Sequential logic Circuits

Sequential logic Circuits

Combinatorial logic circuits alone are not sufficient for the manipulation of information as is done in modern digital systems. The fixed nature of combinatorial circuits — as reflected by the strict correspondence between inputs and outputs — significantly limits the scope of their applications. This is where Sequential circuits take on all their importance. The latter allow the development of systems, the operation of which depends not only on the inputs received, but also on the information previously processed in the course of their operation. It is then understood that a form of memory is implemented, a memory allowing the circuit to remember past events and to process information more adequately. As a first step, we will try to define the tools that allow us to build such circuits. Subsequently, we consider the design techniques that make it possible to draw all the computing power from it.

A sequential logic circuit is a logic circuit having inputs and outputs and having a behavior where the outputs depend not only on the inputs, but also on the sequences of the inputs passed. To do this, the circuit uses a memory part that will allow it to regain the state induced by the inputs passed. The output is therefore calculated based on the present state and the inputs that arrive to the system. This concept of State will be extensively explored in the rest of this chapter ; so we will first focus on the memory type components that we will have to use.

I-THE FLIP-FLOPS

A toggle is a memory circuit that for a combination of logical States of its

inputs presents on its output two stable complementary States.

A toggle is an elementary memory that can only store one bit.

1.1 Rs rocker :

The Rs rocker presents :

* S: setting input to 1 (SET) of Q ;

* R: input setting to 0 (reset° of Q ;

* Q and Q : complementary output

Operating Table :

1.1. RSH rocker :

It is a synchronous rocker with static clock input.

In the Rs toggle, the output changes State, at the time of propagation, at the time when the
combination of the states of the inputs is changed, its mode of operation is asynchronous.
In a synchronous rocker RSH the output state change that corresponds to a
a new combination of input states can only be performed on the active front, rising or falling, of a clock signal.

¤ RSH rocker includes :

- Three entries :

* S: setting to 1 ;

* R: setting to 0 ;

* H: clock input, active on the up or down front of the signal.

- Two outputs: Q and Q whose states are complementary.

Figure 2 - symbols of an RSH toggle

Operating Table of an RSH rocker

1.2. JK rocker :

¤ The rocker JK presents :

The JK rocker has two cascading rocker arms. One is considered to be a master mistress and the other as a slave.

While the slave toggle remains locked, the master toggle changes State on a clock face.

* two inputs J and K ;

* a clock input H ;

* two outputs Q and Q whose states are complementary

a) JK rocker with descending front b) JK rocker with ascending front

1.3. Rocker JK master-slave :

It consists of two JK switches, one master, the other slave, switching to

different levels of the clock signal (Figure 4-a).

The master toggle receives the input information and the active front of the clock signal.

The slave toggle replicates the master toggle on the opposite front of the clock

1.4. Toggle D :

The D toggle is a synchronous toggle with a single data input.

¤ A toggle D Presents :

* an input D (Data)

* a clock input H

* two outputs Q and Q whose states are complementary

1.5. Toggle T :

¤ The rocker T Presents :

* a clock input H ;

* two output Q and Q whose states are complementary

When input H goes to dynamic state 1, the outputs change state.

When input H goes to state 0, the outputs remain in their state.

II-MONO STABLE-A STABLE:

2.1. Mono-stable :

It is a sequential micro-structure, which in output has two complementary States one

on the other.

* One of the two states being stable.

• The other can only be temporarily occupied [pseudo-stable state].

A pulse at the input (i.e. state change from 0 to 1) brings the output to state 1; the

output remains in this state for a period defined by the particular characteristics of

the operator, regardless of how long the input remains in state 1 and then returns to state 0.

2.2. A stable or oscillator :

Sequential Micro-structure which, at output, has two pseudo-stable states

(complementary to each other) ; the transition from one state to another taking place periodically with a

any form factor.

In some uses, a stable swings can take the name multi vibrators

III-COUNTER-COUNTERS:

The counter allows to count within the limit of the scales that constitute it (capacity of the

counter) the pulses applied in input.

The meter can be :

* binary if the gear ratio is equal to 2 or an input power of 2.

* Decimal if the gear ratio is 10 or an input power of 10.

* Modulo n in other cases.