Introduction to VHDL Tutorial R E Haskell and

Introduction to VHDL Tutorial R. E. Haskell and D. M. Hanna T 1: Combinational Logic Circuits

Introduction to VHDL is an acronym for VHSIC (Very High Speed Integrated Circuit) Hardware Description Language IEEE standard specification language (IEEE 1076 -1993) for describing digital hardware used by industry worldwide VHDL enables hardware modeling from the gate level to the system level

Combinational Circuit Example 8 -line 2 -to-1 Multiplexer a(7: 0) b(7: 0) 8 -line 2 x 1 MUX sel y(7: 0) sel 0 1 y a b

An 8 -line 2 x 1 MUX a(7: 0) 8 -line 2 x 1 MUX library IEEE; use IEEE. std_logic_1164. all; b(7: 0) entity mux 2 is sel port ( a: in STD_LOGIC_VECTOR(7 downto 0); b: in STD_LOGIC_VECTOR(7 downto 0); sel: in STD_LOGIC; y: out STD_LOGIC_VECTOR(7 downto 0) ); end mux 2; y(7: 0)

Entity Each entity must begin with these library and use statements library IEEE; use IEEE. std_logic_1164. all; entity mux 2 is port ( a: in STD_LOGIC_VECTOR(7 downto 0); b: in STD_LOGIC_VECTOR(7 downto 0); sel: in STD_LOGIC; y: out STD_LOGIC_VECTOR(7 downto 0) ); end mux 2; port statement defines inputs and outputs

Entity Mode: in or out library IEEE; use IEEE. std_logic_1164. all; entity mux 2 is port ( a: in STD_LOGIC_VECTOR(7 downto 0); b: in STD_LOGIC_VECTOR(7 downto 0); sel: in STD_LOGIC; y: out STD_LOGIC_VECTOR(7 downto 0) ); end mux 2; Data type: STD_LOGIC, STD_LOGIC_VECTOR(7 downto 0);

Standard Logic library IEEE; use IEEE. std_logic_1164. all; type std_ulogic is ( ‘U’, ‘X’ ‘ 0’ ‘ 1’ ‘Z’ ‘W’ ‘L’ ‘H’ ‘-’); -- Uninitialized -- Forcing unknown -- Forcing zero -- Forcing one -- High impedance -- Weak unknown -- Weak zero -- Weak one -- Don’t care

Standard Logic Type std_ulogic is unresolved. Resolved signals provide a mechanism for handling the problem of multiple output signals connected to one signal. subtype std_logic is resolved std_ulogic;

Architecture architecture mux 2_arch of mux 2 is begin mux 2_1: process(a, b, sel) begin a(7: 0) if sel = '0' then y <= a; b(7: 0) else y <= b; end if; end process mux 2_1; end mux 2_arch; 8 -line 2 x 1 MUX y(7: 0) sel Note: <= is signal assignment

Architecture entity name process sensitivity architecture mux 2_arch of mux 2 is list begin mux 2_1: process(a, b, sel) begin Sequential statements if sel = '0' then (if…then…else) must y <= a; be in a process else y <= b; end if; end process mux 2_1; end mux 2_arch; Note begin…end in architecture Note begin…end in process

An 8 -line 4 x 1 multiplexer a(7: 0) b(7: 0) c(7: 0) d(7: 0) 8 -line 4 x 1 MUX sel(1: 0) y(7: 0) Sel “ 00” “ 01” “ 10” “ 11” y a b c d

An 8 -line 4 x 1 multiplexer library IEEE; use IEEE. std_logic_1164. all; entity mux 4 is port ( a: in STD_LOGIC_VECTOR (7 downto 0); b: in STD_LOGIC_VECTOR (7 downto 0); c: in STD_LOGIC_VECTOR (7 downto 0); d: in STD_LOGIC_VECTOR (7 downto 0); sel: in STD_LOGIC_VECTOR (1 downto 0); y: out STD_LOGIC_VECTOR (7 downto 0) ); end mux 4;

Example of case statement architecture mux 4_arch of mux 4 is begin Note implies operator => process (sel, a, b, c, d) begin case sel is Sel y when "00" => y <= a; “ 00” a when "01" => y <= b; “ 01” b when "10" => y <= c; when others => y <= d; “ 10” c end case; “ 11” d end process; end mux 4_arch; Must include ALL posibilities in case statement

VHDL Architecture Structure architecture name_arch of name is Signal assignments begin Processes contain sequential Concurrent statements, but execute concurrently within the Process 1 architecture body Concurrent statements Process 2 Concurrent statements end name_arch;

VHDL Process P 1: process (<sensitivity list) <variable declarations> begin <sequential statements> end process P 1; Within a process: Variables are assigned using : = Optional process label and are updated immediately. Signals are assigned using <= and are updated at the end of the process.

Lab Exercise T 1 Multiplexer Simulation using Aldec Active-HDL