Clock cycle time or clock period: the time interval between two consecutive rising or falling edges of the clock. Difference Between Combinational and Sequential Circuit: Combinational circuit generates an output by relying on the current input, and it stays independent of time. They are stable and their timing can easily be broken down into independent discrete steps, each of which is considered separately.Ī clock signal is a periodic square wave that indefinitely switches from 0 to 1 and from 1 to 0 at fixed intervals. Synchronous sequential circuits that use clock pulses in the inputs are called clocked-sequential circuits. Clock pulses are distributed throughout the system in such a way that the flip-flops are affected only with the arrival of the synchronization pulse. Synchronization is achieved by a timing device called a clock pulse generator. All state transitions in such circuits occur only when the clock value is either 0 or 1 or happen at the rising or falling edges of the clock depending on the type of memory elements used in the circuit. Fig.2 Sequential Logic Circuit Unlike combinational logic circuits whose output depends upon the signals being applied to the inputs at that time, Sequential Logic circuits have some form of memory built in to them so that they are able to take into account their previous input state as well as those at present.
Sequential circuits have a clock signal as one of their inputs. Synchronous sequential circuits use logic gates and flip-flop storage devices. This type of system uses storage elements called flip-flops that are employed to change their binary value only at discrete instants of time. Because of the feedback among logic gates, the system may, at times, become unstable. Gate-type asynchronous systems are basically combinational circuits with feedback paths. This is a system whose outputs depend upon the order in which its input variables change and can be affected at any instant of time. Their classification depends on the timing of their signals: There are two types of sequential circuits. Notice the presence of a CLK signal below. We can connect memory device (s) together with combinational logic device (s) to form a sequential logic device. A sequential circuit is specified by a time sequence of inputs, outputs, and internal states. A memory device on the other hand, is a device of which we can write new values to and is able to remember this value for a period of time. Memory elements next state is also a function of external inputs and present state. The input and the present state of the memory element determines the output. The binary info stored in the memory elements at any given time defines the state of the sequential circuit. The memory elements are devices capable of storing binary info. Combinational logic output depends on the inputs levels, whereas sequential logic output depends on stored levels and also the input levels. Digital electronics is classified into combinational logic and sequential logic.
0 kommentar(er)
