The document discusses registers and counters in digital circuits. It describes how registers are built from flip-flops and can store multiple bits, making them useful for temporary storage. Registers have various modes of operation, including parallel load, serial in/parallel out, and parallel in/parallel out. Counters sequence through states and are either ripple counters with cascading flip-flops or synchronous counters using a common clock. BCD and binary counters are examples provided.

1. -Varsha Kumari
Registers and Counters

2. Register
Flip-flops are limited because they can store only one bit.
Two flip-flops are used for two-bit counters.
Most computers work with integers and single-precision
floating-point numbers that are 32-bits long.
A register is an extension of a flip-flop that can store multiple
bits.
N bit register can store n bit data.
Registers are commonly used as temporary storage in a
processor.
They are faster and more convenient than main memory.
More registers can help speed up complex calculations.

3. Simple 4 bit register
Basic registers are easy to build. We can store multiple bits
just by putting a bunch of flip-flops together!
A 4-bit register is shown on the right, and its internal
implementation is below.
This register uses D flip-flops
it’s easy to store data without worrying about flip-flop input
equations.
All the flip-flops share a common CLK and CLR signal.

5. Adding a parallel load operation
The input D3-D0 is copied to the output Q3-Q0 on every clock
cycle.
How can we store the current value for more than one cycle?
Let’s add a load input signal LD to the register.
If LD = 0, the register keeps its current contents.
If LD = 1, the register stores a new value, taken from inputs
D3-D0.

6. Classification of Registers
Shift Register (bits are shifted from one flip flop to another)
(eg. Left or right shift Register)
Storage Register (bit are stored in flip flop and transferred to
output)
(eg. Register with parallel load)
There are four mode of operations of a shift register.
Serial Input Serial Output
Serial Input Parallel Output
Parallel Input Serial Output
Parallel Input Parallel Output

8. Shift Register (SISO)
A shift register “shifts” its output once every clock cycle.
It is capable of shifting its bits either to left or right
Basic shift register consists of 4 D flip flops. Output of one
flip flop is connected to the intput of next D flip flop.

9. Left shift Register
Each clock pulse transfers the content of register on one bit
position to right.

11. Copy data from Register A to Register B

12. Serial In/Parallel Out

13. Parallel in Serial out

14. Parallel In / Parallel Out

15. Register with parallel load
The transfer of new information into a register is referred to
as loading the register .
 If all the bits of the register are loaded simultaneously with a
common clock pulse, loading is done in parallel.
Clock pulse act as enable the signal which controls the
loading of new information into the register.
Clear input is used to clear all the registers simultaneously.
LOAD INPUT- It determines whether
the next clock pulse will accept new information or leave the
information in the register intact.

17. Bidirectional Shift Register
A register capable of shifting both right and left is called
Bidirectional Shift Register.
If registers has both shift and parallel load capacities then it is
called Shift Register with parallel load.
It consists of 4 D flip flops and 4 multiplexers having 2 select
lines S1 and S0, These select lines controls the mode of
operation of the register as specified in the function table.

20. Counters
Counters are a specific type of sequential circuit.
The output value increases by one on each clock cycle.
After the largest value, the output goes back to 0.
Eg. Using two bits
Counters are of two types :
Ripple counter (Asynchronous)
Synchronous Counters (Clocked Counters)

21. Asynchronous and Syncronous counter
Asynchrous
Counter
Synchronous
Counter
Different flip flops are
applied with different
clocks
All flip flops are applied
with same clock
It is slower in operation It is faster in operation
fixed count sequence
either up or down
any count sequence is
possible

22. Binary Ripple Counter
A Binary Ripple Counter consists of a series of a collectionof
complimentary Flip flops.
Output of each each flip flop , connected to the clock pulse
input of the next higher order Flip flop.
The flip flop holding the least significant bit recieves the
incoming count pulses.

24. When A1 goes from 1 to
0, it complements A2.
When A2 goes from 1 to
0, it complements A3.
When A3 goes from 1 to
0,
It complements A4.

25. Time signal diagram

26. BCD Ripple counter
A decimal counter follows a sequence of ten states and
returns to zero after the count of nine .
4 Flip flops are required to represent each decimal digit.
State diagram of BCD counter

28. Time signal diagram
Clk pulse
X0
X1
X2
X3
X4

29. Steps to design Synchronous counter
1. Decide the no. of flip flop required with the help of
maximum no. in the sequence.
2. Write down the present state and next state of the
sequence.
3. Write the excitation table of the flip flops
4. Obtain the simplified expression using K Map.
5. Draw the logic circuit diagram with the help of expression.