module jcounter
Title 'Johnson Shift Counter'

Declarations

clk pin 43;                // GCK1 on XC9536 CPLD
!q0 pin 22 istype 'reg';   // LED
!q1 pin 21 istype 'reg';   // LED
!q2 pin 20 istype 'reg';   // LED
!q3 pin 19 istype 'reg';   // LED
!q4 pin 18 istype 'reg';   // LED
!q5 pin 16 istype 'reg';   // LED
!q6 pin 14 istype 'reg';   // LED
!q7 pin 13 istype 'reg';   // LED

q = [q7..q0];              // LED Bus Declaration

Equations
//Johnson Shift Counter description

q.clk = clk;
q0 := !q7; q1 := q0; q2 := q1; q3 := q2;
q4 := q3;  q5 := q4; q6 := q5; q7 := q6;

// Test Vectors can be used to functionaly verify equations in-system.

test_vectors([ clk ] -> [ q0, q1, q2, q3, q4, q5, q6, q7]);
            [  .C. ] -> [ 0, 1, 1, 1, 1, 1, 1, 1 ];
            [  .C. ] -> [ 0, 0, 1, 1, 1, 1, 1, 1 ];
            [  .C. ] -> [ 0, 0, 0, 1, 1, 1, 1, 1 ];
            [  .C. ] -> [ 0, 0, 0, 0, 1, 1, 1, 1 ];
            [  .C. ] -> [ 0, 0, 0, 0, 0, 1, 1, 1 ];
            [  .C. ] -> [ 0, 0, 0, 0, 0, 0, 1, 1 ];
            [  .C. ] -> [ 0, 0, 0, 0, 0, 0, 0, 1 ];
            [  .C. ] -> [ 0, 0, 0, 0, 0, 0, 0, 0 ];
            [  .C. ] -> [ 1, 0, 0, 0, 0, 0, 0, 0 ];
            [  .C. ] -> [ 1, 1, 0, 0, 0, 0, 0, 0 ];
            [  .C. ] -> [ 1, 1, 1, 0, 0, 0, 0, 0 ];
            [  .C. ] -> [ 1, 1, 1, 1, 0, 0, 0, 0 ];
            [  .C. ] -> [ 1, 1, 1, 1, 1, 0, 0, 0 ];
            [  .C. ] -> [ 1, 1, 1, 1, 1, 1, 0, 0 ];
            [  .C. ] -> [ 1, 1, 1, 1, 1, 1, 1, 0 ];
            [  .C. ] -> [ 1, 1, 1, 1, 1, 1, 1, 1 ];


end jcounter