Introduction to sequential logic design
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Introduction to Sequential Logic Design. Latches. Terminology. A bistable memory device is the generic term for the elements we are studying. Latches and flip-flops (FFs) are the basic building blocks of sequential circuits.

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Terminology
Terminology

  • A bistable memory device is the generic term for the elements we are studying.

  • Latches and flip-flops (FFs) are the basic building blocks of sequential circuits.

    • latch: bistable memory device with level sensitive triggering (no clock), watches all of its inputs continuously and changes its outputs at any time, independent of a clocking signal.

    • flip-flop: bistable memory device with edge-triggering (with clock), samples its inputs, and changes its output only at times determined by a clocking signal.


S r latch
S-R latch

S sets the Q output to 1, R resets the Q output to 0.

If both R, S are negated, the latch remains in the state that it was forced to (like a bistable element).

QN is normally the complement of Q (but sometimes NOT).



S r latch operation1

Metastability is possibleif S and R are negatedsimultaneously.

S-R latch operation


S r latch timing parameters
S-R latch timing parameters

  • Propagation delay

  • Minimum pulse width



S r latch using nand gates s bar r bar latch
S-R latch using NAND gates(S-bar-R-bar latch)





D latch
D latch

D

Q

C

Q


D latch operation
D-latch operation

When C is asserted, Q follows the D input, the latch is “open” and the path (D-->Q) is “transparent”.

When C is negated, the latch “closes” and Q retains its last value.


D latch timing parameters
D-latch timing parameters

  • Propagation delay (from C or D)

  • Setup time (D before C edge)

  • Hold time (D after C edge)


S r vs d latches
S-R vs. D latches

  • S-R

    • Useful in control applications, “set” and “reset”

    • S=R=1 problem

    • Metastability problem when S, R are negated simultaneously, or a pulse applied to S, R is too short.

  • D

    • Store bits of information

    • No S=R=1 problem

    • Metastability still possible.


Next…

  • Flip-flops

  • Read Ch-7.2


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