   -- Start with LFSR == 0.
   reg.enable        = Ena;
   reg.shiftin       = fb;          -- Shift in msb and shift register left.

   %- Capture previous msb of shift register -%
   msb.clk           = Sys_Clk;
   msb.prn           = /Sys_Rst;    -- Previous msb would have had to be a 1
   msb.ena           = Ena;
   msb.d             = reg.q[A-1];  -- Shift in LFSR msb

   %- LFSR feedback term -%
   fb = reg.q[T1] 
   	!$ reg.q[T2] 
    	!$ reg.q[T3] 
   	!$ reg.q[T4] 
   	!$ reg.q[T5] 
   	!$ reg.q[T6];

   %
      Here is a table of LFSR taps to use for different lengths.  Note
      that tap values are based on LSB being bit 1 and MSB being bit n,
      so the tap values are subtracted by one before being used here.
      The numbers in this table are collected from Xilinx Application 
      Note 52, available at: http://www.xilinx.com/xapp/xapp052.pdf
 

      n  | XNOR from     n  | XNOR from    n  | XNOR from       n XNOR from
      ---+-------------  ---+------------  ---+---------------  ---+----------------
      3  | 3,2           45 | 45,44,42,41  87 | 87,74           129| 129,124
      4  | 4,3           46 | 46,45,26,25  88 | 88,87,17,16     130| 130,127
      5  | 5,3           47 | 47,42        89 | 89,51           131| 131,130,84,83
      6  | 6,5           48 | 48,47,21,20  90 | 90,89,72,71     132| 132,103
      7  | 7,6           49 | 49,40        91 | 91,90,8,7       133| 133,132,82,81
      8  | 8,6,5,4       50 | 50,49,24,23  92 | 92,91,80,79     134| 134,77
      9  | 9,5           51 | 51,50,36,35  93 | 93,91           135| 135,124
      10 | 10,7          52 | 52,49        94 | 94,73           136| 136,135,11,10
      11 | 11,9          53 | 53,52,38,37  95 | 95,84           137| 137,116
      12 | 12,6,4,1      54 | 54,53,18,17  96 | 96,94,49,47     138| 138,137,131,130
      13 | 13,4,3,1      55 | 55,31        97 | 97,91           139| 139,136,134,131
      14 | 14,5,3,1      56 | 56,55,35,34  98 | 98,87           140| 140,111
      15 | 15,14         57 | 57,50        99 | 99,97,54,52     141| 141,140,110,109
      16 | 16,15,13,4    58 | 58,39        100| 100,63          142 | 142,121
      17 | 17,14         59 | 59,58,38,37  101| 101,100,95,94   143 | 143,142,123,122
      18 | 18,11         60 | 60,59        102| 102,101,36,35   144 | 144,143,75,74
      19 | 19,6,2,1      61 | 61,60,46,45  103| 103,94          145 | 145,93
      20 | 20,17         62 | 62,61,6,5    104| 104,103,94,93   146 | 146,145,87,86
      21 | 21,19         63 | 63,62        105| 105,89          147 | 147,146,110,109
      22 | 22,21         64 | 64,63,61,60  106| 106,91          148 | 148,121
      23 | 23,18         65 | 65,47        107| 107,105,44,42   149 | 149,148,40,39
      24 | 24,23,22,17   66 | 66,65,57,56  108| 108,77          150 | 150,97
      25 | 25,22         67 | 67,66,58,57  109| 109,108,103,102 151 | 151,148
      26 | 26,6,2,1      68 | 68,59        110| 110,109,98,97   152 | 152,151,87,86
      27 | 27,5,2,1      69 | 69,67,42,40  111| 111,101         153 | 153,152
      28 | 28,25         70 | 70,69,55,54  112| 112,110,69,67   154 | 154,152,27,25
      29 | 29,27         71 | 71,65        113| 113,104         155 | 155,154,124,123
      30 | 30,6,4,1      72 | 72,66,25,19  114| 114,113,33,32   156 | 156,155,41,40
      31 | 31,28         73 | 73,48        115| 115,114,101,100 157 | 157,156,131,130
      32 | 32,22,2,1     74 | 74,73,59,58  116| 116,115,46,45   158 | 158,157,132,131
      33 | 33,20         75 | 75,74,65,64  117| 117,115,99,97   159 | 159,128
      34 | 34,27,2,1     76 | 76,75,41,40  118| 118,85          160 | 160,159,142,141
      35 | 35,33         77 | 77,76,47,46  119| 119,111         161 | 161,143
      36 | 36,25         78 | 78,77,59,58  120| 120,113,9,2     162 | 162,161,75,74
      37 | 37,5,4,3,2,1  79 | 79,70        121| 121,103         163 | 163,162,104,103
      38 | 38,6,5,1      80 | 80,79,43,42  122| 122,121,63,62   164 | 164,163,151,150
      39 | 39,35         81 | 81,77        123| 123,121         165 | 165,164,135,134
      40 | 40,38,21,19   82 | 82,79,47,44  124| 124,87          166 | 166,165,128,127
      41 | 41,38         83 | 83,82,38,37  125| 125,124,18,17   167 | 167,161
      42 | 42,41,20,19   84 | 84,71        126| 126,125,90,89   168 | 168,166,153,151
      43 | 43,42,38,37   85 | 85,84,58,57  127| 127,126
      44 | 44,43,18,17   86 | 86,85,74,73  128| 128,126,101,99 %

END;