Latches & Flip Flop Short notes | Digital Circuits - Electronics and Communication Engineering (ECE) PDF Download

 Page 1


Latc hes and Flip-Flops
Latc hes and flip-flops are fundamen tal sequen tial logic circuits in digital systems, serving as basic
memory elemen ts. They store binary information and are essen tial for building registers, coun ters, and
state mac hines used in pro cessors, memory units, and con trol systems.
1. In t ro d uction to Latc hes and Flip-Flops
Unlik e com binational logic, sequen tial circuits lik e latc hes and flip-flops ha v e outputs that dep end on
b o th curren t inputs and previous states, in tro ducing memory . Latc hes are lev el-sensitiv e, resp onding to
input c hanges while enabled, while flip-flops are edge-sensitiv e, up dating only on clo c k signal transitions.
Both are constructed using logic gates with feedbac k.
2. Latc h es
A latc h is a bistable circuit that main tains its state un til the inputs c hange and the enable signal is
activ e.
• SR Latc h : Built using NOR or NAND gates with t w o inputs, Set (S) and Reset (R), and t w o
outputs, Q and Q .
– NOR SR L atch :
Q=S+Q, Q=R+Q
– T ruth T able (NOR):
S R Q Q
0 0 Q Q (Hold)
1 0 1 0 (Set)
0 1 0 1 (Reset)
1 1 0 0 (In v alid)
• Gated SR Latc h : A dds an enable input (EN) to con trol when the latc h re sp onds to S and R.
• D Latc h : Eliminates the in v alid state of SR latc h b y using a single data input (D) and enable.
Output Q follo ws D when enabled.
3. Fl ip-Flops
Flip-flops are edge-triggered devices that c hange state only on the rising or falling edge of a clo c k signal,
ensuring sync hronized op eration.
• SR Flip-Flop : Similar to SR latc h but clo c k ed. In v alid state (S =R=1 ) m ust b e a v oided.
• D Flip-Flop : Stores t he v alue of input D on the clo c k edge. T ruth T able:
Clk D Q
? 0 0
? 1 1
- - Q (Hold)
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Latc hes and Flip-Flops
Latc hes and flip-flops are fundamen tal sequen tial logic circuits in digital systems, serving as basic
memory elemen ts. They store binary information and are essen tial for building registers, coun ters, and
state mac hines used in pro cessors, memory units, and con trol systems.
1. In t ro d uction to Latc hes and Flip-Flops
Unlik e com binational logic, sequen tial circuits lik e latc hes and flip-flops ha v e outputs that dep end on
b o th curren t inputs and previous states, in tro ducing memory . Latc hes are lev el-sensitiv e, resp onding to
input c hanges while enabled, while flip-flops are edge-sensitiv e, up dating only on clo c k signal transitions.
Both are constructed using logic gates with feedbac k.
2. Latc h es
A latc h is a bistable circuit that main tains its state un til the inputs c hange and the enable signal is
activ e.
• SR Latc h : Built using NOR or NAND gates with t w o inputs, Set (S) and Reset (R), and t w o
outputs, Q and Q .
– NOR SR L atch :
Q=S+Q, Q=R+Q
– T ruth T able (NOR):
S R Q Q
0 0 Q Q (Hold)
1 0 1 0 (Set)
0 1 0 1 (Reset)
1 1 0 0 (In v alid)
• Gated SR Latc h : A dds an enable input (EN) to con trol when the latc h re sp onds to S and R.
• D Latc h : Eliminates the in v alid state of SR latc h b y using a single data input (D) and enable.
Output Q follo ws D when enabled.
3. Fl ip-Flops
Flip-flops are edge-triggered devices that c hange state only on the rising or falling edge of a clo c k signal,
ensuring sync hronized op eration.
• SR Flip-Flop : Similar to SR latc h but clo c k ed. In v alid state (S =R=1 ) m ust b e a v oided.
• D Flip-Flop : Stores t he v alue of input D on the clo c k edge. T ruth T able:
Clk D Q
? 0 0
? 1 1
- - Q (Hold)
1
• JK Flip-Flop : Extends SR flip-flop, resolving the in v alid state. Inputs J (set) and K (reset) allo w
toggling when J =K =1 . T ruth T able:
J K Q
0 0 Q (Hold)
1 0 1 (Set)
0 1 0 (Reset)
1 1 Q (T oggle)
• T Flip-Flop : T oggles output on eac h clo c k edge when input T = 1 . Deriv ed from JK flip-flop
with J =K =T .
4. Implemen tation
Latc hes and flip-flops are implemen ted using:
• Logic Gates : NOR or NAND gates for latc hes; additional gates for clo c k ed flip-flops.
• CMOS T ec hnology : Lo w-p o w er, high-densit y designs for in tegrated circuits.
• Programmable Logic : FPGAs and CPLDs f or reconfigurable sequen tial circuits.
A D flip-flop, for example, can b e built using a gated D latc h with a master-sla v e configuration to ensure
edge-triggered op eration.
5. Applications
Latc hes and flip-flops are used in:
• Registers : Store data in CPUs and memory units.
• Coun ters : Implemen t sequen tial coun ting in timers and frequency dividers.
• State Mac hines : Store states in con trol units and digital con trollers.
• Sync hronization : Ensure data alignmen t in clo c k ed systems.
• Memory Elemen ts : In RAM and register files.
6. Design Considerations
• Clo c k Sk ew : V ariations in clo c k signal arriv al times can cause incorrect state c hanges in flip-flops.
• Setup and Hold Times : Input data m ust b e stable b efore and after the clo c k edge to ensure
reliable op eration.
• Metastabilit y : Occurs when inputs c hange to o close to the clo c k edge, mitigated b y sync hronizers.
• P o w er Consumption : CMOS flip-flops consume dynamic p o w er during transitions, critical for
lo w-p o w er designs.
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Page 3


