（1）简易电子琴

  module piano(clk,key,speaker);

input clk; //50M

input[3:0] key;

output speaker;

reg speaker,clk5m;

reg[12:0] div,cnt2;

reg[2:0] cnt1;

parameter duo=4777,re=4256,mi=3792,fa=3581;

always@(posedge clk)

begin

if(cnt1==4)  begin cnt1<=0;clk5m<=~clk5m; end

else  cnt1<=cnt1+1;

end

always@(posedge clk5m)

begin

if(div==0) speaker<=1;

else if(cnt2==(div/2)) 

begin cnt2<=0; speaker<=~speaker; end

else cnt2<=cnt2+1; 

end 

always@(key)

case(key)

4'b1110: div<=duo;

4'b1101: div<=re;

4'b1011: div<=mi;

4'b0111: div<=fa;

default: div<=0;

endcase

endmodule