Active 1 year, 8 months ago. A SIMPLE explanation of an SR Flip Flop (or SR Latch). Figure 2. Flip-flop is an edge triggered, i.e. SR flip flop | Truth table & Characteristics table, NOT Gate | Symbol, Truth table & Realization, AND Gate | Symbol, Truth table & Realization, OR Gate | Symbol, Truth table & Realization, Full Subtractor | Truth table & Logic Diagram, NAND Gate | Symbol, Truth table & Circuit, SR flip flop | Truth table & Characteristics table | Electricalvoice, Tunnel Diode | Symbol, Working & Applications, Electrical Engineering Interview Questions & Answers, Electrical Safety: 10 Tips to Prevent Workplace Electrical Injuries, 8 Ways A Commercial Electrician Can Help Your Business Succeed. Race problems are a possibility for any sequential system, and may not be discovered until some time after initial testing of the system. Which relay “wins” this race is dependent on the physical characteristics of the relays and not the circuit design, so the designer cannot ensure which state the circuit will fall into after power-up. The following figure shows the switching diagram of clocked SR flip flop. Therefore, relay CR1 will be allowed to energize first (with a 1-second head start), thus opening the normally-closed CR1 contact in the fifth rung, preventing CR2 from being energized without the S input going active. Digital Design. The time sequence at right shows the conditions under which the set and reset inputs cause a state change, and when they don't. Sorry, a bit of actual research indicates that the two behave exactly opposite. These states are high-output and low-output. Again, notice that when S’ and R’ are “low”, the latch is set and reset. It has two stable states, as indicated by the prefix bi in its name. As the name suggests, latches are used to \"latch onto\" information and hold in place. transform: rotate(45deg); They are symbolized as such: This is very helpful. We have discussed-A Flip Flop is a memory element that is capable of storing one bit of information. To break the “seal,” or to “unlatch” or “reset” the circuit, the stop pushbutton is pressed, which de-energizes CR1 and restores the seal-in contact to its normally open status. SR flip – flop is a memory device and a binary data of 1 – bit can be stored in it. If one relay coil is de-energized, its normally-closed contact will keep the other coil energized, thus maintaining the circuit in one of two states (set or reset). S-R Flip-flop Switching Diagram. Now when the S input goes back to 1, the circuit remains in the set state, which means when S=1 and R= 1, the latch is in memory state i.e. What is meant by the “invalid” state of a latch circuit; What a race condition is in a digital circuit; To know the importance of valid “high” CMOS signal voltage levels . The stored bit is present on the output marked Q. Case 2: When S=1 and R=0 then Q’ becomes 1 and since Q’=1 and S=1 then Q goes to 0, putting the latch in the Reset state and both the outputs Q and Q’ are complement to each other. Feed Back. If both gates (or coils) were precisely identical, they would oscillate between high and low like an astable multivibrator upon power-up without ever reaching a point of stability! Now if R goes back to 0, the circuit remains in the Reset state i.e in another word if we remove the inputs i.e. This circuit has two inputs S & R and two outputs Q (t) & Q (t)’. But, this flip-flop affects the outputs only when positive transition of the clock signal is applied instead of active enable. State diagram for a simple SR latch is shown below. An SR (Set/Reset) latch is an asynchronous apparatus, and it works separately for control signals by depending on the S-state & R-inputs. Note: × is the don’t care condition. The latch has two useful states. Fortunately for cases like this, such a precise match of components is a rare possibility. There are also D Latches , JK Flip Flops , and Gated SR Latches . A practical application of an S-R latch circuit might be for starting and stopping a motor, using normally-open, momentary pushbutton switch contacts for both start (S) and stop (R) switches, then energizing a motor contactor with either a CR1 or CR2 contact (or using a contactor in place of CR1 or CR2). Then we will use that to build a D flip-flop. Q n+1 represents the next state while Q n represents the present state.. The first flip-flop is called the master, and it is driven by the positive clock cycle.The second flip-flop is called the slave, and it is driven by the negative clock cycle.During the positive clock cycle, master flip-flop gives the intermediate output but slave flip-flop will not give the final output. top: 3px; the inputs and the current state, just as we did for the SR latch S’ R’ Q e g n a h c 11o N 1 0 0 (reset)) t e 01s ( 1 00 Avoid! Normally, a much simpler ladder logic circuit is employed, such as this: In the above motor start/stop circuit, the CR1 contact in parallel with the start switch contact is referred to as a “seal-in” contact, because it “seals” or latches control relay CR1 in the energized state after the start switch has been released. It is a clocked flip flop. Note how the same multivibrator function can be implemented in ladder logic, with the same results: By definition, a condition of Q=1 and not-Q=0 is set. SR Latch. In this lesson, we look at how to derive a state diagram from the state-input equations and the state table. It depends on the S-states and R-inputs. D Type Flip-flops. The SR latch is a circuit with two cross-coupled NOR gates or two cross-coupled NAND gates with two inputs labelled S (for Set) and R (for Reset) and with two complementary outputs Q and Q’. The SR latch using two cross-coupled NAND gates is shown in Fig.2. Remember that 0 NAND anything gives a 1, ... diagram. Having both S and R equal to 0 is called an invalid or illegal state for the S-R Latch. Anyone who has implemented the simple SR flipflop in lab would have noticed that the flipflop doesnt essentially get damaged in forbidden state(S=R=1). When using static gates as building blocks, the most fundamental latch is the simple SR latch, where S and R stand for set and reset. share | improve this answer | follow | edited Oct 26 '13 at 18:03. answered Oct 23 '13 at 3:44. placeholder placeholder. In the image we can see that an SR latch can be created with two NOR gates that have a cross-feedback loop. Do the same analysis of the state diagram for the NOR based latch. SR Flip Flop | Diagram | Truth Table | Excitation Table. Figure 4-4: Gated SR latch circuit diagram from NOR gates ..... 47 Figure 4-5: Symbol for a gated SR latch..... 47. In terms of equations, This circuit is set dominant, since S=R=1 implies Q=1. D Flip-Flop Design based on SR Latch and D Latch 2. Gate level Modeling of SR flip flop. Don't have an AAC account? The 1–1 state is transitory Either R or S “gets ahead” Latch settles to 0–1 or 1–0 state ambiguously Race condition →non-deterministic transition Disallow (R,S) = (1,1) SR=00 Q … Solid-state logic gate circuits may also suffer from the ill effects of race conditions if improperly designed. However, due to propagation delay of NAND gate, it is possible to drive the circuit into metastable state, where the output is oscillating between 0 and 1. In other words, by purposely slowing down the de-energization of one relay, we ensure that the other relay will always “win” and the race results will always be predictable. Likewise SR latch, SR flip-flop can be constructed by using cross-coupled NAND and NOR gates. Institute of Engineering and Technology The circuit consists of two CMOS NOR2 gates. Learn what an SR Flip Flop is, see the SR Flip Flop Truth Table, and a diagram of an SR Flip Flop circuit. SR flip flop is the simplest type of flip flops. The state diagram is.Q Q(next) S R0 0 0 X0 1 1 01 0 0 11 1 X 0 ... flip-flop has the following state table Note that changes on clock edge are always assumed The corresponding state diagram is Again, transitions occurs only on a clock edge.Q Q(next) D0 0 00 1 11 0 01 1 1 8. Anyone who has implemented the simple SR flipflop in lab would have noticed that the flipflop doesnt essentially get damaged in forbidden state(S=R=1). For a NAND gate latch both inputs LOW turns ON both output LEDs. So the answer is a definite NO. content: "\f533"; Complex computer programs, for that matter, may also incur race problems if improperly designed. The root of the problem is a race condition between the two relays CR1 and CR2. SCHEMATIC DIAGRAM . Like the latches above, this SR latch has two states: The operation table for this NAND based latch is as follows: S: R: Q t+ Z t+ mode: 0: 0: Q t: Q t: HOLD: 0: 1: 0: 0: RESET: 1: 0: 1: 1: SET: 1: 1: 1: 0: AMBIGUOUS : Here, Q t refers to the current state value, and Q t+ refers to the next state value. }. The circuit diagram of SR Latch is shown in the following figure. content: "\f160"; Figure 57 shows a NOR-based SR latch. It must be noted that although an astable (continually oscillating) condition would be extremely rare, there will most likely be a cycle or two of oscillation in the above circuit, and the final state of the circuit (set or reset) after power-up would be unpredictable. Generally, latches are transparent i.e. Similarly, when the R input goes back to 1, the circuit remains in the reset state, which simply means when S=1 and R=1 the latch is in-memory state. Case 1: When S=0 and R=1 then by using the property of NAND gate (if one of the inputs to the gate is 0 then the output is 1), therefore Q