sequential?/combinational?

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

Sam

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.

Good luck on your test!

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

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

"Jonathan Kirwan" schreef in bericht news: snipped-for-privacy@4ax.com...

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

Jeroen

If you read Ratch's quote, you will have your answer. If it amounts 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

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 your own words.