What distinguishes sequential logic from combinational logic?
Sequential logic has feedback, meaning outputs depend on current inputs and previous inputs (the system has state).
What causes a sequential system to have state?
The output of a gate is fed back as input to an earlier gate, so past inputs influence current outputs.
What are the inputs and outputs of an SR latch?
Inputs: S (Set), R (Reset). Outputs: Q and Q′ (complements).
What logic gates are used to build a basic SR latch?
Two NOR gates connected with feedback.
What does the SR latch store?
One bit of memory — the value of Q.
What happens when S = 0 and R = 0 in an SR latch?
Lower NOR inputs: (Q, S) = (previous Q, 0) → Q′ becomes the complement of Q ( Q’ = 1)
Upper NOR inputs: (R, Q′) = (0, complement of Q) → Q remains the same
Therefore the latch holds its previous state.
What happens when S = 1 and R = 0 in an SR latch?
Lower NOR inputs: (Q, S) = (anything, 1) → Q′ = 0
Upper NOR inputs: (R, Q′) = (0, 0) → Q = 1
Therefore the latch sets: Q becomes 1.
What happens when S = 0 and R = 1 in an SR latch?
Upper NOR inputs: (R, Q′) = (1, anything) → Q = 0
Lower NOR inputs: (Q, S) = (0, 0) → Q′ = 1
Therefore the latch resets: Q becomes 0.
Why is the input combination S = 1 and R = 1 not allowed in an SR latch?
Both Q and Q′ become 0, violating the rule that they should be a complement of each other.
What is a characteristic table in the context of latches?
A truth table showing the next state (Qₙ₊₁) given inputs and current state (Qₙ).
Why are SR latches considered asynchronous?
They change state immediately when inputs change; no clock controls transitions.
Why can asynchronous SR latches be problematic in larger circuits?
Output may depend on the order/timing of input changes, causing nondeterministic behaviour.
How does an inverted SR latch differ from a NOR-based SR latch?
It uses NAND gates and is active-low: S = 0 sets, R = 0 resets.
What input combination is forbidden in an inverted SR latch?
S = 0 and R = 0 simultaneously.
What are the two main problems with simple SR latches?
They are asynchronous
Non-deterministic transitions associated with certain inputs
How do we solve asynchronous transition issues in latches?
Add a clock to create a gated latch or flip-flop.
How do we avoid nondeterministic transitions in SR latches?
Add gates to control inputs or use a different flip-flop design.
What is the purpose of adding a clock to a latch?
To synchronise state transitions across sequential circuits.
What are the three flip-flop types introduced in the lecture?
SR, D, and J-K flip-flops.
How does a clocked SR flip-flop differ from a simple SR latch?
It only responds to S and R when the clock signal is active.
What is another name for the clocked SR flip-flop?
Gated SR latch.
Why was the D flip-flop introduced?
To eliminate the invalid S = 1, R = 1 condition in SR flip-flops.
How does the D flip-flop ensure S and R are always different?
It uses an inverter so the two inputs to the internal gates are always opposite.
What does the D in D flip-flop stand for?
Data or Delay.
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History of Computer Science32
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Von Neumann and Harvard Architecture45
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Parallelism, Memory and Amdahl's Law36
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Data Representation45
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Data Storage23
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Representing Real Data19
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Real Numbers Instructions54
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Instruction Execution - week 719
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Operand Modes - Week 713
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Control Instructions - Week 738
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Pipelining and Superscalar Architectures - Week 817
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Input/Output - Week 834
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Handling Input/Output - Week 944
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Boolean Logic and Circuits - Week 916
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Boolean Logic and Circuits - Week 1011
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Application of Karnaugh Maps - Week 102
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Implementing Arithmetic - Week 1028
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Sequential Logic - Week 1128
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Flip Flops - Week 1127
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Intro to Operating Systems34
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OS lecture 8 - Design36
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OS Lecture 9 - Threads and Processes (1)48
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OS lecture 10 - Threads and Processes (2)54
