# Computer Architecture CS 215

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Computer Architecture CS 215
Latches & Flip-flops Computer Architecture CS 215

Sequential Circuits Storage elements Combinatorial Logic Inputs
Combinational Logic Storage Elements Inputs Outputs State Next Storage elements Latches or Flip-Flops Combinatorial Logic

Sequential Circuits Inputs Outputs Combinational Logic
Storage Elements Inputs Outputs State Next Combinatorial Logic Next state function Next State = f(Inputs, State) Output function (Mealy) Outputs = g(Inputs, State) Output function (Moore) Outputs = h(State) Output function type depends on specification and affects the design significantly

Sequential Circuits Synchronous Asynchronous
Behavior defined from knowledge of its signals at discrete instances of time Storage elements observe inputs and can change state only in relation to a timing signal (clock pulses from a clock) Asynchronous Behavior defined from knowledge of inputs an any instant of time and the order in continuous time in which inputs change If clock just regarded as another input, all circuits are asynchronous! Nevertheless, the synchronous abstraction makes complex designs tractable!

Discrete Event Simulation
Rules: Gates modeled by an ideal (instantaneous) function and a fixed gate delay Any change in input values is evaluated to see if it causes a change in output value Changes in output values are scheduled for the fixed gate delay after the input change At the time for a scheduled output change, the output value is changed along with any inputs it drives

Simulated NAND Gate  t (ns) A B F(I) F Comment – 1
Example: A 2-Input NAND gate with a 0.5 ns. delay Assume A and B have been 1 for a long time At time t=0, A changes back to 1. to a 0 at t= 0.8 ns, F A B DELAY 0.5 ns. F(Instantaneous) t (ns) A B F(I) F Comment 1 A=B=1 for a long time Þ Ü F(I) changes to 1 0.5 F changes to 1 after a 0.5 ns delay 0.8 F(Instantaneous) changes to 0 0.13 F changes to 0 after a 0.5 ns delay

Basic (NAND) S–R Latch S (set) “Cross-Coupling” two NAND gates gives the S-R Latch S = 0, R = 0 is forbidden as input pattern Q Q R (reset) Time R S Q Comment 1 ? Stored state unknown “Set” Q to 1 Now Q “remembers” 1 “Reset” Q to 0 1 1 1 Now Q “remembers” 0 1 1 Both go high 1 1 ? ? Unstable!

Basic (NOR) S–R Latch S (set) R (reset) Q Cross-coupling two NOR gates gives the S – R Latch R S Q Comment ? Stored state unknown 1 “Set” Q to 1 Now Q “remembers” 1 “Reset” Q to 0 Now Q “remembers” 0 Both go low Unstable! Time

Clocked S-R Latch Adding two NAND gates to the basic S-R NAND latch gives the clocked S–R latch: Has a time sequence behavior similar to the basic S-R latch except that the S and R inputs are only observed when the line C is high. C means “control” or “clock”. S R Q C

Clocked S-R Latch The Clocked S-R Latch can be described by a table
The table describes what happens after the clock [at time (t+1)] based on: current inputs (S,R) and current state Q(t). S R Q C

D Latch Adding an inverter to the S-R Latch, gives the D Latch:
Q C Adding an inverter to the S-R Latch, gives the D Latch: Note that there are no “indeterminate” states! Q D Q(t+1) Comment No change 1 Set Q Clear Q No Change C D Q

Flip-Flops Master-slave flip-flop Edge-triggered flip-flop
Standard symbols for storage elements

S-R Master-Slave Flip-Flop
C S R Q Two clocked S-R latches in series with the clock on the second latch inverted The input is observed by the first latch with C = 1 The output is changed by the second latch with C = 0 The path from input to output is broken by the difference in clocking values (C = 1 and C = 0). The behavior demonstrated by the example with D driven by Y given previously is prevented since the clock must change from 1 to 0 before a change in Y based on D can occur.