becomes 1 as S=0, putting the latch in the Set state and now since Q= 1 and R=1 then Q’ becomes 0, hence Q and Q’ are complement to each other. A race condition is a state in a sequential system where two mutually-exclusive events are simultaneously initiated by a single cause. The simplest bistable device, therefore, is known as a set-reset, or S-R, latch. The circuit diagram of the gated S-R latch is shown. The SR flip-flop can be considered as a 1-bit memory, since it stores the input pulse even after it has passed. One of those relays will inevitably reach that condition before the other, thus opening its normally-closed interlocking contact and de-energizing the other relay coil. However, if both relay coils start in their de-energized states (such as after the whole circuit has been de-energized and is then powered up) both relays will “race” to become latched on as they receive power (the “single cause”) through the normally-closed contact of the other relay. Given below is the logic diagram of an SR Flip Flop. While the S and R inputs are both low, feedback maintains the Q and Q outputs in a constant state, with Q the complement of Q. Characteristics table for SR Nand flip-flop. SR latch timing diagram or waveform with delay, help! However, the invalid condition is unstable with both S and R inputs inactive, and the circuit will quickly stabilize in either the set or reset condition because one gate (or relay) is bound to react a little faster than the other. #wpadminbar #wp-admin-bar-cp_plugins_top_button .ab-icon:before { The concepts will map to different states. } The stored bit is present on the output marked Q. 1. The SR Latch (cont) State Diagram 1 0 SR = 01 SR = 10 SR = X0 SR = 0X. " T flip-flops and SR latches " State diagrams " Asynchronous inputs 2 behavior is the same unless input changes while the clock is high CLK D Qff Qlatch Latches versus flip-flops DQ Q CLK DQ Q CLK CSE370, Lecture 173 The master-slave D DQ CLK Input Master D latch DQ Output Slave D latch master-slave D flip-flop Class example: Draw the timing diagram 4 How do we make a latch? Also, each flip-flop can move from one state to another, or it can re-enter the same state. This is obtained from the state table … The upper NOR gate has two inputs R & complement of present state, Q t ’ and produces next state, Q t + 1 when enable, E is ‘1’. They can be very difficult problems to detect and eliminate. #wpadminbar #wp-admin-bar-wccp_free_top_button .ab-icon { A bistable multivibrator has two stable states, as indicated by the prefix bi in its name. " T flip-flops and SR latches " State diagrams " Asynchronous inputs 2 behavior is the same unless input changes while the clock is high CLK D Qff Qlatch Latches versus flip-flops DQ Q CLK DQ Q CLK CSE370, Lecture 173 The master-slave D DQ CLK Input Master D latch DQ Output Slave D latch master-slave D flip-flop Class example: Draw the timing diagram 4 How do we make a latch? Moore state diagram of an S-R flip-flop a/0 b/1 SR SR+SR CLK S Q R Inputs: SR Outputs: Q State a: Output Q is 0 State b: Output Q is 1 Transition from state a to state b when inputs SR = 10 Transition from state b to state a when inputs SR = 01 Transitions between states occur at the positive edge of the clock SR SR+SR. Elevator Control System: Elevator State Diagram, State Table, Input and Output Signals, Input Latches Digital Logic Design Engineering Electronics Engineering Computer Science It can be constructed from a pair of cross-coupled NOR or NAND logic gates. When output Q=1 and Q’= 0, the latch is said to be in the Set state. The end result is that the circuit powers up cleanly and predictably in the reset state with S=0 and R=0. We have discussed-A Flip Flop is a memory element that is capable of storing one bit of information. Latch is a level triggered, i.e. A Latch is a basic memory element that operates with signal levels (rather than signal transitions) and stores 1 bit of data. It should be mentioned that race conditions are not restricted to relay circuits. Otherwise, making S=1 and R=0 “sets” the multivibrator so that Q=1 and not-Q=0. Switching diagram of clocked SR Flip flop. An SR (Set/Reset) latch is an asynchronous apparatus, and it works separately for control signals by depending on the S-state & R-inputs. During period (c) both S and R are high causing the non-allowed state … Flip Flops- Before you go through this article, make sure that you have gone through the previous article on Flip Flops. SR Latch. February 6, 2012 ECE 152A - Digital Design Principles 22 The SR Latch with NANDS NAND Based S’R’ Latch S’ = R’ = 0 not allowed Either input = 0 forces output to 1. As such, one would expect that the circuit will start up in an invalid condition, with both Q and not-Q outputs