Basics of sequential circuits

1. DIGITAL
ELECTRONICS
BASICS OF SEQUENTIAL LOGIC CIRCUITS
PREPARED BY- SHREYA SAHU
M.Tech (C&I), Delhi Technological
University

2.  The present output not only depends on present input; it also depends on past output.
 EXAMPLE- Logic gates with feedback connections (Latches).
 It consists of a combinational circuits, which accepts digital signals from external inputs ands from the
outputs of memory elements connected in feedback path and generates signals for external outputs
and for the inputs to memory element.
Combinational
Circuit
Memory Elements
INPUTS OUTPUTS

3. COMPARISON OF COMBINATIONAL
AND SEQUENTIAL CIRCUITS
COMBINATIONAL CIRCUITS
1. The output variables at any instant of time
depends only on the present input variables.
2. These circuits does not require any memory
element, hence called memory less system.
3. Combinational circuits are faster.
4. They are easy to design
SEQUENTIAL CIRCUITS
1. The output variables at any instant of time
depends not only on the present input variables
but also the past history of the system.
2. To store the past history of the input variables,
memory unit is required.
3. Sequential circuits are slower.
4. They are comparatively harder to design.

4. The sequential circuits are classified as Synchronous and Asynchronous
sequential circuits depending upon on the timing of their signals.
NOTE: Clock is periodically recurring pulse. It is generated by pulse generator.
SYNCHRONOUS
 The change in input signals can
affect memory element upon
activation of clock signals.
 The maximum operational speed of
clock depends on time delays
involved.
 In this circuit, memory elements are
“clocked flip-flops”.
 It is easier to design.
 It is generally “edge triggered”.
ASYNCHRONOUS
 The change in input signals can affect
memory element at any instant of time.
 Because of absence of clock, circuit can
operate faster than synchronous circuit.
 In this circuit, memory elements are either
“unclocked flip flops” or time delay
elements.
 More difficult to design.
 It is generally “level triggered”.

5. A basic memory cell is a circuit that stores one bit of information and this one bit memory
element is called flip-flop. Flip-flops are bistable multivibrator, has two stable states and it
can remain in either of the states which can be changed by applying proper triggering
signal.
In its simplest form, flip-flop is called a ‘Latch’, since it latches (or locks) data in it. In latch
there is no facility to read its contents. They are temporary storage devices, ideally suited for
storing information between processing units and input units.

6. The below table lists the basic difference between flip-flop
and latches.
LATCHES
1. Latches use level triggering.
2. Asynchronous inputs.
3. The output changes as per the input
till enable is high.
FLIP-FLOPS
1. Flip-flops use edge triggering.
2. Synchronous inputs.
3. The output changes as per the input
only at triggering point.

7. Triggering: It is used to initiate the operation of latches or flip-flops. Its main
purpose is to synchronize latches or flip-flops.
It is classified as:-
1. Level triggering
2. Edge triggering
In level triggering, input signals affect
the flip-flop only when the clock is at
logic ‘1’.
In edge triggering, input signals affect
the flip-flop only if they are present at
the positive going or negative going
edge of the clock pulse.
In level triggering circuit the output
may change several times in a single
clock, whereas in edge triggering circuit
the output will change only once in a
single clock.

8. THANKYOU