Edge-Triggered D Flip-Flop
C S R Q D The edge-triggered D flip-flop is the same as the master- slave D flip-flop It can be formed by: Replacing the first clocked S-R latch with a clocked D latch or Adding a D input and inverter to a master-slave S-R flip-flop The delay of the S-R master-slave flip-flop can be avoided since the 1s-catching behavior is not present with D replacing S and R inputs The change of the D flip-flop output is associated with the negative edge at the end of the pulse It is called a negative-edge triggered flip-flop

Positive-Edge Triggered D Flip-Flop
Formed by adding inverter to clock input Q changes to the value on D applied at the positive clock edge within timing constraints to be specified Our choice as the standard flip-flop for most sequential circuits C S R Q D

Standard Symbols for Storage Elements
Master-Slave: Postponed output indicators Edge-Triggered: Dynamic indicator (a) Latches S R SR D with 0 Control D C D with 1 Control (b) Master-Slave Flip-Flops Triggered D Triggered SR (c) Edge-Triggered Flip-Flops

State Diagrams The sequential circuit function can be represented in graphical form as a state diagram with the following components: A circle with the state name in it for each state A directed arc from the Present State to the Next State for each state transition A label on each directed arc with the Input values which causes the state transition, and A label: On each circle with the output value produced, or On each directed arc with the output value produced.

State Diagrams Label form: On circle with output included:
state/output Moore type output depends only on state On directed arc with the output included: input/output Mealy type output depends on state and input

State Diagram: SR Latch

Other Flip-Flop Types J-K and T flip-flops
Behavior Implementation Basic descriptors for understanding and using different flip-flop types Characteristic tables Characteristic equations Excitation tables For actual use, see Reading Supplement - Design and Analysis Using J-K and T Flip-Flops

J-K Flip-flop Behavior
Same as S-R flip-flop with J analogous to S and K analogous to R Except that J = K = 1 is allowed, and For J = K = 1, the flip-flop changes to the opposite state As a master-slave, has same “1s catching” behavior as S-R flip-flop If the master changes to the wrong state, that state will be passed to the slave E.g., if master falsely set by J = 1, K = 1 cannot reset it during the current clock cycle

J-K Flip-flop Build this! J C K JK=10 or 11 JK=00 D J Q = 0 Q = 1 K C
Implementation To avoid 1s catching behavior, one solution used is to use an edge-triggered D as the core of the flip-flop J C K Build this! D C K J Q = 0 Q = 1 JK=00 JK=10 or 11 JK=01 or 11

T Flip-flop Behavior Same as a J-K flip-flop with J = K = T
Has a single input T For T = 0, no change to state For T = 1, changes to opposite state Same as a J-K flip-flop with J = K = T As a master-slave, has same “1s catching” behavior as J-K flip-flop Cannot be initialized to a known state using the T input Reset (asynchronous or synchronous) essential

T Flip-flop Build this! T C T = 1 T=0 D T Q = 0 Q = 1 C Implementation
To avoid 1s catching behavior, one solution used is to use an edge-triggered D as the core of the flip-flop T C Build this! Q = 0 Q = 1 T=0 T = 1 C D T

Basic Flip-Flop Descriptors
Used in analysis Characteristic table - defines the next state of the flip-flop in terms of flip-flop inputs and current state Characteristic equation - defines the next state of the flip-flop as a Boolean function of the flip-flop inputs and the current state Used in design Excitation table - defines the flip-flop input variable values as function of the current state and next state

D Flip-Flop Descriptors
Characteristic Table Characteristic Equation Q(t+1) = D Excitation Table D 1 Operation Reset Set Q(t 1) + Q(t +1) 1 D Operation Reset Set

T Flip-Flop Descriptors
Characteristic Table Characteristic Equation Q(t+1) = T Å Q Excitation Table No change Complement Operation 1 T Q(t 1) Q ( t ) + Q(t + 1) Q ( t ) 1 T No change Complement Operation

S-R Flip-Flop Descriptors
Characteristic Table Characteristic Equation Q(t+1) = S + R Q, S.R = 0 Excitation Table 1 Operation S R No change Reset Set Undefined ? Q(t + 1) Q ( t ) Operation No change Set Reset S X 1 Q(t+ 1) Q(t) R

J-K Flip-Flop Descriptors
Characteristic Table Characteristic Equation Q(t+1) = J Q + K Q Excitation Table 1 No change Set Reset Complement Operation J K Q(t + 1) Q ( t ) Q(t + 1) 1 Q(t) Operation X K J No change Set Reset No Change