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Using building blocks to make bigger circuits

Using building blocks to make bigger circuits. So far we have defined basic building blocks as Inverter, AND, and OR gates storage elements (D-Flip-flop) These building blocks can be used to bigger circuits We may give them additional functionality

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Using building blocks to make bigger circuits

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  1. Using building blocks to make bigger circuits • So far we have defined basic building blocks as • Inverter, AND, and OR gates • storage elements (D-Flip-flop) • These building blocks can be used to bigger circuits • We may give them additional functionality • We can either add more inputs or wider inputs, and outputs • Or we may perform a more complex operation • Or both, like multiplexers, decoders, …. • Examples: • 3, 4, 5 …. n bit input gates • Or you may have multiple bit data inputs • AND, OR, XOR, STORE, READ n-bit data

  2. D0 D1 D2 D3 D Q D Q D Q D Q Q0 Q1 Q2 Q3 C P C P C P C P Clock Making a register • Flip-flops can be connected to act as a register • All clock signals are connected together to one clock • All flip-flops get different input, each storing one-bit information • A 4-bit register is shown -- It uses 4 D-FFs • Has a 4 bit inputs and 1 clock and produces 4 bit output

  3. D0 D Q D Q D Q D Q Q0 Q1 Q2 Q3 C P C P C P C P Clock Making a shift-register • Flip-flops can also be connected to act as a shift register • All clock signals are connected together to one clock • First flip flop gets a new input • Others get input from previous flip-flop • A 4-bit shift register is shown • It has one bit and one clock input and produces 1 bit output

  4. A B C S Using 1-bit building blocks to make n-bit circuit • Design a 1 bit circuit with proper “glue logic” to use it for n-bits • It is called a bit slice • The basic idea of bit slice is to design a 1-bit circuit and then piece together n of these to get an n-bit component • Previous two examples showed how to use 1-bit components • However, there was no other glue signal or logic • Next, we consider other kind of examples • A half-adder adds two 1-bit inputs • Two half adders can be used to add 3 bits • A 3-bit adder is a full adder

  5. A B C S A A1 A2 A0 A3 A C B B Cout0 Cout2 Cout1 Cout3 Cout Full Adder Full Adder Full Adder Full Adder Full Adder B0 B B3 B1 B2 Sum0 Sum Sum3 Sum2 Sum1 C1 C3 C Ci C2 Cout C C S S Sum Full adder and multi-bit adder • Two half adders can be used to add 3 bits • n-bit adder can use full adders • n can be arbitrary large

  6. Glue Logic • Normally the glue logic is part of 1-bit adder • A basic building block has • Primary inputs • Primary outputs • Cascading inputs • Cascading outputs • The cascading signals interact directly with the glue logic • Carry in 1-bit adder is a primary as well as a cascading output • A and B are primary inputs • C is a cascading input • Cout is cascading output • S is primary output

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