module dstate2b
title 'State machine showing how an arbitrary default state can be used
to recover from illegal states.  Dave Pellerin - Data I/O Corp.'

        dstate device 'p16r4';
        clock pin 1;
        reset pin 2;
        get_lost pin 3;
        q0,q1,q2,q3 pin 14,15,16,17;
        q0,q1,q2,q3 istype 'reg';     "Forces state diagram to produce
                                      "pin-to-pin equations. See note below.
        sreg = [q3..q0];

        S1 = [0,0,0,1];
        S2 = [0,0,1,1];
        S3 = [0,1,1,1];
        S4 = [0,1,1,0];
        S5 = [1,1,1,0];
        S6 = [1,1,0,1];
        S7 = [1,0,0,1];
        SX = [0,1,0,1];         "Illegal state

equations

        sreg.clk = clock;
        sreg := reset & S1;

  "This equation will reset us to S1 from any state, including those
  "that are not defined in the state diagram. Note that this design
  "relies on the D-type flip-flops resetting to register-zero states
  "after illegal transitions. This means that @DCSET can not be used
  "in this design to mimimize logic.  In this version of the design,
  "the reset equation is written using a ':=' (pin-to-pin) equation
  "for the state register set.  Pin-to-pin is allowed in this design
  "because the state diagram produces pin-to-pin (':='/.FB) equations
  "as a result of specifying ISTYPE 'reg' in the declarations section.
  "If no ISTYPE 'reg' is specified, the default attribute for pins
  "14-17 of the P16R4 is 'reg_D', which results in detailed equations
  "from state diagrams as seen in DSTATE2A.ABL.

state_diagram sreg

        state S1: if get_lost then SX else S2;
        state S2: if get_lost then SX else S3;
        state S3: if get_lost then SX else S4;
        state S4: if get_lost then SX else S5;
        state S5: if get_lost then SX else S6;
        state S6: if get_lost then SX else S7;
        state S7: if get_lost then SX else S1;

        " Note: there is no transition provided out of SX

test_vectors([clock, get_lost, reset]->sreg)
             [ .c. ,    0    ,   1  ]-> S1;
             [ .c. ,    0    ,   0  ]-> S2;
             [ .c. ,    0    ,   0  ]-> S3;
             [ .c. ,    1    ,   0  ]-> SX;    "Lost until a reset happens...
             [ .c. ,    0    ,   0  ]-> .x.;
             [ .c. ,    0    ,   1  ]-> S1;
             [ .c. ,    0    ,   0  ]-> S2;
             [ .c. ,    0    ,   0  ]-> S3;
             [ .c. ,    0    ,   0  ]-> S4;
             [ .c. ,    0    ,   0  ]-> S5;
             [ .c. ,    1    ,   0  ]-> SX;
             [ .c. ,    0    ,   0  ]-> .x.;
             [ .c. ,    0    ,   0  ]-> .x.;
             [ .c. ,    0    ,   1  ]-> S1;
end