introduce HDL and give some examples about the Verilog language

1. Overview of HDLs

    HDL is hardware description language is means that it is a language to describe hardware. 

HDLs have two main styles: Verilog and VHDL.  Verilog: is C-based and VHDL is Ada based.  Those days, Verilog is mainly used in electronic and industry owing to the programming is based on C language. 

2. two different ways to specific the modules

    Structural and Behavioral.

  Figure 1

  Structural : first it represent the simple element then connect those elements together.

   For example: for figure 1

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   module figure1(x1,x2,x3,f);

input x1,x2,x3

output f;

and(g,x1,x2);

not (k,x2);

and(h,k,x3);

or(f,g,h);

endmodule

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 When represent large circuits, we should use Behavioral which still has two types: logic expression and procedural statements.

for logic expression:  the statements are concurrent

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     module figure1(x1,x2,x3,f);

input x1,x2,x3;

output f;

assign f=(x1 & x2)|（~x2 & x3);

endmodule

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for procedural statements: the statments should access in order, so it needalways block 

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module figure1(x1,x2,x3,f);

input x1,x2,x3;

output f;

reg f;

always @(x1 or x2 or x3)

if(x2==1)

f=x1;

else

f=x3;

endmodule

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