Intro to sequential circuits
 

  • In the next following slides, I want to show you that:

      • A digital circuit can have a state

        (I.e.: the outputs of such digital circuit is not completely determined by its inputs_

  • What exactly is a input of a (digital) circuit:

      • Input of a digital circuit = a signal that can be controlled (completely) by external forces (= your actions)

The SR-latch circuit
 

Consider the following digital circuit:

 
 
  • This digital circuit is called the "SR-latch" (more details later)

  • The input signals are called S (=set) and R (=reset) (i.e.: we can change S or R at will !)

  • The output signals are named Q and Q (=NOT(Q)) (we cannot change Q or Q at will !)

Feedback loop: outputs used as inputs
 

Notice that the SR-latch has a feedback loop:

 
 

The output signals (can we cannot control) are used as input signals into the SR-latch circuit

Feedback loop: outputs used as inputs
 

In other words:   the input values for the Nor gates are:

 
 

The output signals (that we cannot control) are used as input signals into the SR-latch circuit

Studying the behavior of the SR-latch
 

We will now study the behavior of the SR-latch:

 
 

Note: the behavior of sequential circuits is dependent on the current state of the circuit !!

Therefore: to observe a certain behavior, we must "go through" more than 1 state of the circuit !

Studying the behavior of the SR-latch
 

What is the outputs of the SR-latch for S=1 and R=0 ?

 
 

 

Studying the behavior of the SR-latch
 

What is the outputs of the SR-latch for S=1 and R=0 ?   Answer:

 
 

Notice that the state is stable (the feedback loop does not alter the input values)

Note: the inputs S=1 and R=0 represents the "set" function (S = set) that makes the output Q = 1

Studying the behavior of the SR-latch
 

While the SR-latch is in this state, we change the input values to S=0 and R=0:

 
 

What are the new output values of the SR-latch circuit ?

Studying the behavior of the SR-latch
 

While the SR-latch is in this state, we change the input values to S=0 and R=0:   -   Answer:

 
 

The output (state) remains unchanged !

Note: S=0 and R=0 represents the "remember" function (outputs are unchanged)

Studying the behavior of the SR-latch
 

What is the outputs of the SR-latch for S=0 and R=1 ?

 
 

 

Studying the behavior of the SR-latch
 

What is the outputs of the SR-latch for S=0 and R=1 ?   Answer:

 
 

Notice that the state is also stable (the feedback loop does not alter the input values)

Note: S=0 and R=1 represents the "reset" function that makes the Q output = 0

Studying the behavior of the SR-latch
 

While the SR-latch is in this state, we change the input values to S=0 and R=0:

 
 

What are the new output values of the SR-latch circuit ?

Studying the behavior of the SR-latch
 

While the SR-latch is in this state, we change the input values to S=0 and R=0: -   Answer:

 
 

The output (state) remains unchanged !

Note: S=0 and R=0 represents the "remember" function (outputs are unchanged)

Why is the SR-latch NOT a combinatorial circuit ?
 

In combinatorial circuits: the output values is only dependent on the (current) input values

The outputs (Q and Q) are different for the same input values S=0, R=0 !!

Therefore, the SR-latch cannot be a combinatorial circuit !!!

Final comments
 

  • We do not use the S=1 and R=1 with the SR-latch

    (because they don't give any meaningful functions)

  • Some digital circuits with feedback loops can be unstable for certain input values

    The unstable behavior is when a change in the output values causes a change in some input values that then cause a new change in output values that causes a change in some input values that... and so on..

    (But the SR-latch is stable)

    (We won't delve too deeply into this phenomenon in CS355)