// 三級CIC抽取器實例：cic3_decimator.V
module cic3_decimator(clk, x_in, y_out);
	parameter	STATE_HOLD = 1'b0, STATE_SAMPLE = 1'b1;
	input 		  clk;				// 輸入時鐘	
	input 	[7:0]   x_in;			// 輸入8位數據
	output [25:0] y_out;		// 輸出26位數據

	reg  	    state, derived_clk;
	reg [4:0] counter;
	// 有限狀態機，用于實現下采樣
	always @(negedge clk) begin: FSM_DECIMATOR
		case(state)
			STATE_HOLD: begin
				if(counter == 31)	
					state <= STATE_SAMPLE;
			end
			STATE_SAMPLE: begin
				ComReg0[0] <= IntReg[2];
				state <= STATE_HOLD;
			end
			default:
				state <= STATE_HOLD;
		endcase
		if((counter>8)&&(counter<16))	// 生成下采樣后的時鐘
			derived_clk <= 1;
		else
			derived_clk <= 0;
		counter <= counter + 1;
	end

	wire [25:0] sxtx;		// Sign extended input
	assign sxtx = {{18{x[7]}},x};	// 符號擴展
	
	reg  [7:0]  x;					// Registered input
	reg [25:0] IntReg[2:0];			// I section 0,1 and 2
  	// 積分器實現模塊
	always @(posedge clk) begin: INTEGRATOR
		x  <= x_in;				
		IntReg[0] <= IntReg[0] + sxtx;
		IntReg[1] <= IntReg[1] + IntReg[0];
		IntReg[2] <= IntReg[2] + IntReg[1];
	end
	
	reg [25:0] ComReg0[2:0],ComReg1[2:0],ComReg2[2:0],ComReg3;	
	//梳狀器實現模塊 
	always @(posedge derived_clk)begin:COMB
		ComReg0[1] <= ComReg0[0];
		ComReg0[2] <= ComReg0[1];
		ComReg1[0] <= ComReg0[0] - ComReg0[2];
		ComReg1[1] <= ComReg1[0];
		ComReg1[2] <= ComReg1[1];
		ComReg2[0] <= ComReg1[0] - ComReg1[2];
		ComReg2[1] <= ComReg2[0];
		ComReg2[2] <= ComReg2[1];
		ComReg3     <= ComReg2[0] - ComReg2[2];
	end

	assign y_out = ComReg3;		//輸出
endmodule