ff_example.v

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      //////////////////////////////////////////
     // FF_EXAMPLE.V Version 1.0             //
    // Example of implementing different    //
   // types of registers                   //
  // HDL Synthesis Design Guide for FPGAs //
 // May 1997                             //
//////////////////////////////////////////

module ff_example (RESET, CLOCK, ENABLE, D_IN, 
		   A_Q_OUT, B_Q_OUT, C_Q_OUT, D_Q_OUT);

input         RESET, CLOCK, ENABLE;
input 	[7:0] D_IN;
output	[7:0] A_Q_OUT;
output	[7:0] B_Q_OUT;
output	[7:0] C_Q_OUT;
output	[7:0] D_Q_OUT;

reg 	[7:0] A_Q_OUT;
reg 	[7:0] B_Q_OUT;
reg 	[7:0] C_Q_OUT;
reg 	[7:0] D_Q_OUT;

    // D flip-flop
    always @(posedge CLOCK)
    begin
        A_Q_OUT <= D_IN;
    end 

    // Flip-flop with asynchronous reset
    always @(posedge RESET or posedge CLOCK)
    begin 
        if (RESET)
            B_Q_OUT <= 8'b00000000;
	else
  	    B_Q_OUT <= D_IN;
    end 
  
    // Flip-flop with asynchronous set
    always @(posedge RESET or posedge CLOCK) 
    begin
	if (RESET)
	     C_Q_OUT <= 8'b11111111;
	else
	     C_Q_OUT <= D_IN;
    end 

    // Flip-flop with asynchronous reset and clock enable
    always @(posedge RESET or posedge CLOCK)
    begin
        if (RESET)
	    D_Q_OUT <= 8'b00000000;
	else if (ENABLE) 
	    D_Q_OUT <= D_IN;
    end 

endmodule