Solution:

VHDL description of Logic Gates:

Hardware description languages (HDLs) differ from software programming languages because HDLs include ways of describing logic connections and characteristics.

• An HDL implements a logic design in hardware (PLD), whereas a software programming language, such as C or BASIC, instructs existing hardware what to do.

• The two standard HDLs used for programming PLDs are VHDL and Verilog. Both of these HDLs have their advocates, but VHDL will be used in this textbook.

• Shows VHDL programs for gates described in this VHDL description. Two gates are left as Checkup exercises. VHDL has an entity/architecture structure.

• The entity defines the logic element and its inputs/outputs or ports; the architecture describes the logic operation. Keywords that are part of the VHDL syntax are shown bold for clarity.

(a) OR gate:

• entity OR gate is,

• port (A, B: in bit; X: out bit);

• end entity OR gate;

• architecture OR function of OR gate is,

• begin

• X 6= A OR B;

• end architecture OR function;

(b) AND gate:

• entity AND gate is,

• port (A, B: in bit; X: out bit);

• end entity AND gate;

• architecture AND function of AND gate is,

• begin

• X 6= A AND B;

end architecture AND function;

(c) NAND gate:

• entity NAND gate is,

• port (A, B, C: in bit; X: out bit);

• end entity NAND gate;

• architecture NAND function of NAND gate is,

• begin

• X 6= A NAND B NAND C;

• end architecture NAND function;

(d) XNOR gate:

• entity XNOR gate is,

• port (A, B: in bit; X: out bit);

• end entity XNOR gate;

• architecture XNOR function of XNOR gate is,

• begin

• X 6= A XNOR B;

• end architecture XNOR function;

(e) NOT gate:

• entity Inverter is,

• port (A: in bit; X: out bit);

• end entity Inverter;

• architecture NOT function of Inverter is

• begin

• X 6= NOT A;

• end architecture NOT function;