235:. This problem is not as severe in synchronous circuits because the outputs of the memory elements only change at each clock pulse. The interval between clock signals is designed to be long enough to allow the outputs of the memory elements to "settle" so they are not changing when the next clock comes. Therefore, the only timing problems are due to "asynchronous inputs"; inputs to the circuit from other systems which are not synchronized to the clock signal.
152:. The interval between clock pulses must be long enough so that all the logic gates have time to respond to the changes and their outputs "settle" to stable logic values before the next clock pulse occurs. As long as this condition is met (ignoring certain other details) the circuit is guaranteed to be stable and reliable. This determines the maximum operating speed of the synchronous circuit.
159:
The maximum possible clock rate is determined by the slowest logic path in the circuit, otherwise known as the critical path. Every logical calculation, from the simplest to the most complex, must complete in one clock cycle. So logic paths that complete their calculations quickly are idle much of
72:
with "channel up" and "channel down" buttons. Pressing the "up" button gives the television an input telling it to switch to the next channel above the one it is currently receiving. If the television is on channel 5, pressing "up" switches it to receive channel 6. However, if the television is on
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is not synchronized by a clock signal; the outputs of the circuit change directly in response to changes in inputs. The advantage of asynchronous logic is that it can be faster than synchronous logic, because the circuit doesn't have to wait for a clock signal to process inputs. The speed of the
173:
The clock signal must be distributed to every flip-flop in the circuit. As the clock is usually a high-frequency signal, this distribution consumes a relatively large amount of power and dissipates much heat. Even the flip-flops that are doing nothing consume a small amount of power, thereby
73:
channel 8, pressing "up" switches it to channel "9". In order for the channel selection to operate correctly, the television must be aware of which channel it is currently receiving, which was determined by past channel selections. The television stores the current channel as part of its
160:
the time, waiting for the next clock pulse. Therefore, synchronous logic can be slower than asynchronous logic. One way to speed up synchronous circuits is to split complex operations into several simple operations which can be performed in successive clock cycles, a technique known as
221:
However, asynchronous logic is more difficult to design and is subject to problems not encountered in synchronous designs. The main problem is that digital memory elements are sensitive to the order that their input signals arrive; if two signals arrive at a
178:
in the chip. In battery-powered devices, additional hardware and software complexity is required to reduce the clock speed or temporarily turn off the clock while the device is not being actively used, in order to maintain a usable battery
141:
of the circuit. The state of the synchronous circuit only changes on clock pulses. At each cycle, the next state is determined by the current state and the value of the input signals when the clock pulse occurs.
132:. The output of each flip-flop only changes when triggered by the clock pulse, so changes to the logic signals throughout the circuit all begin at the same time, at regular intervals, synchronized by the clock.
226:
or latch at almost the same time, which state the circuit goes into can depend on which signal gets to the gate first. Therefore, the circuit can go into the wrong state, depending on small differences in the
145:
The main advantage of synchronous logic is its simplicity. The logic gates which perform the operations on the data require a finite amount of time to respond to changes to their inputs. This is called
79:. When a "channel up" or "channel down" input is given to it, the sequential logic of the channel selection circuitry calculates the new channel from the input and the current channel.
238:
Asynchronous sequential circuits are typically used only in a few critical parts of otherwise synchronous systems where speed is at a premium, such as parts of microprocessors and
65:, a basic building block in all digital circuitry. Virtually all circuits in practical digital devices are a mixture of combinational and sequential logic.
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The design of asynchronous logic uses different mathematical models and techniques from synchronous logic, and is an active area of research.
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The output of all the storage elements (flip-flops) in the circuit at any given time, the binary data they contain, is called the
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547:(xiii+1+123+7 pages) (NB. The back cover of this book erroneously states volume 4, whereas it actually is volume 101.)
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which is distributed to all the memory elements in the circuit. The basic memory element in synchronous logic is the
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types. In synchronous sequential circuits, the state of the device changes only at discrete times in response to a
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Asynchronous
Operators of Sequential Logic: Venjunction & Sequention — Digital Circuits Analysis and Design
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507:. Lecture Notes in Electrical Engineering (LNEE). Vol. 101 (1 ed.). Berlin / Heidelberg, Germany:
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94:. In asynchronous circuits the state of the device can change at any time in response to changing inputs.
