So far we have defined basic building blocks as

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
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
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
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
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