[11] | 1 | /* |
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| 2 | * This code implements a game of ping pong (table tennis) between 2 players, with |
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| 3 | * a slight twist: each player puts a ball into play at the same time. There can |
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| 4 | * be an indefinite delay from the time one player hits a ball until the time that |
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| 5 | * the other player returns it. If both balls are in play, then both players |
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| 6 | * always hit each ball at the same instant. |
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| 7 | * |
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| 8 | * While both balls are in play, exactly one player may choose not to return his |
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| 9 | * ball. This leaves one ball in play. If just one ball is in play, the ball |
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| 10 | * should remain in play forever. |
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| 11 | */ |
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| 12 | |
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| 13 | typedef enum {BALL, NO_BALL} player_status; |
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| 14 | typedef enum {HIT, NO_HIT} action_type; |
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| 15 | |
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| 16 | module ping_pong(clk); |
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| 17 | input clk; |
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| 18 | |
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| 19 | /* signal declarations */ |
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| 20 | action_type wire action_A, action_B; |
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| 21 | player_status wire state_A, state_B; |
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| 22 | |
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| 23 | /* the ping pong players */ |
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| 24 | player player_A(clk, action_B, action_A, state_A); |
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| 25 | player player_B(clk, action_A, action_B, state_B); |
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| 26 | |
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| 27 | never_no_balls nnb(clk, state_A, state_B); |
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| 28 | infinitely_often_hit ioh(clk, action_A); |
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| 29 | |
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| 30 | endmodule |
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| 31 | |
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| 32 | |
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| 33 | /* |
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| 34 | * Two state process. If the player doesn't have the ball (state NO_BALL) and the |
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| 35 | * opponent hits the ball, then go to state BALL. If the player has the ball, then |
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| 36 | * if he doesn't hit the ball, then he remains in state BALL. If he hits the ball, |
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| 37 | * and the opponent doesn't also hit the ball, then go to state NO_BALL. |
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| 38 | */ |
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| 39 | module player(clk, opponent, me, state); |
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| 40 | input clk; |
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| 41 | input opponent; |
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| 42 | output me; |
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| 43 | output state; |
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| 44 | |
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| 45 | action_type wire opponent, me, random; |
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| 46 | player_status reg state; |
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| 47 | |
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| 48 | initial state = BALL; |
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| 49 | |
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| 50 | /* set the value of the output of the player */ |
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| 51 | |
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| 52 | assign random = $ND(HIT, NO_HIT); |
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| 53 | assign me = (state == BALL) ? random : NO_HIT; |
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| 54 | |
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| 55 | always @(posedge clk) begin |
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| 56 | case(state) |
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| 57 | BALL: |
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| 58 | begin |
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| 59 | if ((me == HIT) && (opponent == NO_HIT)) |
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| 60 | state = NO_BALL; |
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| 61 | end |
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| 62 | |
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| 63 | NO_BALL: |
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| 64 | begin |
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| 65 | if (opponent == HIT) |
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| 66 | state = BALL; |
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| 67 | end |
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| 68 | |
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| 69 | default:; |
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| 70 | endcase; |
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| 71 | end |
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| 72 | endmodule |
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| 73 | |
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| 74 | /* |
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| 75 | * If it's ever the case that neither player has a ball, then go to state BAD |
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| 76 | * and stay there. |
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| 77 | */ |
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| 78 | typedef enum {GOOD, BAD} prop1_status; |
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| 79 | |
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| 80 | module never_no_balls(clk, state_A, state_B); |
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| 81 | input clk; |
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| 82 | input state_A; |
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| 83 | input state_B; |
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| 84 | |
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| 85 | player_status wire state_A, state_B; |
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| 86 | prop1_status reg state; |
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| 87 | |
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| 88 | initial state = GOOD; |
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| 89 | |
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| 90 | always @(posedge clk) begin |
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| 91 | case(state) |
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| 92 | GOOD: |
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| 93 | begin |
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| 94 | if ((state_A == NO_BALL) && (state_B == NO_BALL)) |
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| 95 | state = BAD; |
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| 96 | end |
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| 97 | |
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| 98 | BAD: |
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| 99 | begin |
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| 100 | state = BAD; |
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| 101 | end |
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| 102 | |
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| 103 | default:; |
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| 104 | endcase; |
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| 105 | end |
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| 106 | endmodule |
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| 107 | |
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| 108 | |
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| 109 | /* |
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| 110 | * If player does not hit the ball now, then he will sometime in the future. |
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| 111 | */ |
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| 112 | typedef enum {OKAY, PENDING} prop2_status; |
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| 113 | |
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| 114 | module infinitely_often_hit(clk, player_action); |
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| 115 | input clk; |
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| 116 | input player_action; |
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| 117 | |
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| 118 | action_type wire player_action; |
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| 119 | prop2_status reg state; |
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| 120 | |
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| 121 | initial state = OKAY; |
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| 122 | |
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| 123 | always @(posedge clk) begin |
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| 124 | case(state) |
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| 125 | OKAY: |
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| 126 | begin |
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| 127 | if (player_action == NO_HIT) |
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| 128 | state = PENDING; |
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| 129 | end |
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| 130 | |
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| 131 | PENDING: |
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| 132 | begin |
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| 133 | if (player_action == HIT) |
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| 134 | state = OKAY; |
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| 135 | end |
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| 136 | |
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| 137 | default:; |
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| 138 | endcase; |
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| 139 | end |
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| 140 | endmodule |
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| 141 | |
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