# sequential?/combinational?

Discussion in 'Electronic Basics' started by [email protected], May 11, 2005.

1. ### Guest

I have read a few different definitions of sequential and combinational
and i still do not understand the difference between them. If anybody
could put the major difference/s into simpler terms i would be grateful

cheers
sam

2. ### Lord GarthGuest

Sequential logic means one operation after another happens in sequence.
This
is controlled by using a timing pulse or a clock. Clocks can be synchronous
meaning all devices are pulsed at the same time or the clock can ripple
through
the circuit meaning that the output from the first stage clocks the second
and so on.
It is generally better to design synchronous logic. Combinational logic
does not
use a clock. It uses a combination of logic gates to perform a task and is
asynchronous
to any system clock.

3. ### Guest

Thankyou very much thats a few extra marks i will gain on my exam
tommorrow!

Sam

5. ### RatchGuest

From Digital Design by M. Morris Mano

"A combinational circuit consists of logic gates whose outputs at any
time are determined directly from the present combination of inputs without
regard to previous inputs. A combinational circuit performs a specific
information processing operation fully specified logically by a set of
Boolean functions. Sequential circuits employ memory elements in addition
to logic gates. Their outputs are a function of the inputs and the state of
the memory elements. The state of the memory elements, in turn, is a
function of previous inputs. As a consequence, the outputs of a sequential
circuit depend not only on present inputs, but also on past inputs, and the
circuit behavior must be specified by a time sequence of inputs and internal
states." Synchronous/asynchronous clocking pertain only to sequential
logic. Ratch

6. ### Jonathan KirwanGuest

When I saw the OP's glee at getting an answer for a test from here
(without trying to really understand it) I decided against saying
anything more just then. It appears the test may be over now, so:

To put it simply without distorting too much, combinatorial logic
doesn't have state and sequential logic does.

Jon

7. ### JeroenGuest

What about a 2 gate S/R flipflop? Or does logic with feedback not qualify as
combinatorial?

Jeroen

8. ### Jonathan KirwanGuest

to retained state, the knowledge of which is required (as well as the
state of the inputs) in order to understand the outputs, then it is
not combinatorial.

Jon

9. ### Barry JonesGuest

Combinational circuits are not clocked. You put some set of inputs into
it, and the circuit produces results (outputs) just as fast as it can.
Change the inputs, and the outputs follow, with a very small propagation
delay.

Sequential circuits are clocked. You control when the outputs will
change with a clock. You can change the inputs over and over, and it
won't make any difference to the outputs until the next clock pulse. At
that time, whatever the inputs were at that moment get reflected in the
outputs. Flip-flops are used to maintain the state of the outputs
between clock pulses. Flip-flop outputs are generally considered to be
the circuit outputs.

Sequential circuits generally include a combinational component or
section that provides the logic that determines what the outputs should
be after the next clock pulse. The combinational component usually has
external inputs and feedback inputs from the flip-flop outputs.

I hope you work on understanding this rather than copying it. Put in in