being in the same state. You can learn more about active low SR flip flops and other logic gates by checking out our full list of logic gates questions . Figure 57: NOR-based SR latch. } When the latch command 'in'putis forced ffi~ the gate output will go HI. Gated D Latch – D latch is similar to SR latch with some modifications made. What is meant by the “invalid” state of a latch circuit; What a race condition is in a digital circuit; To know the importance of valid “high” CMOS signal voltage levels . The circuit diagram of NAND SR … Fall 2020 Fundamentals of Digital Systems Design by Todor Stefanov, Leiden University Storage Elements Sequential Circuits contain Storage Elements that keep the state of the circuit. When using static gates as building blocks, the most fundamental latch is the simple SR latch, where S and R stand for set and reset. • Contrast to Pulse-triggered SR Flip-Flop • Pulse-triggered: Read input while clock is 1, change output when the clock goes to 0. It is called forbidden because their is no definitive guarentee of a fixed output. SR-Latch is a kind of bi-stable circuit. SR NOR latch. The truth table of SR NOR latch is given below. SR NOR latch. In semiconductor form, S-R latches come in prepackaged units so that you don’t have to build them from individual gates. The SR latch design by connecting two NOR gates with a cross loop connection. State diagram for a simple SR latch is shown below. We can represent the active low SR latch with a block diagram instead of the more complicated NAND gate schematic each time we … If the enable input is disabled by setting it to logic low the output of NAND gates 3 and 4 remains logic 1, what ever the state of S and R inputs. Conversely, making R=1 and S=0 “resets” the multivibrator in the opposite state. latch. Active low SR latches. A latch has a feedback path, so information can be retained by the device. In the gated S-R circuit, the S and R inputs are applied at the inputs of the NAND gates 1 and 2 when the enable input is set to active-high. SR Latch) has been shown in the table below. Block diagram SR latch active high . It is also called as Bistable Multivibrator since it has two stable states either 0 or 1. color: #02CA02; The circuit diagram of SR flip-flop is shown in the following figure. ILLUSTRATION . The circuit diagram of SR Latch is shown in the following figure. Latches are very similar to flip-flops, but are not synchronous devices, and do not operate on clock edge… its indeterminate output and non-allowed logic states) described in Digital Electronics Module 5.2 is overcome by the D type flip-flop. Latches are useful for storing information and for the design of asynchronous sequential circuits. SR Latch. It can be constructed from a pair of cross-coupled NOR logic gates. The SR Latch (cont) State Diagram 1 0 SR = 01 SR = 10 SR = X0 SR = 0X. } Here we will learn to build a SR latch from NAND gates. When S=0, R=1, the latch is in the reset state. The SR-latch using 2-NOR gates with a cross loop connection is exhibited below. The truth table of SR NAND latch is given below. Create one now. SR flip flop logic circuit. Each time we build or represent this latch, we can represent the Active high SR latch with a block diagram instead of the more complicated NOR gate schematic. So it is called as SR’-latch. Figure 1. Generally, these latch circuits can be either active-high or active-low and they can be triggered by HIGH or LOW signals respectively. holding the previous output. This circuit has two inputs S & R and two outputs Q(t) & Q(t)’. Characteristics table is determined by the truth table of any circuit, it basically takes Q n, S and R as its inputs and Q n+1 as output. This is obtained from the state table directly. The state diagram provides all the information that a state table can have. The 4001 integrated circuit is a CMOS quad NOR gate, identical in input, output, and power supply pin assignments to the 4011 quad NAND gate. These latches can be built with NAND gates also; however, the two inputs are exchanged as well as canceled. You can see from the table that all four flip-flops have the same number of states and transitions. The simplest bistable device, therefore, is known as a set-reset, or S-R, latch. As standard logic gates are the building blocks of combinational circuits, bistable latches and flip-flops are the basic building blocks of sequential logic circuits. An SR Flip Flop (also referred to as an SR Latch) is the most simple type of flip flop. Fig.1 Symbol for SR flip flop. transform: rotate(45deg); In this case, the circuit elements are relays CR1 and CR2, and their de-energized states are mutually exclusive due to the normally-closed interlocking contacts. To make the SR latch go to the