Topic 6

6.0 REGISTER
Noorziza Binti Abdul Aziz
Jabatan Kejuruteraan Elektrik
Politeknik Ibrahim Sultan

Introduction
• Shift registers are a type of sequential logic circuit,
mainly for storage of digital data. They are a group of
flip-flops connected in a chain so that the output from
one flip-flop becomes the input of the next flip-flop.
• Most of the registers possess no characteristic internal
sequence of states.
• Registers are synchronous circuits thus all flip-flops are
controlled by a common clock line and all are set or
reset simultaneously.
• As registers are often use to collect serial data they are
also called accumulators.

6.0 Register
Register:
• A set of n flip-flops
• Each flip-flop stores one bit
• An n-bit register has a group of n flip-flops.
• Two basic functions: data storage and data movement.
Shift Register:
• A register that allows each of the flip-flops to pass the stored
information to its adjacent neighbour
Counter:
• A register that goes through a predetermined sequence of
states

• The usage of shift register can be seen
in calculator functions.
For example, try to press the key 1, then
a number 1 will be displayed. When
the second number is pressed, key
8, then number 1 will be shifted to the
left and number 18 will be displayed.
• From the
example above, we find that there have
a logic circuit that was stored in
calculator and will store
the first number (for
temporary) and then the
number will shifted if the second
number was entered.

D flip flop as a memory devices
As a flip-flop can store only one
bit of data, the device registers
or memory capacity is equal
to the number of flipflops
are used. This means
that if we want to build a 2-bit
shift register, the two flip-flops
are needed.
2 Bits Shift Register

Types of shift register
• the basic types of shift registers
(i) Serial In - Serial Out (SISO)
(ii) Serial In - Parallel Out (SIPO)
(iii) Parallel In – Serial Out (PISO)
(i) Parallel In - Parallel Out (PIPO)

Functions of shift register
i. Storing data
ii. Moving data
iii. Convert serial data to parallel data.
iv. Convert parallel data to serial data .
v. Arithmetic logic in division and
multiplying

Serial In - Serial Out
• Block diagram:
The diagram shows four flip-flops connected to form a
SERIAL IN, SERIAL OUT shift register.

• Upon the arrival of a clock pulse,
data at the D input of each flip-flop
is transferred to its Q output.
1
CLK
1
• At the start, the contents of the
register can be set to zero by means
of the CLEAR line.
• If a 1 is applied to the input of the
first flip-flop, then upon the arrival
of the first clock pulse, this 1 is
transferred to the output of flip-flop
1 (input of flip-flop 2).
• After four clock pulses this 1 will be
at the output of flip-flop 4.
1
1
CLK
2
CLK
3
• In this manner, a four bit number
can be stored in the register.
• After four more clock pulses, this
data will be shifted out of the
register.
1
CLK
4

Data movement using
SISO shift register

Serial In - Parallel Out
• Data is fed into the SERIAL IN/PARALLEL OUT shift
register bit by bit, in the same way as for the SISO shift
register.
• However the four bits are all shifted out simultaneously,
in parallel, as one word.

Serial In
10010110

Parallel In – Serial Out
• With the PARALLEL IN/SERIAL OUT shift
register, four bits are shifted into the register
simultaneously, in parallel.
• They are then clocked out, one after the other,
in serial form.

Parallel In
Serial Out

Parallel In - Parallel Out
• The PARALLEL IN/PARALLEL OUT shift register is loaded with four
bits simultaneously, in parallel.
• They are also clocked out simultaneously, in parallel.

Parallel In
Example of data moving
Parallel Out

(i)
Shift register as divider circuits
(Right Shift Register)
• The number shifted from MSB (most significant bit) to LSB
(least significant bit) which shifted from left to right.
• Bit ‘0’ is inserted to MSB
• Example:
Binary
Decimal
Original number 01011000 88
Shift#1 00101100 44
Shift#2 00010110 22

Right shift register circuit
Serial input
D
clk
QA
QA
D
clk
QB
QB
D
clk
QC
QC
D QD
clk
QD
Serial Output
CLK

(ii)
Shift register as multiplier circuits
(Left Shift Register)
• The number is shifted from LSB to MSB (shifted
from right)
• Bit ‘0’ is inserted to LSB.
Binary
Decimal
Original number 00010110 22
Shift#1 00101100 44
Shift#2 01011000 88

Left shift register circuit
Serial Output
Serial Input
D
clk
QA
QA
D
clk
QB
QB
D
clk
QC
QC
D QD
clk
QD
CLK

Shift Register Counters
• They are basically shift registers with the serial
outputs connected back to the serial inputs in
order to produce particular sequences.
• These registers are classified as counters
because they exhibit a specified sequence of
states.

(i) Ring Counter
• A ring counter is basically a circulating shift
register in which the output of the most
significant stage is fed back to the input of the
least significant stage.
• The following is a 4-bit ring counter constructed
from D flip-flops.
• The output of each stage is shifted into the next
stage on the positive edge of a clock pulse.
• If the CLEAR signal is high, all the flipflops except
the first one FF0 are reset to 0. FF0 is preset to 1
instead.

4 bit Ring Counter

Data movement in
Ring Counter 4 bit

(i) Johnson Counter
• Johnson counters are a variation of
standard ring counters, with the inverted
output of the last stage fed back to the
input of the first stage.
• They are also known as twisted ring
counters. An n-stage Johnson counter
yields a count sequence of length 2n, so it
may be considered to be a mod-2n
counter.

6 bit Johnson Counter

4 bit Johnson Counter
data movement