Systems Sequential Circuits Shankar Balachandran* Associate - - PowerPoint PPT Presentation

Shankar Balachandran* Associate Professor, CSE Department Indian Institute of Technology Madras

*Currently a Visiting Professor at IIT Bombay

Digital Circuits and Systems

Spring 2015 Week 4 Module 22

Sequential Circuits

Sequential Logic Elements 2

Registers

 A register is a group of storage cells suitable for holding

binary information.

 Flip-flop or latches can be used as storage cells in a

• register. Typically, D flip-flops or latches are used.

 There are different types of registers based on the way in

which binary data is stored and accessed.

 Parallel-in Parallel-out (Data Load)  Serial-in Serial-out (Shift register)  Universal Shift Register

Sequential Logic Elements 3

Parallel-In Parallel-Out Register

 All cells of a register loaded in one clock cycle.

Parallel Load

Sequential Circuits 4

Loaden_n

Sequential Logic Elements 5

Shift Register

 The data is stored by serially sending it in, i.e., n clocks are needed

to load an n-bit register.

t t

1

t

2

t

3

t

4

t

5

t

6

t

7

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 R

3

R

2

R

1

R Out = In

Different Kinds of Shift Registers

Sequential Logic Elements 6

in in in

Sequential Logic Elements 7

Shift Register

 Shift registers can be used to perform

multiplication by powers of 2.

 For example, multiplying a binary number by 2 is equivalent to

shifting it left by 1 position

 Multiplying by 2i is equivalent to shifting i bit positions to the left

and padding the lower significant bits by 0s.

 For dividing by powers of 2

 Shift right by 1 position  Dividing by 2i is equivalent to shifting i bit positions to the right

and padding the upper significant bits by 0s.

 Will lose the reminder though unless the shifted bits are stored

also

Parallel Access Shift Register

Think about how you will make the flipflops hold the value. They are constantly shifting or loading values now.

Sequential Logic Elements 9

Bidirectional Shift Register with Load

 All the above register functions can be combined into a single circuit

which works as a universal shift register, i.e., bidirectional shift register with parallel load operation:

How would you implement universal shift register using D flip-flops and logic gates?

End of Week 4: Module 22

Thank You

Sequential Circuits 10