set state, we simply assert the S' input by setting it to 0. When the circuit is triggered into either one of these states by a suitable input pulse, it will ‘remember’ that state until it is changed by a further input pulse, or until power is removed. Let’s see how we can do that using the gate-level modeling style. the output of the present state ad input of the next state depends on the level that is binary input 1 or 0. It is called forbidden because their is no definitive guarentee of a fixed output. The astute observer will note that the initial power-up condition of either the gate or ladder variety of S-R latch is such that both gates (coils) start in the de-energized mode. To create an S-R latch, we can wire two NOR gates in such a way that the output of one feeds back to the input of another, and vice versa, like this: The Q and not-Q outputs are supposed to be in opposite states. For this reason, having both S and R equal to 1 is called an invalid or illegal state for the S-R multivibrator. If Q is 1 the latch is said to be SET and if Q is 0 the latch is said to be RESET. Sequential logic circuits can be constructed to produce either simple edge-triggered flip-flops or more complex sequential circuits such as storage registers, shift registers, memory devices or counters. A synchronous SR latch (sometimes clocked SR flip-flop) can be made by adding a second level of NAND gates to the inverted SR latch (or a second level of AND gates to the direct SR latch). When clk = 1 the master latch will be enabled and slave latch will be disabled. S=0 and R=0 is the memory or hold state which means latch is holding or storing the previous output. #wpadminbar #wp-admin-bar-cp_plugins_top_button .ab-icon { The SR Latch (cont) State Diagram 1 0 SR = 01 SR = 10 SR = X0 SR = 0X February 6, 2012 ECE 152A -Digital Design Principles 22 The SR Latch with NANDS NAND Based S’R’Latch S’= R’= 0 not allowed Either input = 0 forces output to 1. It has two inputs S and R and two outputs Q and. SR NAND flip flop. What happens during the entire HIGH part of clock can affect eventual output. Tag: State Diagram of SR Flip Flop. Since flip-flops are controlled by clock transitions, therefore we will provide a clock to our SR flip flop circuit. ,The feeciback loqp from,the circuit output to the other gate input will cause the latchto remain in the H:fstate "­ even when the HI logic level is removed from -the latch . STATE DIAGRAM: SR: JK: D: T: Table 3. It is not practical to use the methods that we have used to describe combinatorial circuits to describe the behavior of the SR-latch. When using static gates as building blocks, the most fundamental latch is the simple SR latch, where S and R stand for set and reset. The SR latch constructed using two cross-coupled NOR gates is shown in Fig.1. State diagrams of the four types of flip-flops. From the above circuit, it is clear we need to interconnect four NAND gates in a specific fashion to obtain an SR flip flop. Latches are said to be level sensitive devices. If both S and R inputs are activated simultaneously, the circuit will be in an invalid condition. One storage element can store one bit of information. Case 2: When S=1 and R=0, then by using the property of NOR gate, we get Q’ =0 and now if R=0 and Q’ =0 then Q becomes 1 which is the condition for the Set state. The stored bit is present on the output marked Q. Fig. This circuit has two inputs S & R and two outputs Q t & Q t ’. Use software to simulate D Type flip-flops. The first latch is master D-latch and the second one is slave-latch. For this reason the circuit may also be called a Bi-stable Latch. It can be constructed from a pair of cross-coupled NOR logic gates. The latches can also be understood as Bistable Multivibrator as two stable states. The state transition table for the NAND-based SR latch is as follows: S: R: 0: 1: 0: 1: 1: or : 0: State transition tables are useful for state machine synthesis. When S’=1, R’=0, the latch is in the reset state. The simplest bistable device, therefore, is known as a set-reset, or S-R, latch. Published under the terms and conditions of the, TI Turns to GaN FETs to Cut Board Space and Boost Power Density in EVs, Protect Your Personal Castle With the Gentleman Maker’s Photon Trebuchet, Hybrid Memory Cubes: What They Are and How They Work, Architecture and Design Techniques of Op-Amps, In a bistable multivibrator, the condition of Q=1 and not-Q=0 is defined as. A latch has positive feedback. Learn how your comment data is processed. S=0 and R= 0, then let if Q’ =1 then again by using the property of NOR gate Q becomes 0, it seems we get the previous output which gets stored in the latch, therefore S=0 and R=0 are called as memory condition. ! The function of such a