Chapter 22 - Asynchronous Sequential Circuits
We will look at the relation of setup and hold time to the design of the asynchronous circuits that realize flip-flops in more detail in Chapter 23. In
A DESIGN METHOD OF ASYNCHRONOUS SEQUENTIAL
A systematic asynchronous design method based on a flow diagram is shown. The realization utilizes a so-called phase-register coded lout of 11.
CS8351 DIGITAL PRINCIPLES AND SYSTEM DESIGN UNIT IV
The analysis of asynchronous sequential circuits consists of obtaining a table or a diagram that describes the sequence of internal states and outputs as a
Asynchronous Sequential Circuits Design
Asynchronous sequential circuits do not use clock signals as synchronous circuits do. Instead the circuit is driven by the pulses of the inputs which means
Asynchronous Sequential Circuits
The delay elements can be viewed as providing short term memory for the sequential circuit. Page 3. Asynchronous Sequential Circuits. • During the design of
IE1204 Digital Design F12: Asynchronous Sequential Circuits (Part 1)
Asynchronous Mealy compatible. IE1204 Digital Design Autumn2016. ·Asynchronous sequential circuits have similar structure as synchronous sequential circuits. ·
A Modern Approach to the Asynchronous Sequential Circuit Synthesis
The switching circuit design is composed of the two classes of the devices: combinational circuits and sequential circuits. The class of the sequential circuits.
Pass-transistor asynchronous sequential circuits
Abstract -The vast majority of sequential circuits currently imple- mented with VLSI circuit technology are designed using synchronous design theory.
Design of asynchronous esign of asynchronous sequential circuits
There are two distinct models by which a synchronous sequential logic circuit can be designed. In Mealy Model the output is derived from present state as well
COE 202: Digital Logic Design Sequential Circuits Part 1
• Latches are useful in asynchronous sequential circuits. • Flip-Flips are built with latches. Page 8. Latches. • A latch is binary storage element. • Can store
Design of asynchronous esign of asynchronous sequential circuits
There are two distinct models by which a synchronous sequential logic circuit can be designed. In Mealy Model the output is derived from present state as well
IE1204 Digital Design F12: Asynchronous Sequential Circuits (Part 1)
gates and summarize their delays to a single block with delay ?. Asynchronous sequential circuit: SR-latch with NOR gates. IE1204 Digital Design Autumn2016.
Asynchronous Sequential Circuits
The analysis of asynchronous sequential circuits An asynchronous sequential circuit may become ... Design a gated latch circuit with two inputs G.
Chapter 22 - Asynchronous Sequential Circuits
Design a toggle circuit like the one in Section 22.2 except that pulses on the input alternate over three outputs. 22–6 Three-Way Edge Toggle. Page 12. 394.
State assignments for non-normal asynchronous sequential circuits
designing an asynchronous sequential circuit is obtaining an internal state assignment. The internal state assignment problem consists basically of encoding
Chapter 9 - Asynchronous Sequential Logic
the behavior of the circuit can be analyzed by observing the stale transition as a function of changes in the input variables. Flow Table. During the design of
CS8351 DIGITAL PRINCIPLES AND SYSTEM DESIGN UNIT IV
The memory elements in asynchronous sequential circuits are either unclocked flip-flops (Latches) or time-delay elements. 3. Page 4. S No. Synchronous
Chapter 9 Asynchronous Sequential Logic Outline
Asynchronous Sequential Circuits. ? Analysis Procedure. ? Circuits with Latches. ? Design Procedure. ? Reduction of State and Flow Tables.
Using Petri Nets in the Design Process for Interacting Asynchronous
Each asynchronous sequential circuit can be described by a state tranllition graph and the .'~e . ign of the circuit froro the given graph is routine.
