本帖最后由 dan_xb 于 2011-6-17 14:00 编辑
给你一个可实现的例子：
这个产生大约1.5MHz的时钟（Spartan-3），随温度变化
module ring_osc(osc_out); //about 1.5MHz out Clock
output osc_out;
(* s = "TRUE" *) wire delay1,delay2,delay3,delay4;
(* s = "TRUE" *) wire delay5,delay6,delay7,delay8;
(* s = "TRUE" *) wire delay9,delay10,delay11,delay12;
(* s = "TRUE" *) wire delay13,delay14,delay15,delay16;
(* s = "TRUE" *) wire invert;
wire clk_div2,clk_div4,clk_div8,clk_div16,clk_div32;
(* LOC = "SLICE_X0Y0" *) LUT1 #(.INIT(2'b10)) delay1_lut (.O(delay1) ,.I0(invert) );
(* LOC = "SLICE_X0Y1" *) LUT1 #(.INIT(2'b10)) delay2_lut (.O(delay2) ,.I0(delay1) );
(* LOC = "SLICE_X0Y2" *) LUT1 #(.INIT(2'b10)) delay3_lut (.O(delay3) ,.I0(delay2) );
(* LOC = "SLICE_X0Y3" *) LUT1 #(.INIT(2'b10)) delay4_lut (.O(delay4) ,.I0(delay3) );
(* LOC = "SLICE_X0Y4" *) LUT1 #(.INIT(2'b10)) delay5_lut (.O(delay5) ,.I0(delay4) );
(* LOC = "SLICE_X0Y5" *) LUT1 #(.INIT(2'b10)) delay6_lut (.O(delay6) ,.I0(delay5) );
(* LOC = "SLICE_X0Y6" *) LUT1 #(.INIT(2'b10)) delay7_lut (.O(delay7) ,.I0(delay6) );
(* LOC = "SLICE_X0Y7" *) LUT1 #(.INIT(2'b10)) delay8_lut (.O(delay8) ,.I0(delay7) );
(* LOC = "SLICE_X0Y8" *) LUT1 #(.INIT(2'b10)) delay9_lut (.O(delay9) ,.I0(delay8) );
(* LOC = "SLICE_X0Y9" *) LUT1 #(.INIT(2'b10)) delay10_lut (.O(delay10),.I0(delay9) );
(* LOC = "SLICE_X0Y10" *) LUT1 #(.INIT(2'b10)) delay11_lut (.O(delay11),.I0(delay10));
(* LOC = "SLICE_X0Y11" *) LUT1 #(.INIT(2'b10)) delay12_lut (.O(delay12),.I0(delay11));
(* LOC = "SLICE_X0Y12" *) LUT1 #(.INIT(2'b10)) delay13_lut (.O(delay13),.I0(delay12));
(* LOC = "SLICE_X0Y13" *) LUT1 #(.INIT(2'b10)) delay14_lut (.O(delay14),.I0(delay13));
(* LOC = "SLICE_X0Y14" *) LUT1 #(.INIT(2'b10)) delay15_lut (.O(delay15),.I0(delay14));
(* LOC = "SLICE_X0Y15" *) LUT1 #(.INIT(2'b10)) delay16_lut (.O(delay16),.I0(delay15));
(* LOC = "SLICE_X0Y16" *) LUT1 #(.INIT(2'b01)) invert_lut (.O(invert) ,.I0(delay16));
FDCPE #(.INIT(1'b0)) toggle1_flop (.Q(clk_div2), .C(invert), .CE(1'b1),.CLR(1'b0),.D(~clk_div2), .PRE(1'b0)); //24MHz
FDCPE #(.INIT(1'b0)) toggle2_flop (.Q(clk_div4), .C(clk_div2), .CE(1'b1),.CLR(1'b0),.D(~clk_div4), .PRE(1'b0)); //12MHz
FDCPE #(.INIT(1'b0)) toggle3_flop (.Q(clk_div8), .C(clk_div4), .CE(1'b1),.CLR(1'b0),.D(~clk_div8), .PRE(1'b0)); //6MHz
FDCPE #(.INIT(1'b0)) toggle4_flop (.Q(clk_div16),.C(clk_div8), .CE(1'b1),.CLR(1'b0),.D(~clk_div16),.PRE(1'b0)); //3MHz
FDCPE #(.INIT(1'b0)) toggle5_flop (.Q(clk_div32),.C(clk_div16),.CE(1'b1),.CLR(1'b0),.D(~clk_div32),.PRE(1'b0)); //1.5MHz
BUFG BUFG1 (.O(osc_out),.I(clk_div32));
endmodule
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