1. Digital Logic Structures: Chapter 3 COMP 2610 Dr. James Money COMP 2610 1

2. Sequential Logic Circuits So far we’ve discussed logic structures that depend solely on the input at the moment These are called combination logic circuits There is no capacity to store information from prior values

3. Sequential Logic Circuits We also discussed logic circuits that would store information These included Gated D latches, registers, and memory We want to now to discuss logic structures that both process and store information

4. Sequential Logic Circuits These types of circuits base their decision on the current inputs values and what has happened before These are called sequential logic circuits These are used to implement finite state machines

5. Sequential Logic Circuits State Machine Combinational Logic Circuit Storage Elements InputsOutputs

6. Combination Lock A simple example of these types of structures is a combination lock Suppose we wish to lock a bike, but do not want to require a physical key A typical approach is to use a combination of numbers

7. Combination Lock Two types of “combination” locks 4184 30 15 5 1020 25 Combinational Success depends only on the values, not the order in which they are set. Sequential Success depends on the sequence of values (e.g, R-13, L-22, R-3).

8. Concept of State For a sequential lock to work properly, we need to keep track of sequence of rotations We have to track the correct sequence versus all the other incorrect ones R13-L22-R3 opens, but R13-L29-R3 does not

9. Concept of State There are several situations we identify: – (A) The lock is not open and NO operations have been performed – (B) The lock is not open, but the user has completed the R13 operation – (C) The lock is not open, but the user has completed R13 followed by L22 – (D) The lock is open

10. Concept of State We refer to A-D as the state of the lock This notion of state is important in computer engineering The state of the system is a snapshot of that system at a given point in time

11. Concept of State In our case, there are four states : A-D Is there a fifth state? No!

12. State Diagram