Asynchronous Sequential Circuits
The delay elements can be viewed as providing short term memory for the sequential circuit. Page 3. Asynchronous Sequential Circuits. • During the design of
IE1204 Digital Design
F12: Asynchronous
Sequential Circuits
(Part 1)Masoumeh(Azin) Ebrahimi (masebr@kth.se)
Elena Dubrova (dubrova@kth.se)
KTH / ICT / ES
BV pp. 584-640This lecture
IE1204 Digital Design, Autumn2016
Asynchronous Sequential
Machines
An asynchronous sequential machine is
a sequential machine without flip-flopsAsynchronous sequential machines are
constructed by analyzing combinational logic circuits with feedback Assumption: Only one signal in acircuit can change its value at any timeIE1204 Digital Design, Autumn2016
Golden rule
IE1204 Digital Design, Autumn2016
Asynchronous state machines
Asynchronous state machines are used
when it is necessary to keep the information about a state, but no clock is available - All flip-flops and latches are asynchronous state machines - Useful to synchronize events in situations where metastability is/can be a problemIE1204 Digital Design, Autumn2016
To analyze the behavior of an
asynchronous circuit, we use ideal gates and summarize their delays to a single block with delay ǻAsynchronous sequential circuit:SR-latch with NOR gates
IE1204 Digital Design, Autumn2016
R SQYyDelayIdeal gates
(Delay = 0)Q aRSSR-latch
By using a delay block, we can treat
- y as the current state - Y as the next stateAnalysis of a sequential asynchronous circuitIE1204 Digital Design, Autumn2016
R S QYyThus, we can produce a state table where the
next state Y depends on the inputs and the current state yState tableIE1204 Digital Design, Autumn2016
R S QY y)(ySRYState table
IE1204 Digital Design, Autumn2016
)(ySRYPresentNextstate
stateSR= 0001 1011
y YYYY0001011010
ySRYRSyFrom statefunction to truth tableNote: BV usesthisbinary codeSince we do not have flip-flops, but only
combinational circuits, a state change can cause additional state changesA state is
- stable if Y(t) = y(t + ǻ) - unstable if Y(t) y(t + ǻ)Stable statesIE1204 Digital Design, Autumn2016
Present
Nextstate
stateSR= 0001 1011
y YYYY0001011010
yYstableStable states (next state = present state) are
circledExcitation tableIE1204 Digital Design, Autumn2016
Present
Nextstate
stateSR= 0001 1011
y YYYY0001011010R
S QYy 00 0 10R S QYy 10 0 01When dealing with asynchronous
sequential circuits, a different terminology is used - The state table calledflow table - The state-assigned state table is called excitation tableTerminologyIE1204 Digital Design, Autumn2016
Flow table (Moore)
IE1204 Digital Design, Autumn2016
Present
NextstateOutput
stateSR= 0001 1011Q
AAABA0
BBABA1
100011010010
A0 B11101SR
Flow Table (Mealy)
IE1204 Digital Design, Autumn2016
Present
NextstateOutput,Q
stateSR = 0001 1011 00 01 1011
AAABA000
BBABA11-
-10/100/111/0
01/000/010 /-
AB 01-11-SR/ QDo not care ('-') has been chosen for output decoder since output changes
directly after the state transition (basic implementation)Asynchronous Moore compatible
IE1204 Digital Design, Autumn2016
Asynchronous sequential circuits have similar
structure as synchronous sequential circuitsInstead of flip-flops one have a "delay block"
R S QYy 10 01 0Asynchronous Mealy compatible
IE1204 Digital Design, Autumn2016
Asynchronous sequential circuits have similar
structure as synchronous sequential circuitsInstead of flip-flops one have a "delay block"
Analysis of Asynchronous Circuits
The analysis is done using the following steps:
1)Replace the feedbacks in the circuit with a
delay element.The input of the delay element represents the next state Y while the output y represents the current state.2) Find out thenext-state and output expressions
3) Set up the correspondingexcitation table
4) Create aflow tableand replace the encoded
states with symbolic states5) Draw astate diagramif necessary
IE1204 Digital Design, Autumn2016
D-latch state function
IE1204 Digital Design, Autumn2016
Q 1D C1QD CQ D CYy CyCDYMaster-slave D flip-flop is designed
using two D-latchesExampleMaster-slave D flip-flop
IE1204 Digital Design, Autumn2016
D ClkQQ D CQ ysymMasterSlaveQ D ClkQQ s ssmm yQCyDCYyCCDYThe equations for the next state:
From these equations, we can directly
deduce excitation tableExcitation tableIE1204 Digital Design, Autumn2016
Present
Nextstate
stateCD= 0001 1011Output
y mys YmYsQ0000 00 00100
0100 00 01111
1011 11 00 100
1111 11 01 111s
ssmm yQCyDCYyCCDYExcitation table
We define four states S1, S2, S3, S4
and get the following flow tableFlow tableIE1204 Digital Design, Autumn2016
Present
NextstateOutput
stateCD= 00 01 1011Q
S1S1 S1 S1S30
S2S1 S1 S2S41
S3S4 S4 S1 S30
S4S4S4S2S41Present
Nextstate
stateCD= 00 01 1011Output y mys YmYsQ0000 00 00100
0100 00 01111
1011 11 00 100
11111101111
Excitation table
Flow table
Remember: Only one input can be
changed simultaneouslyThus, some transitions never occur!Flow table
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