DigitalLogicCircuits-2

DigitalLogicCircuits-2 - Kingdom Of Saudi Arabia Al-Imam...

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Digital Logic Circuits Chapter 1 in “Computer System Architecture” Kingdom Of Saudi Arabia Al-Imam Muhammad Ibn Saud Islamic University College of Computer and Information Sciences Information System Department 1 st semester, 2010 – 2011 CS 224: Computer Organization 1 T. Hala A. Al-Rumaih
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Flip-Flops Although every digital system is likely to have a combinational circuit, most systems encountered in practice also include storage elements, which require that the system be described in terms of sequential circuits. The most common type of sequential circuit is the synchronous type. Synchronous sequential circuits employ signals that affect the storage elements only at discrete instants of time. Synchronization is achieved by a timing device called a clock pulse generator that produces a periodic train of clock pulses. The clock pulses are distributed throughout the system in such a way that storage elements are affected only with the arrival of the synchronization pulse. 2
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Flip-Flops (Cont.) Clocked synchronous sequential circuits are the type most frequently encountered in practice . They seldom manifest instability problems and their timing is easily broken down into independent discrete steps, each of which may be considered separately . Clocked Sequential Circuit: The storage elements employed in clocked sequential circuits are called flip- flops . A flip-flop is a binary cell capable of storing one bit of information . It has two outputs, one for the normal value and one for the complement value of the bit stored in it . A flip-flop maintains a binary state until directed by a clock pulse to switch states. The difference among various types of flip-flops is in the number of inputs they possess and in the manner in which the inputs affect the binary state. 3
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Flip-Flops (Cont.) SR Flip-Flop: The graphic symbol of the SR flip-flop is : It has three inputs, labeled S (for set), R (for reset), and C (for clock). It has an output Q and sometimes the flip-flop has a complemented output, which is indicated with a small circle at the other output terminal. 4
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(Cont.) There is an arrowhead-shaped symbol in front of the letter C to designate a dynamic input. The dynamic indicator symbol denotes the fact that the flip-flop responds to a positive transition (from 0 to 1) of the input clock signal The operation of the SR flip-flop is as follows: If there is no signal at the clock input C, the output of the circuit cannot change irrespective of the values at inputs S and R. Only when the clock signal changes from 0 to 1 can the output be affected according to the values in inputs S and R.
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DigitalLogicCircuits-2 - Kingdom Of Saudi Arabia Al-Imam...

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