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Concurrent vs Sequential

• Combinational vs Sequential logic– Combinational logic is that in which the output

of the circuit depends solely on the current input

– Sequential logic is defined as that in which the output does depend on previous inputs.

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Concurrent statement

• Operators

• The WHEN statement– (When/Else or With/select/when)– The GENERATE statement– The BLOCK statement

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Operators

• Most basic way of creating concurrent code.

• Operators can be used to implement any combinational circuit.

• Operators– AND, OR, +, - , * , sll, sra, etc

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Operators

Operator Type

Operators Data types

Logical NOT,AND,NAND, NOR, OR, XOR,XNOR

Bit, Bit_vector

Std_logic,std_logic_vector

Std_ulogic_vector, std_ulogic

Arithmetic +, -, *, /, ** Integer,signed, unsigned

Comparison =,/=, <, >, <=,>= All above

Shift Sll,srl,sla,sra,rol,ror Bit_vector

Concatenation &, (,,,) Same as above

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When (Simple and selected)

• Fundamental concurrent statement

• Syntax assignment WHEN condition ELSE

assignment WHEN condition ELSE

……….;

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Important Aspect of WHEN

• WHEN value -- Single value

• WHEN value1 to value2 – Range, for enumerated data types

• WHEN value1|value2|………- value1 or value2

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Circuit of 2:4 decoder

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3:8 Decoder• entity decodedr38 is• Port ( a : in std_logic_vector(2 downto 0);• b : out std_logic_vector(7 downto 0);• en : in std_logic);• end decodedr38;

• architecture Behavioral of decodedr38 is• signal temp : std_logic_vector(7 downto 0);• begin• temp <= "00000001" when a="000" else• "00000010" when a="001" else• "00000100" when a="010" else• "00001000" when a="011" else• "00010000" when a="100" else• "00100000" when a="101" else• "01000000" when a="110" else• "10000000";

• b <= temp when en='1' else• (others => '0');• end Behavioral;

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5:32 Decoder5x32 decoder

2:4

3:8

3:8

3:8

3:8

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5:32 decoder• entity decoder532 is• Port ( x : in std_logic_vector(4 downto 0);• y : out std_logic_vector(31 downto 0));• end decoder532;

• architecture Behavioral of decoder532 is• component decoder24 is• Port ( din : in std_logic_vector(1 downto 0); -- decoder input• dout : out std_logic_vector(3 downto 0)• );-- decoder output• end component decoder24;• component decodedr38 is• Port ( a : in std_logic_vector(2 downto 0);• b : out std_logic_vector(7 downto 0);• en : in std_logic);• end component decodedr38;• signal temp : std_logic_vector(3 downto 0);

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5:32 decoder (cont..)

• begin

• u1: decoder24 port map(din(1)=>x(4),din(0)=>x(3),dout(3)=>temp(3),dout(2)=>temp(2),dout(1)=>temp(1),dout(0)=>temp(0));

• u2: decodedr38 port map(en=>temp(0),a(2)=>x(2),a(1)=>x(1),a(0)=>x(0),b(7)=>y(7),b(6)=>y(6),b(5)=>y(5),b(4)=>y(4),b(3)=>y(3),b(2)=>y(2),b(1)=>y(1),b(0)=>y(0));

• u3: decodedr38 port map(en=>temp(1),a(2)=>x(2),a(1)=>x(1),a(0)=>x(0),b(7)=>y(15),b(6)=>y(14),b(5)=>y(13),b(4)=>y(12),b(3)=>y(11),b(2)=>y(10),b(1)=>y(9),b(0)=>y(8));

• u4: decodedr38 port map(en=>temp(2),a(2)=>x(2),a(1)=>x(1),a(0)=>x(0),b(7)=>y(23),b(6)=>y(22),b(5)=>y(21),b(4)=>y(20),b(3)=>y(19),b(2)=>y(18),b(1)=>y(17),b(0)=>y(16));

• u5: decodedr38 port map(en=>temp(3),a(2)=>x(2),a(1)=>x(1),a(0)=>x(0),b(7)=>y(31),b(6)=>y(30),b(5)=>y(29),b(4)=>y(28),b(3)=>y(27),b(2)=>y(26),b(1)=>y(25),b(0)=>y(24));

• end Behavioral;

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Conditional Signal Assignment statement

• The conditional signal assignment statement selects different values for the target signal based on the specified, possibly different, conditions. A typical syntax –

target-signal <= [waveform-elements when condition else

[waveform-elements when condition else..Waveform-elements when condition];

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Example 4: 1 Mux using when statement

library ieee;use ieee.std_logic_1164.all;

entity mux4_1_when isport (a: in std_logic_vector(3 downto 0);s: in std_logic_vector(1 downto 0);y: out std_logic);end mux4_1_when;

architecture mux_behave of mux4_1_when isbeginy <= a(0) when s = "00" else a(1) when s = "01" else a(2) when s = "10" else a(3);end mux_behave;

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Symbol of 2:1 Mux

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Selected signal Assignment statement

• The selected signal assignment statement selects different values for a target signal based on the value of a select expression.

• A typical syntax –With expression select

target-signal <= waveform-element when choices,

waveform-element when choices,..

waveform-element when choices;

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Example 4:1 mux using with-select

library ieee;use ieee.std_logic_1164.all;

entity mux4_1_with isport (a: in std_logic_vector(3 downto 0);s: in std_logic_vector(1 downto 0);y: out std_logic);end mux4_1_with;

architecture mux_behave of mux4_1_with isbeginwith s selecty <= a(0) when "00", a(1) when "01", a(2) when "10", a(3) when others;end mux_behave;

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Mux architecture

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Circuit of Encoder

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Truth table of encoder

A3` A2 A1 A0 F1 F0

0 0 0 1 0 0

0 0 1 0 0 1

0 1 0 0 1 0

1 0 0 0 1 1

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7-segment display

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  Input Output

No. X3 X2 X1 X0 G F E D C B A

0 0 0 0 0 0 1 1 1 1 1 1

1 0 0 0 1 0 0 0 0 1 1 0

2 0 0 1 0 1 0 1 1 0 1 1

3 0 0 1 1 1 0 0 1 1 1 1

4 0 1 0 0 1 1 0 0 1 1 0

5 0 1 0 1 1 1 0 1 1 0 1

6 0 1 1 0 1 1 1 1 1 0 0

7 0 1 1 1 0 0 0 0 1 1 1

8 1 0 0 0 1 1 1 1 1 1 1

9 1 0 0 1 1 1 0 0 1 1 1

Truth table of 7-segment display decoder