1 Lecture 13 VHDL 3/16/09

2 VHDL VHDL is a hardware description language. The behavior of a digital system can be described (specified) by writing a VHDL program. The VHDL description can be simulate to verify the systems behavior and it can then be synthesized and programmed into an FPGA. A VHDL program typically consists of at least two parts. These parts are called the entity and architecture. Entity – Declaration of a modules input and output. oAnalogous to schematic symbol showing signal pin outs. Architecture – Describes actual behavior and implementation details. –An architecture can have only one entity associated with it, but several architectures may have the same entity. –This means we can have several architectures.

3 Example of Entity Architecture pair for a simple gate ENTITY AND2 is port (X, Y: in BIT; Z: out BIT); end ENTITY AND2; ARCHITECTURE ARC1 OF AND2 IS BEGIN -- After a delay of 2 ns Z <= X and Y after 2 ns; -- Z assumes the value of X and Y. END ARCHITECTURE ARC1;

4 Constants, variables, and signals VHDL objects include constants, variables, and signals. The VHDL use of constants and variables is similar to that in other languages. Signals are new and are typically used to represent voltages on wires.

5 Port signal modes ENTITY AND2 is port (X, Y: in BIT; Z: out BIT); end ENTITY AND2; Type BIT can assume values of only “0” and “1”. Possible entity port modes in – The signal is an input to the entity. out – The signal is an output to the entity. buffer – The signal is an output to the entity, but its value can be read inside the entity’s architecture. inout – The signal can be used as an input or an output. Used for tri- state I/O busses.

6 Standard logic should be use instead of bit Type STD_ULOGIC is ( ‘U’,-- Uninitialized ‘X’,-- Forcing Unknown ‘0’,-- Forcing 0 ‘1’,-- Forcing 1 ‘W’,-- Weak unknown (seldom used) ‘L’,-- Weak 0 (seldom used) ‘H’, -- Weak 1 (seldom used) '-'-- Don’t care );

7 Example using standard logic type LIBRARY IEEE; USE work.all; USE IEEE.Std_Logic_1164.all; -- VHDL library that defines std_logic entity AND2 is port (X, Y: in std_logic; Z: out std_logic); end entity AND2; ARCHITECTURE ARC1 OF AND2 IS BEGIN Z <= X and Y after 2 ns; END ARCHITECTURE ARC1;

8 Example using std_logic_vector input AND gate example LIBRARY IEEE; USE work.all; USE IEEE.Std_Logic_1164.all; entity AND4 is port (X: in std_logic_vector (3 downto 0); Z: out std_logic); end entity AND4; ARCHITECTURE ARC OF AND4 IS BEGIN Z <= X(3) and X(2) and X(1) and X(0); END ARCHITECTURE ARC;

9 74LS138

10 Boolean Operators AND OR NAND NOR XOR XNOR NOT

11 Process The process is a fundamental concept in VHDL Processes are contained within an architecture. –An architecture may contain several processes. Processes execute concurrently (at the same time) Combinational circuits can be modeled without using process. Sequential circuits need processes. Processes provide powerful statements not available outside processes. Example process (A, B) F <= A xor B; end process; Sensitivity list

12 Using VHDL for a BCD to 2421 decoder. library IEEE; use IEEE.std_logic_1164.all; entity VBCD2421 is port ( SWT: in STD_LOGIC_VECTOR (4 downto 1); LED: out STD_LOGIC_VECTOR (8 downto 1) ); end VBCD2421;

13 architecture VBCD2421_arch of VBCD2421 is signal W, X, Y, Z: STD_LOGIC; signal F: STD_LOGIC_VECTOR(3 downto 0); signal V: STD_LOGIC_VECTOR(3 downto 0); signal A, B, C, D: STD_LOGIC; begin W <= SWT(1); X <= SWT(2); Y <= SWT(3); Z <= SWT(4); A <= F(3); B <= F(2); C <= F(1); D <= F(0); LED(8 downto 5) <= D&C&B&A; LED(4 downto 1) <= Z&Y&X&W; V <= W&X&Y&Z;

14 process(V) begin case V is when "0000" => F <= "0000"; when "0001" => F <= "0001"; when "0010" => F <= "0010"; when "0011" => F <= "0011";

15 when "0100" => F <= "0100"; when "0101" => F <= "1011"; when "0110" => F <= "1100"; when "0111" => F <= "1101"; when "1000" => F <= "1110"; when "1001" => F <= "1111"; when others => F <= "----"; -- Don't Cares end case; end process; end VBCD2421_arch;

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