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DigitalCircuits

DigitalCircuits - Digital Circuits Basic Concepts Simple...

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Jon Turner/ David M. Zar Simple binary calculator Basic circuit elements Specifying circuits with hardware description languages Using simulation to verify circuit operation Logic equations and combinational circuits Digital Circuits – Basic Concepts

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‹#› A Simple Binary Calculator Three operations » clear stored value » load new value » add to stored value Components » flip flops store information » multiplexor selects 1-of-2 inputs » adder computes sum D in add 1 0 Adder load clear D >C Q result 8 8 8 8 clk D flip-flops multiplexor 1 0 1 0 x00
‹#› Basic Circuit Elements AND gate A B A · B A B A + B OR gate A A inverter 0 1 C X 0 A 1 B A B X=C · A + C · B C 2:1 multiplexor D flip-flop D >C Q D clk store value of D on rising clock edge AB A·B A+B A’ 00 0 0 1 01 0 1 1 10 0 1 0 11 1 1 0 truth table for logic gates 2 2 2 is shorthand for 2 2 is shorthand for for gates and mux, outputs change as inputs change for flip flop, output changes only at clock edge

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‹#› VHDL Specification of Calculator entity calculator is port ( clk: in std_logic; clear, load, add: in std_logic; -- operation signals dIn: in std_logic_vector(7 downto 0); -- input data result: out std_logic_vector(7 downto 0)); -- output result end calculator; architecture a1 of calculator is signal dReg: std_logic_vector(7 downto 0); begin process (clk) begin if rising_edge(clk) then if clear = '1' then dReg <= x"00"; elsif load = '1' then dReg <= dIn; elsif add = '1' then dReg <= dReg + dIn; end if; end if; end process; result <= dReg; end a1; stored value synchronization condition synchronized assignments
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