[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 arbitrary, but finite, delay from the time one player hits a ball until |
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| 5 | * the time that the other player returns it. If both balls are in play, then |
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| 6 | * both players 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 | |
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| 14 | typedef enum {HIT, WAIT_GOING, WAIT_COMING} player_status; |
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| 15 | typedef enum {HIT, IDLE} action_type; |
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| 16 | typedef enum {TO_A, TO_B, OUT_OF_PLAY} ball_status; |
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| 17 | |
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| 18 | module ping_pong(clk); |
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| 19 | input clk; |
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| 20 | |
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| 21 | /* signal declarations */ |
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| 22 | action_type wire action_A, action_B; |
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| 23 | player_status wire state_A, state_B; |
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| 24 | ball_status wire state_ball_1, state_ball_2; |
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| 25 | |
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| 26 | /* the ping pong players */ |
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| 27 | player player_A(clk, action_B, action_A, state_A); |
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| 28 | player player_B(clk, action_A, action_B, state_B); |
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| 29 | |
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| 30 | /* the balls */ |
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| 31 | ball ball_1(clk, action_A, action_B, state_ball_1, TO_A); |
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| 32 | ball ball_2(clk, action_A, action_B, state_ball_2, TO_B); |
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| 33 | |
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| 34 | endmodule |
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| 35 | |
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| 36 | /* |
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| 37 | * Three state process. If the player is waiting while the ball is going away from |
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| 38 | * him (state WAIT_GOING) and the opponent hits the ball, then go to state WAIT_COMING. |
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| 39 | * The player can remain in state WAIT_COMING an arbitrary, but finite, amount of time. |
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| 40 | * The player non-deterministically moves from state WAIT_COMING to state HIT. From |
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| 41 | * state HIT, the player immediately hits the ball. Depending on the action of the |
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| 42 | * opponent, the player either moves to state WAIT_GOING or to state WAIT_COMING. |
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| 43 | * |
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| 44 | * Note that in state WAIT_GOING, there is only one ball still in play. Also, in state |
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| 45 | * WAIT_COMING, it's possible that there is a ball going away, as well as coming towards |
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| 46 | * the player. However, in state WAIT_GOING, there cannot be a ball coming towards |
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| 47 | * the player. |
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| 48 | */ |
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| 49 | module player(clk, opponent, out, state); |
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| 50 | input clk; |
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| 51 | input opponent; |
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| 52 | output out; |
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| 53 | output state; |
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| 54 | |
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| 55 | action_type wire opponent, out; |
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| 56 | player_status reg state; |
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| 57 | player_status wire r_state; |
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| 58 | |
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| 59 | assign out = (state==HIT) ? HIT : IDLE; |
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| 60 | assign r_state = $ND(WAIT_COMING, HIT); |
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| 61 | |
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| 62 | initial state = HIT; |
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| 63 | |
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| 64 | always @(posedge clk) begin |
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| 65 | case(state) |
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| 66 | HIT: |
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| 67 | begin |
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| 68 | if (opponent == IDLE) |
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| 69 | state = WAIT_GOING; |
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| 70 | else if (opponent == HIT) |
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| 71 | state = WAIT_COMING; |
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| 72 | end |
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| 73 | |
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| 74 | WAIT_GOING: |
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| 75 | begin |
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| 76 | if (opponent == HIT) |
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| 77 | state = WAIT_COMING; |
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| 78 | end |
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| 79 | |
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| 80 | WAIT_COMING: |
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| 81 | begin |
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| 82 | state = r_state; |
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| 83 | end |
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| 84 | |
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| 85 | default:; |
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| 86 | endcase; |
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| 87 | end |
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| 88 | endmodule |
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| 89 | |
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| 90 | |
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| 91 | /* |
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| 92 | * Three state process. The ball is either going towards player A |
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| 93 | * (state TO_A) or towards B. If it's ever the case that the ball |
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| 94 | * is going towards A, and A is IDLE and B HITS another ball, then |
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| 95 | * this ball goes OUT_OF_PLAY, since two balls are now going |
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| 96 | * towards A, and the assumption is that A cannot return both balls. |
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| 97 | * The symmetic case holds when in TO_B. |
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| 98 | * |
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| 99 | * Note that this module does not produce any outputs (except for its |
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| 100 | * state, which is read only by the property). Thus, if the |
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| 101 | * property being checked just deals directly with the players, then |
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| 102 | * the balls can be safely ignored. |
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| 103 | */ |
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| 104 | module ball(clk, action_A, action_B, state, init); |
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| 105 | input clk; |
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| 106 | input action_A, action_B; |
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| 107 | input init; |
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| 108 | output state; |
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| 109 | |
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| 110 | action_type wire action_A, action_B; |
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| 111 | ball_status reg state; |
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| 112 | ball_status wire init; |
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| 113 | |
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| 114 | initial state = init; |
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| 115 | |
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| 116 | always @(posedge clk) begin |
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| 117 | case(state) |
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| 118 | TO_A: |
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| 119 | begin |
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| 120 | if (action_A == HIT) |
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| 121 | state = TO_B; |
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| 122 | else if ((action_A == IDLE) && (action_B == HIT)) |
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| 123 | state = OUT_OF_PLAY; |
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| 124 | end |
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| 125 | |
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| 126 | TO_B: |
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| 127 | begin |
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| 128 | if (action_B == HIT) |
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| 129 | state = TO_A; |
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| 130 | else if ((action_B == IDLE) && (action_A == HIT)) |
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| 131 | state = OUT_OF_PLAY; |
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| 132 | end |
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| 133 | |
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| 134 | OUT_OF_PLAY: |
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| 135 | begin |
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| 136 | state = OUT_OF_PLAY; |
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| 137 | end |
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| 138 | |
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| 139 | default:; |
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| 140 | endcase; |
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| 141 | end |
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| 142 | endmodule |
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| 143 | |
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