B.S Anangpuria Institute of Technology & Management
Branch: CSE/IT (4th SEM)
Session-2009
Computer Architecture and
Organization
CSE – 210-E
Unit -1
Basic Principles
Lecture – 3:
• Sequential logic Blocks
Latches
Flip flops
Registers
Counters
Submitted by:
Prerna Mittal
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• Sequential logic Blocks : logic blocks whose output logic value
depends on the input values and the state of the blocks
– In this we have the concept of memory which was not
applicable for combinational circuits.
The various sequential blocks or circuits are:
Latches:
•
A latch is a kind of bistable multivibrator, an electronic circuit which has two
stable states and thereby can store one bit of information. Today the word is
mainly used for simple transparent storage elements, while slightly more
advanced non-transparent (or clocked) devices are described as flip-flops.
Informally, as this distinction is quite new, the two words are sometimes used
interchangeably.
S-R latch:
To overcome the restricted combination, one can add gates to the inputs that would
convert (S,R) = (1,1) to one of non-restricted combinations. That can be:
Q = 1 (1,0) — referred to as an S-latch
Q = 0 (0,1) — referred to as an R-latch
Keep state (0,0) — referred to as an E-latch
D-LATCH
Forbidden input values are forced not to occur by using an inverter between the inputs.
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D
Q
Q
E
(enable)
E
Q’
D(data)
D
0
1
Q(t+1)
0
1
D
E
Q’
Q
Q’
Flip Flops:
D – flip flop:
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If you compare the D-flip flop and D – latch the only difference you find in the circuit is
that latches do not have clocks and flip – flops have it.
So you can note down the difference between latches and flip – flops as:
• Latch is an edge triggered device whereas Flip flop is a level triggered.
• The output of a latch changes independent of a clock signal whereas the Output of
a Flip - Flop changes at specific times determined by a clocking signal.
• In Latch We do not require clock pulses and flip flops are clocked devices.
Characteristics
- State transition occurs at the rising edge or
falling edge of the clock pulse
Latches
respond to the input only during these periods
Edge-triggered Flip Flops (positive)
respond to the input only at this time
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Counters: A counter is a device which stores (and sometimes displays) the number of
times a particular event or process has occurred, often in relationship to a clock signal.
4 – bit binary counter:
A0
A1
A2
A3
Q
Q
Q
Q
J
K
J
K
J
K
J
K
Clock
Counter
Enable
Output
Carry
RING COUNTER:
In Ring Counter the output of 1st flip flop is moved to the input of 2nd flip flop.
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JOHNSON COUNTER :
In Johnson counter the output of last flip flop is inverted and given to the first flip flop.
Registers: It refers to a group of flip-flops operating as a coherent unit to hold data. This
is different from a counter, which is a group of flip-flops operating to generate new data
by tabulating it.
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Shift register: A register that is capable of shifting data one bit at a time is called a shift
register. The logical configuration of a serial shift register consists of a chain of flip-flops
connected in cascade, with the output of one flip-flop being connected to the input of its
neighbor. The operation of the shift register is synchronous; thus each flip-flop is
connected to a common clock. Using D flip-flops forms the simplest type of shiftregisters.
Bi- directional shift register with parallel load
A0
A1
A2
A3
Q
Q
Q
Q
D C
D C
D C
D C
4x1
MUX
Clock
S0S1 SeriaI I0
Input
4x1
MUX
4x1
MUX
I1
4x1
MUX
I2
Serial I3
Input
Summary:
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

Sequential circuits: output logic value depends on the input values and the
state of the blocks. These circuits have memory.
Various combinational circuits are:
o Latches: An electronic circuit which has two stable states and
thereby can store one bit of information
o Flip flops: It also has 2 stable states but with memory.
o Counter: A device which stores number of times a particular event
or process has occurred.
o Registers: A group of flip-flops operating as a coherent unit to hold
data.
Important Questions derived from this:
Q1. What is the difference in latch and flip flop?
Q2. Explain Johnson counter?
Q3. Draw shift register with parallel load.
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