Post on 05-Jan-2016
description
FPGAs and VHDL
Lecture L12.1
FPGAs and VHDL
• Field Programmable Gate Arrays (FPGAs)• VHDL
– 2 x 1 MUX– 4 x 1 MUX– An Adder– Binary-to-BCD Converter– A Register– Fibonacci Sequence Generator
Block diagram of Xilinx Spartan IIE FPGA
Each Spartan IIE CLB contains two of these CLB slices
Look Up Tables
Capacity is limited by number of inputs, not complexity
Choose to use each function generator as 4 input logic (LUT) or as high speed sync.dual port RAM
• Combinatorial Logic is stored in 16x1 SRAM Look Up Tables (LUTs) in a CLB
• Example:
A B C D Z
0 0 0 0 00 0 0 1 00 0 1 0 00 0 1 1 10 1 0 0 10 1 0 1 1 . . .1 1 0 0 01 1 0 1 01 1 1 0 01 1 1 1 1
Look Up Table
Combinatorial Logic
AB
CD
Z
4-bit address
GFunc.Gen.
G4G3G2G1
WE
2(2 )4
= 64K !
Introduction to VHDL
• 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
8-line
2 x 1 MUX
a(7:0)
b(7:0)y(7:0)
sel
sel y
0 a
1 b
library IEEE;use IEEE.std_logic_1164.all; entity mux2 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 mux2;
An 8-line 2 x 1 MUX
a(7:0)
b(7:0)
y(7:0)
sel
8-line2 x 1MUX
library IEEE;use IEEE.std_logic_1164.all; entity mux2 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 mux2;
Entity Each entity must begin with these library and use
statements
port statement defines inputs and outputs
library IEEE;use IEEE.std_logic_1164.all; entity mux2 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 mux2;
Entity
Mode: in or out
Data type: STD_LOGIC,STD_LOGIC_VECTOR(7 downto 0);
architecture mux2_arch of mux2 isbegin mux2_1: process(a, b, sel) begin if sel = '0' then y <= a; else y <= b; end if; end process mux2_1;end mux2_arch;
Architecture
a(7:0)
b(7:0)
y(7:0)
sel
8-line2 x 1MUX
Note: <= is signal assignment
architecture mux2_arch of mux2 isbegin mux2_1: process(a, b, sel) begin if sel = '0' then y <= a; else y <= b; end if; end process mux2_1;end mux2_arch;
Architecture entity name
process sensitivity list
Sequential statements (if…then…else) must
be in a process
Note begin…end
in processNote begin…end
in architecture
An 8-line 4 x 1 multiplexer
a(7:0)
b(7:0)y(7:0)
sel(1:0)
8-line4 x 1MUXc(7:0)
d(7:0)
Sel y
“00” a
“01” b
“10” c
“11” d
An 8-line 4 x 1 multiplexer
library IEEE;use IEEE.std_logic_1164.all; entity mux4 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 mux4;
Example of case statement
architecture mux4_arch of mux4 isbegin process (sel, a, b, c, d) begin case sel is when "00" => y <= a; when "01" => y <= b; when "10" => y <= c; when others => y <= d; end case; end process;end mux4_arch; Must include ALL posibilities
in case statement
Note implies operator =>
Sel y
“00” a
“01” b
“10” c
“11” d
An Adder-- Title: adderlibrary IEEE;use IEEE.std_logic_1164.all;use IEEE.std_logic_unsigned.all; entity adder is generic(width:positive); port (
a: in STD_LOGIC_VECTOR(width-1 downto 0);b: in STD_LOGIC_VECTOR(width-1 downto 0);
y: out STD_LOGIC_VECTOR(width-1 downto 0) );end adder; architecture adder_arch of adder isbegin add1: process(a, b) begin y <= a + b; end process add1;end adder_arch;
adder
y(n-1:0)
b(n-1:0) a(n-1:0)
Note: + sign synthesizesan n-bit full adder!
Binary-to-BCD Converter
-- Title: Binary-to-BCD Converterlibrary IEEE;use IEEE.std_logic_1164.all;use IEEE.std_logic_unsigned.all; entity binbcd is port ( B: in STD_LOGIC_VECTOR (7 downto 0); P: out STD_LOGIC_VECTOR (9 downto 0) );end binbcd;
architecture binbcd_arch of binbcd isbegin bcd1: process(B) variable z: STD_LOGIC_VECTOR (17 downto 0); begin for i in 0 to 17 loop
z(i) := '0'; end loop; z(10 downto 3) := B; for i in 0 to 4 loop
if z(11 downto 8) > 4 then z(11 downto 8) := z(11 downto 8) + 3; end if; if z(15 downto 12) > 4 then z(15 downto 12) := z(15 downto 12) + 3; end if; z(17 downto 1) := z(16 downto 0);
end loop;
P <= z(17 downto 8); end process bcd1; end binbcd_arch;
A Register
-- A width-bit registerlibrary IEEE;use IEEE.std_logic_1164.all; entity reg is generic(width: positive); port ( d: in STD_LOGIC_VECTOR (width-1 downto 0); load: in STD_LOGIC; clr: in STD_LOGIC;
clk: in STD_LOGIC; q: out STD_LOGIC_VECTOR (width-1 downto 0) );end reg;
regclk
clrload
d(n-1:0)
q(n-1:0)
architecture reg_arch of reg isbegin process(clk, clr) begin if clr = '1' then for i in width-1 downto 0 loop q(i) <= '0'; end loop; elsif (clk'event and clk = '1') then if load = '1' then q <= d; end if; end if; end process; end reg_arch;
regclk
clrload
d(n-1:0)
q(n-1:0)
Register architecture
Infers a flip-flop for alloutputs (q)
W clrclk
1
+B A
S
hunds
88
8
8
R1set
clk1
binbcdunitstens
set
clr
r
s
tr
r
Fibonacci Sequence
-- Title: Fibonacci Sequencelibrary IEEE;use IEEE.STD_LOGIC_1164.all;use IEEE.std_logic_unsigned.all; entity fib is
port(clr : in std_logic;clk : in std_logic;P : out std_logic_vector(9 downto 0)
);end fib;
P
architecture fib_arch of fib is
component addergeneric(
width : POSITIVE);port(
a : in std_logic_vector((width-1) downto 0);b : in std_logic_vector((width-1) downto 0);y : out std_logic_vector((width-1) downto 0));
end component;
component reggeneric(
width : POSITIVE);port(
d : in std_logic_vector((width-1) downto 0);load : in std_logic;clr : in std_logic;set : in std_logic;clk : in std_logic;q : out std_logic_vector((width-1) downto 0));
end component;
Declare components
component binbcdport(
B : in std_logic_vector(7 downto 0);P : out std_logic_vector(9 downto 0));
end component;
signal r, s, t: std_logic_vector(7 downto 0);signal one, zero: std_logic;constant bus_width: positive := 8;
W clrclk
1
+B A
S
hunds
88
8
8
R1set
clk1
binbcdunitstens
set
clr
r
s
tr
r
begin one <= '1'; zero <= '0'; U1: adder generic map(width => bus_width) port map (a => t, b => r, y => s); R1: reg generic map(width => bus_width) port map (d => r, load =>one, clr => zero, set => clr,
clk =>clk, q => t); W: reg generic map(width => bus_width) port map (d => s, load => one, clr => clr, set => zero,
clk =>clk, q => r); U2: binbcd port map (B => r, P => P); end fib_arch;
W clrclk
1
+B A
S
hunds
88
8
8
R1set
clk1
binbcdunitstens
set
clr
r
s
tr
r
Wire up the circuit
Fibonacci Sequence Works!