Single bit comparator

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Single bit comparator
Single bit comparator
gr a(1) gta(0)
A(1)
sm a(1) lta(0)
A(0)
eq a(1)a(0)
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Truth table
A(1) A(0) Gr Sm Eq
0 0 0 0 1
0 1 0 1 0
1 0 1 0 0
1 1 0 0 1
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Boolean equation

• Gr lt a(1) and (not a(0))
• Le lt a

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VHDL Code

• entity singlebitcomparator is
• Port ( a in std_logic_vector(1 downto 0)
• en in std_logic
• gt out std_logic
• sm out std_logic
• eq out std_logic)
• end singlebitcomparator
• architecture Behavioral of singlebitcomparator is
• begin
• process (en,a)
• begin
• if (a(1)gta(0)) then
• gt lt '1' sm lt '0' eq lt '0'
• elsif (a(1) lt a(0)) then
• gt lt '0' sm lt '1' eq lt '0'
• else
• gt lt '0' sm lt '0' eq lt '1'
• end if

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Waveform of single bit comparator
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4-bit comparator
4-bit comparator
1-bit comp
A
1-bit comp
B
1-bit comp
1-bit comp
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Boolean equation for 4-bit comparator

• Let Aa3a2a1a0
• Let Bb3b2b1b0
• Intermediate signal i3,i2,i1 and i0
• AeqB i3i2i1i0
• AgtB a3(b3bar)i3a2(b2bar)i3i2a1(b1bar)i3i2i1a
  0(b0bar)
• AltB Not(AeqBAgtB)

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VHDL code of 4-bit comp

• entity comp4bit is
• Port ( x in std_logic_vector(3 downto 0)
• y in std_logic_vector(3 downto 0)
• en in std_logic
• greater out std_logic
• smaller out std_logic
• equal out std_logic)
• end comp4bit
• architecture Behavioral of comp4bit is
• component singlebitcomparator is
• Port ( a in std_logic_vector(1 downto 0)
• en in std_logic
• gt out std_logic
• sm out std_logic
• eq out std_logic)
• end component singlebitcomparator
• signal temp std_logic_vector(10 downto 0)

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• begin
• u1 singlebitcomparator port map(a(1)gtx(3),a(0)
  gty(3),engten,gtgttemp(0),smgttemp(1),eqgttemp(2))
  
• u2 singlebitcomparator port map(a(1)gtx(2),a(0)
  gty(2),engttemp(2),gtgttemp(3),smgttemp(4),eqgttemp
  (5))
• u3 singlebitcomparator port map(a(1)gtx(1),a(0)
  gty(1),engttemp(5),gtgttemp(6),smgttemp(7),eqgttemp
  (8))
• u4 singlebitcomparator port map(a(1)gtx(0),a(0)
  gty(0),engttemp(8),gtgttemp(9),smgttemp(10),eqgtequ
  al)
• greater lt temp(0) or temp(3) or temp(6) or
  temp(9)
• smaller lt temp(1) or temp(4) or temp(7) or
  temp(10)
• end Behavioral

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Waveform of 4-bit comparator