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Electrical, Electronics, and
Digital Hardware Essentials for Scientists and Engineers
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whose output depends on the present value of its input signals and on the
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19:"Sequential circuit" redirects here. For the synthesizer company, see
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design and helps to improve the performance of modern processors.
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A familiar example of a device with sequential logic is a
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of past inputs, the input history. This is in contrast to
485:"Asynchronous logic elements. Venjunction and sequention"
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Vasyukevich, Vadim O. (2011). Written at Riga, Latvia.
122:) generates a sequence of repetitive pulses called the
82:Digital sequential logic circuits are divided into
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155:Synchronous logic has two main disadvantages:
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210:device is potentially limited only by the
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333:Sequential Logic: Analysis and Synthesis
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166:. This technique is extensively used in
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275:Application-specific integrated circuit
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61:Sequential logic is used to construct
58:) while combinational logic does not.
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231:of the logic gates. This is called a
102:Nearly all sequential logic today is
680:Three-dimensional integrated circuit
462:Switching and Finite Automata Theory
110:logic. In a synchronous circuit, an
460:Kohavi, Zvi; Jha, Niraj K. (2009).
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692:Erasable programmable logic device
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398:Digital Design: A Systems Approach
14:
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727:Complex programmable logic device
497:from the original on 2011-07-22.
739:Field-programmable object array
675:Mixed-signal integrated circuit
483:Vasyukevich, Vadim O. (2009).
1:
865:Hardware description language
733:Field-programmable gate array
395:; Harting, R. Curtis (2012).
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184:Asynchronous sequential logic
98:Synchronous sequential logic
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877:Formal equivalence checking
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855:Transaction-level modeling
466:Cambridge University Press
403:Cambridge University Press
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1013:Logic in computer science
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798:Digital signal processing
783:Logic in computer science
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709:Programmable logic device
669:Hybrid integrated circuit
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517:10.1007/978-3-642-21611-4
439:Contemporary Logic Design
330:Cavanagh, Joseph (2006).
240:digital signal processing
810:Switching circuit theory
715:Programmable Array Logic
703:Programmable logic array
299:Vai, M. Michael (2000).
860:Register-transfer level
1008:Automata (computation)
751:Tensor Processing Unit
361:Lipiansky, Ed (2012).
966:Electronic literature
920:Hardware acceleration
788:Computer architecture
686:Emitter-coupled logic
623:Printed circuit board
112:electronic oscillator
63:finite-state machines
16:Type of logic circuit
892:Finite-state machine
870:High-level synthesis
805:Circuit minimization
393:Dally, William James
265:Asynchronous circuit
190:Asynchronous circuit
1018:Digital electronics
939:Digital photography
721:Generic Array Logic
643:Combinational logic
618:Printed electronics
582:Digital electronics
260:Synchronous circuit
255:Combinational logic
45:combinational logic
21:Sequential Circuits
887:Asynchronous logic
663:Integrated circuit
628:Electronic circuit
435:Borriello, Gaetano
229:propagation delays
212:propagation delays
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944:Digital telephone
915:Computer hardware
882:Synchronous logic
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475:978-0-521-85748-2
149:propagation delay
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499:(118 pages)
431:Katz, Randy
302:VLSI Design
216:logic gates
174:generating
108:synchronous
84:synchronous
1002:Categories
987:Runt pulse
959:television
653:Logic gate
598:Transistor
590:Components
543:2011929655
281:References
242:circuits.
203:self-timed
176:waste heat
163:pipelining
843:Placement
633:Flip-flop
613:Capacitor
535:1876-1100
338:CRC Press
307:CRC Press
224:flip-flop
199:clockless
130:flip-flop
114:called a
608:Inductor
603:Resistor
492:Archived
437:(2005).
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40:sequence
848:Routing
682:(3D IC)
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369:Wiley
179:life.
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116:clock
76:state
51:state
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