Lecture 119: Digital Electronics - SR Latch

Overview of Sequential Circuits

• Difference between combinational and sequential circuits is the presence of a memory block in sequential circuits.
• Memory element stores past outputs, aiming to store single bit numbers (0 or 1).

Key Storage Element: Latch

• Basic storage element is called a latch.
• Focus of this lecture is on the SR latch.
• Types of SR Latch:
  • NOR SR Latch
  • NAND SR Latch

Functionality of SR Latch

• Inputs: S (Set) and R (Reset).
  • S = 1 sets output Q to 1.
  • R = 1 resets output Q to 0.

Important Characteristics of SR Latch

• SR latch operates similarly to a door latch:
  • In a door latch, once closed, the door cannot let anything in or out.
  • In an SR latch, changing inputs can change stored data.
• Importance of using a clock to control data storage (to be explained in future presentations).

Truth Table for NOR Gate

• For two inputs A and B, the output Y for a NOR gate:
  • 0 0 = 1
  • 0 1 = 0
  • 1 0 = 0
  • 1 1 = 0

Case Analysis of SR Latch

Case 1: S = 0, R = 1

• Q = 0, Q' = 1 (Q complement).
  • Reset state: outputs are complementary.

Memory State

• When S = 0 and R = 0 (inputs removed), the outputs remain the same (stored state).
• Proven that outputs store previous states.

Case 2: S = 1, R = 0

• Q = 1, Q' = 0.
  • Set state: outputs are complementary.
• When inputs are removed, the outputs (1, 0) remain stored.

Case 3: S = 1, R = 1 (Not Used State)

• Q = Q' = 0.
  • Contradictory situation.
  • Leads to undefined behavior (not allowed).
• If inputs are removed (S = 0, R = 0), conflicting outputs arise.

Summary Table of SR Latch States

NAND SR Latch

• Shift from NOR to NAND gates.
• Key changes: S and R are now set and reset in the NAND configuration.
• Truth table: Similar steps followed as NOR gate.
  • 0 0 for S and R results in memory state.
  • If S = 0, R = 1 then Q = 1, Q' = 0, etc.

Conclusion

• Understanding SR latch is critical for grasping future topics like flip-flops, counters, and registers.
• Questions are encouraged for clarification.
• Importance of understanding the material for success in the course.