1. Addis Ababa Institute of Technology
(AAIT) Department of Electrical and
Computer Engineering
ECEG-3201 Digital Logic Design
2. AAIT, Department of
Electrical and Computer
Engineering
Nebyu Yonas Sutri2
Learning Outcomes
At the end of the lecture, students should get
familiarized with;
Registers.
Counters.
3. 3
AAIT, Department of
Electrical and Computer
Engineering
Nebyu Yonas Sutri
Registers
A flip-flop stores one bit of information. When a set of
n flip-flops are used to store n bits of information,
such as an n-bit number, we refer to these flip-flops
as a register.
Register is a digital circuit with two basic functions;
data storage and data movement.
A common clock is used for each flip-flop in a register.
A register that provides the ability to shift its contents
is called a shift register.
4. 4
AAIT, Department of
Electrical and Computer
Engineering
Nebyu Yonas Sutri
Registers
Transmitting n bits of information at once using n
separate wires is called parallel transfer.
But it is also possible to transfer all bits using a single
wire, by performing the transfer one bit at a time, in n
consecutive clock cycles. We refer to this scheme as
serial transfer.
5. 5
AAIT, Department of
Electrical and Computer
Engineering
Nebyu Yonas Sutri
Shift Registers
A shift register is a sequential logic device made up of
flip-flops that allows parallel or serial loading and
serial or parallel outputs as well as shifting bit by bit.
A shift register is an n-bit register with a provision for
shifting its stored data by one bit position at each tick
of the clock.
There are different types of shift registers.
Serial in Serial Out
Serial in Parallel Out
Parallel in Serial Out
Parallel in Parallel Out
6. 6
AAIT, Department of
Electrical and Computer
Engineering
Nebyu Yonas Sutri
Shift Registers
This sequential device loads the data present on its
inputs and then moves or “shifts” it to its output once
every clock cycle, hence the name “Shift register”.
Common tasks of shift registers:
Serial/Parallel data conversion
UART (universal asynchronous receiver/transmitter)
Time delay
Ring counter
Twisted-ring counter or Johnson counter
Memory device
7. 7
AAIT, Department of
Electrical and Computer
Engineering
Nebyu Yonas Sutri
Characteristics of Shift Registers
Number of bits (4-bit, 8-bit, etc.)
Loading
Serial
Parallel (asynchronous or synchronous)
Common modes of operation.
Parallel load
Shift right-serial load
Shift left-serial load
Hold
Clear
Recirculating or non-recirculating
8. 8
AAIT, Department of
Electrical and Computer
Engineering
Nebyu Yonas Sutri
Serial/Parallel Data Conversion
1 0 1 0 1 1 1 1Serial in Serial out
Shift registers can be used to convert from serial-
to-parallel or the reverse from parallel-to-serial.
1 0 1 0 1 1 1 1Serial in
Parallel out
1 0 1 0 1 1 1 1 Serial out
Parallel in
1 0 1 0 1 1 1 1
Parallel in
Parallel out
9. 9
AAIT, Department of
Electrical and Computer
Engineering
Nebyu Yonas Sutri
Serial In Serial Out Shift Registers
Serial-In Serial-Out (SISO) - the data is shifted
serially “IN” and “OUT” of the register, one bit at a
time in either a left or right direction under clock
control.
16. 16
AAIT, Department of
Electrical and Computer
Engineering
Nebyu Yonas Sutri
Parallel In Parallel Out Shift Registers
Clk pulse
no.
FF0 FF1 FF2 FF3
0 0 0 0 0
1 1 0 1 0
2 0 1 0 1
3 0 0 1 0
4 0 0 0 1
5 0 0 0 0
Data IN = 1010
17. 17
AAIT, Department of
Electrical and Computer
Engineering
Nebyu Yonas Sutri
Universal Shift Register
Universal Shift Register is a register that has both
serial and parallel input and output capability.
Today, there are many high speed bi-directional
“universal” type Shift Registers available such as the
TTL 74LS194, 74LS195,74HC194 or the CMOS 4035
which are available as 4-bit multi-function devices that
can be used in either serial-to-serial, left shifting, right
shifting, serial-to-parallel, parallel-to-serial, or as a
parallel-to-parallel multifunction data register, hence
the name “Universal”.
18. 18
AAIT, Department of
Electrical and Computer
Engineering
Nebyu Yonas Sutri
Universal Shift Register
Universal shift registers are very useful digital
devices. They are frequently used in arithmetic
operations to shift data to the left or right for
multiplication or division.
19. 19
AAIT, Department of
Electrical and Computer
Engineering
Nebyu Yonas Sutri
Shift Register based Counters
A shift register counter is basically a shift
register with the serial output connected to the
serial input to produce a special sequence.
There are two basic types of counters which are
based on shift registers
Ring counters
Johnson counters
20. 20
AAIT, Department of
Electrical and Computer
Engineering
Nebyu Yonas Sutri
Ring Counters
In a ring counter the output of the most
significant stage is fed back to the input of the
least significant stage.
E.g. 4 bit ring counter
21. 21
AAIT, Department of
Electrical and Computer
Engineering
Nebyu Yonas Sutri
4 bit Ring Counters
It got only 4 distinct states, hence mod-4 counter. Only
4 of the maximum 16 states are used, making ring
counters very inefficient in terms of state usage.
But the major advantage of a ring counter over a
binary counter is that it is self-decoding. No extra
decoding circuit is needed to determine what state the
counter is in.
24. 24
AAIT, Department of
Electrical and Computer
Engineering
Nebyu Yonas Sutri
Johnson Counters
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 modulus, so it may be
considered to be a mod-2n counter.
E.g. 4 bit and 5 bit Johnson counters
29. 29
AAIT, Department of
Electrical and Computer
Engineering
Nebyu Yonas Sutri
Counters
There are two basic types of counters
Asynchronous counters
Synchronous counters
30. 30
AAIT, Department of
Electrical and Computer
Engineering
Nebyu Yonas Sutri
Asynchronous counters
Counters that do not have a fixed time relationship
with each other and generally do not count at the
same time.
An asynchronous counter is one in which the
flip-flops within it do not change states at exactly
the same time because they do not have a
common clock pulse.
Also known as ripple counters.
34. 34
AAIT, Department of
Electrical and Computer
Engineering
Nebyu Yonas Sutri
Synchronous Counter
The counter have a fixed time relationship with
each other and generally count at the same time.
A synchronous counter is one which all the
flip-flops are connected to the same clock.
38. 38
AAIT, Department of
Electrical and Computer
Engineering
Nebyu Yonas Sutri
Reading assignment.
Read about synchronous counters design, on
Fundamentals of digital logic with verilog design,
Section 8.7, page 495-505.
What to Do this Week?