Latc hes and Flip-Flops
Latc hes and flip-flops are fundamen tal sequen tial logic circuits in digital systems, serving as basic
memory elemen ts. They store binary information and are essen tial for building registers, coun ters, and
state mac hines used in pro cessors, memory units, and con trol systems.
1. In t ro d uction to Latc hes and Flip-Flops
Unlik e com binational logic, sequen tial circuits lik e latc hes and flip-flops ha v e outputs that dep end on
b o th curren t inputs and previous states, in tro ducing memory . Latc hes are lev el-sensitiv e, resp onding to
input c hanges while enabled, while flip-flops are edge-sensitiv e, up dating only on clo c k signal transitions.
Both are constructed using logic gates with feedbac k.
2. Latc h es
A latc h is a bistable circuit that main tains its state un til the inputs c hange and the enable signal is
activ e.
• SR Latc h : Built using NOR or NAND gates with t w o inputs, Set (S) and Reset (R), and t w o
outputs, Q and Q .
– NOR SR L atch :
Q=S+Q, Q=R+Q
– T ruth T able (NOR):
S R Q Q
0 0 Q Q (Hold)
1 0 1 0 (Set)
0 1 0 1 (Reset)
1 1 0 0 (In v alid)
• Gated SR Latc h : A dds an enable input (EN) to con trol when the latc h re sp onds to S and R.
• D Latc h : Eliminates the in v alid state of SR latc h b y using a single data input (D) and enable.
Output Q follo ws D when enabled.
3. Fl ip-Flops
Flip-flops are edge-triggered devices that c hange state only on the rising or falling edge of a clo c k signal,
ensuring sync hronized op eration.
• SR Flip-Flop : Similar to SR latc h but clo c k ed. In v alid state (S =R=1 ) m ust b e a v oided.
• D Flip-Flop : Stores t he v alue of input D on the clo c k edge. T ruth T able:
Clk D Q
? 0 0
? 1 1
- - Q (Hold)
1
• JK Flip-Flop : Extends SR flip-flop, resolving the in v alid state. Inputs J (set) and K (reset) allo w
toggling when J =K =1 . T ruth T able:
J K Q
0 0 Q (Hold)
1 0 1 (Set)
0 1 0 (Reset)
1 1 Q (T oggle)
• T Flip-Flop : T oggles output on eac h clo c k edge when input T = 1 . Deriv ed from JK flip-flop
with J =K =T .
4. Implemen tation
Latc hes and flip-flops are implemen ted using:
• Logic Gates : NOR or NAND gates for latc hes; additional gates for clo c k ed flip-flops.
• CMOS T ec hnology : Lo w-p o w er, high-densit y designs for in tegrated circuits.
• Programmable Logic : FPGAs and CPLDs f or reconfigurable sequen tial circuits.
A D flip-flop, for example, can b e built using a gated D latc h with a master-sla v e configuration to ensure
edge-triggered op eration.
5. Applications
Latc hes and flip-flops are used in:
• Registers : Store data in CPUs and memory units.
• Coun ters : Implemen t sequen tial coun ting in timers and frequency dividers.
• State Mac hines : Store states in con trol units and digital con trollers.
• Sync hronization : Ensure data alignmen t in clo c k ed systems.
• Memory Elemen ts : In RAM and register files.
6. Design Considerations
• Clo c k Sk ew : V ariations in clo c k signal arriv al times can cause incorrect state c hanges in flip-flops.
• Setup and Hold Times : Input data m ust b e stable b efore and after the clo c k edge to ensure
reliable op eration.
• Metastabilit y : Occurs when inputs c hange to o close to the clo c k edge, mitigated b y sync hronizers.
• P o w er Consumption : CMOS flip-flops consume dynamic p o w er during transitions, critical for
lo w-p o w er designs.
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7. Practical Considerations
• Propagation Dela y : Time from clo c k edge to output c hange, affecting system sp eed (e.g., CMOS:
10–50 ns).
• F an-in/F an-out : Limited b y logic family , impacting scalabilit y .
• Noise Margin : Must b e su?icien t for reliable op eration in noisy en vironmen ts.
• Reset/Preset : Man y flip-flops include async hronous reset or pres et inputs for initialization.
8. Master-Sla v e Configuration
T o ac hiev e edge-triggered b eha vior, flip-flops often use a master-sla v e structure:
• T w o latc hes in series, with the master latc h enabled on one clo c k phase and the sla v e on the opp osite
phase.
• Ensures the output c hanges only on the clo c k edge, prev en ting race conditions.
9. Conclusion
Latc hes and flip-flops are critical sequen tial logic elemen ts that pro vide memory and sync hronization in
digital systems. Latc hes offer simple lev el-sensitiv e storage, while flip-flops pro vide robust edge-triggered
op eration for clo c k ed designs. Their e?icien t implemen tation and careful consideration of timing and
p o w er constrain ts are essen tial for building reliable digital circuits in pro cessors, memory , and con trol
applications.
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