練習九.利用狀態機的嵌套實現層次結構化設計目的：１.運用主狀態機與子狀態機產生層次化的邏輯設計；
　２.在結構化設計中靈活使用任務（task）結構。
在上一節，我們學習了如何使用狀態機的實例。實際上，單個有限狀態機控制整個邏輯電路的運轉在實際設計中是不多見，往往是狀態機套用狀態機，從而形成樹狀的控制核心。這一點也與我們提倡的層次化、結構化的自頂而下的設計方法相符，下面我們就將提供一個這樣的示例以供大家學習。
該例是一個簡化的EPROM的串行寫入器。事實上，它是一個EPROM讀寫器設計中實現寫功能的部分經刪節得到的，去除了EPROM的啟動、結束和EPROM控制字的寫入等功能，只具備這樣一個雛形。工作的步驟是：１.地址的串行寫入；２.數據的串行寫入；3.給信號源應答，信號源給出下一個操作對象；４.結束寫操作。通過移位令并行數據得以一位一位輸出。
模塊源代碼：
module writing(reset,clk,address,data,sda,ack);
? input reset,clk;
? input[7:0] data,address;
? output sda,ack;　//sda負責串行數據輸出；
? 　　　　　　　　 //ack是一個對象操作完畢后，模塊給出的應答信號。
? reg link_write;　//link_write 決定何時輸出。
? reg[3:0] state;　//主狀態機的狀態字。
? reg[4:0] sh8out_state;　//從狀態機的狀態字。
? reg[7:0] sh8out_buf;??? //輸入數據緩沖。
? reg finish_F;?????????? //用以判斷是否處理完一個操作對象。
? reg ack;
? parameter
??? idle=0,addr_write=1,data_write=2,stop_ack=3;
? parameter
??? bit0=1,bit1=2,bit2=3,bit3=4,bit4=5,bit5=6,bit6=7,bit7=8;
? assign?? sda = link_write? sh8out_buf[7] : 1'bz;
? always @(posedge clk)
??? begin
??????? if(!reset)?????????????? //復位。
????????? begin
???????????? link_write<= 0;
???????????? state??? <= idle;
???????????? finish_F <= 0;
???????????? sh8out_state<=idle;
?????????????????? ack<= 0;
???????????? sh8out_buf<=0;
????????? end
??????? else
????????? case(state)
????????? idle:??????????????????????
??????????? begin
??????????????? link_write? <= 0;
?????????????? state??? <= idle;
?????????????? finish_F <= 0;
?????????????? sh8out_state<=idle;
???????????????????? ack<= 0;
?????????????? sh8out_buf<=address;?
????????????? state??? <= addr_write;
??????????? end
????????? addr_write:???????? //地址的輸入。
??????????? begin
??????????????? if(finish_F==0)
????????????????? begin? shift8_out; end
??????????????? else
????????????????? begin
???????????????????? sh8out_state <= idle;
???????????????????? sh8out_buf?? <= data;
??????????????????????????? state <= data_write;
???????????????????????? finish_F <= 0;
????????????????? end
??????????? end
????????? data_write:?????? //數據的寫入。
??????????? begin
??????????????? if(finish_F==0)
????????????????? begin? shift8_out; end
??????????????? else
????????????????? begin
????????????????????? link_write <= 0;
?????????????????????????? state <= stop_ack;
??????????????????????? finish_F <= 0;?
???????????????????????????? ack <= 1;
????????????????? end
??????????? end????
????????? stop_ack:???????????? //完成應答。
??????????? begin
????????????????? ack <= 0;
????????????? state <= idle;
??????????? end
?????????
????????? endcase??????
??? end??????????????????????????
task shift8_out;??????????????? //串行寫入。
? begin
???? case(sh8out_state)
???? idle:
?????? begin
?????????? link_write? <= 1;
????????? sh8out_state <= bit0;
??????? end
???? bit0:
?????? begin
?????????? link_write <= 1;
???????? sh8out_state <= bit1;????????????????????????????????
?????????? sh8out_buf <= sh8out_buf<<1;
?????? end
???? bit1:
?????? begin
???????? sh8out_state<=bit2;
???????? sh8out_buf<=sh8out_buf<<1;
?????? end
???? bit2:
?????? begin
???????? sh8out_state<=bit3;
???????? sh8out_buf<=sh8out_buf<<1;
?????? end
???? bit3:
?????? begin
???????? sh8out_state<=bit4;
???????? sh8out_buf<=sh8out_buf<<1;
?????? end
???? bit4:
?????? begin
???????? sh8out_state<=bit5;
???????? sh8out_buf<=sh8out_buf<<1;
?????? end
????
???? bit5:
?????? begin
???????? sh8out_state<=bit6;
???????? sh8out_buf<=sh8out_buf<<1;
?????? end
???? bit6:
?????? begin
???????? sh8out_state<=bit7;
???????? sh8out_buf<=sh8out_buf<<1;
?????? end
???? bit7:
?????? begin
???????? link_write<= 0;
???????? finish_F<=finish_F+1;???????????????????????????????
?????? end?
??????
???? endcase
? end
endtask
endmodule?????????
測試模塊源代碼：
`timescale 1ns/100ps
`define clk_cycle 50
module writingTop;
? reg reset,clk;
? reg[7:0] data,address;
? wire ack,sda;
? always #`clk_cycle? clk = ~clk;
? initial
??? begin
??????????? clk=0;
??????????? reset=1;
??????????? data=0;
??????????? address=0;
??????????? #(2*`clk_cycle) reset=0;
??????????? #(2*`clk_cycle) reset=1;
?????? #(100*`clk_cycle) $stop;
??? end
? always @(posedge ack)????? //接收到應答信號后，給出下一個處理對象。
??? begin
??????????? data=data+1;
??????????? address=address+1;
??? end????????
? writing writing(.reset(reset),.clk(clk),.data(data),
????????????????? .address(address),.ack(ack),.sda(sda));?
endmodule???????
仿真波形：[[wysiwyg_imageupload:252:height=174,width=496]]
練習：仿照上例，編寫一個實現EPROM內數據串行讀取的模塊。編寫測試模塊，給出仿真波形。