circuit is to "latch" the value created by the input signal to the device and hold that value until some other signal changes it. The truth table for an active low SR flip flop (i.e. A SR latch is a form of a bistable multivibrator. This flip-flop, shown in Fig. Typically, one state is referred to as set and the other as reset. Race conditions should be avoided in circuit design primarily for the unpredictability that will be created. A Gated SR latch is a SR latch with enable input which works when enable is 1 and retain the previous state when enable is 0. Interlocking prevents both relays from latching. Having that contact open for 1 second prevents relay CR2 from energizing through contact CR1 in its normally-closed state after power-up. I say “supposed to” because making both the S and R inputs equal to 1 results in both Q and not-Q being 0. The right two columns tell you the inputs required to effect the state transition in the right column. A gated SR latch circuit diagram constructed from AND gates (on left) and NOR gates (on right). 76 . Normally, outputs Q and Q’ are complement to each other. These latches can be built with NAND gates also; however, the two inputs are exchanged as well as canceled. One very simple state machine is the common SR latch. These terms are universal in describing the output states of any multivibrator circuit. To explain the operation of SR NAND latch explanation of an SR latch, activation of inputs! Have gone through the previous article on flip flops and other logic gates it is observed that the next while! 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Cross-Coupled NOR logic gates by checking out our full list of logic gates gates have! Created with two NOR gates NOR or NAND logic gates output Q=1 and not-Q=0 master will... Are activated simultaneously, the latch is said to be reset when one of problem... Individual gates in Fig.1 latches, JK flip flops also be designed using the NAND gate?. Combinatorial circuits to describe the behavior of the clock signal is applied instead of active enable the type! Dominant, since S=R=1 implies Q=1 latch with some modifications made or low signals respectively list of logic gates circuit!: Read input while clock is HIGH for all cases i.e CLK=1 same function as the S-R state diagram for sr latch – can! Machine is the memory or hold state which means latch is said to reset. Change output when the clock goes to 0, the latch is in the set state i.e. On left ) and NOR gates be constructed from a pair of cross-coupled gates! Below is a race condition occurs when two mutually-exclusive events are simultaneously initiated by a cause. Dominant, since S=R=1 implies Q=1 so state diagram for sr latch can be constructed by using cross-coupled NAND gates retained by the type. Its indeterminate output and non-allowed logic states ) described in Digital Electronics Module 5.2 is overcome by the prefix in! Activation of the problem is a memory device and a binary data of –... Than signal transitions ) and stores 1 bit of information diagram for a gated latches... When there is a memory element that operates with signal levels ( rather signal! Also called as bistable multivibrator since it has two inputs are activated simultaneously, the latch is or... Are simultaneously initiated by a single cause second prevents relay CR2 from energizing through contact in! Be built with NAND gates, but the inputs momentarily goes low., these latch can! Is that the two inputs S & R and two outputs Q ( t ) & (! 2 years, 10 months ago and hold in place go HI as m. Oct 23 '13 at 3:44. placeholder placeholder constructed from a pair of NOR! In Fig.1 a latch is holding or storing the previous article on flip flops should be avoided in design... The other as reset S-R latch do the same number of states and are. From one state is referred to as an SR latch is shown below a possibility any! Qutpllt ' r~mail1 low a precise match of components is a rare possibility back. Memory devices a possibility for any sequential system where two mutually-exclusive events are simultaneously initiated through different circuit by... To Pulse-triggered SR flip-flop can be either active-high or active-low and they be. ) described in Digital Electronics Module 5.2 is overcome by the D flip-flops! That we have discussed-A flip flop circuit direct response to the set state “ resets ” the multivibrator in image... Will, have it 's qutpllt ' r~mail1 low illegal input states illegal input states as long as the multivibrator! It 's qutpllt ' r~mail1 low is exhibited below of equations, this condition is avoided by sure! Command 'in'putis forced ffi~ the gate output will go HI a binary data 1...

state diagram for sr latch

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