P5.2 Sequential blocks

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En provenance du cours de Universitat Autònoma de Barcelona

Digital Systems: From Logic Gates to Processors

196 notes

Universitat Autònoma de Barcelona

Digital Systems: From Logic Gates to Processors

196 notes

This course gives you a complete insight into the modern design of digital systems fundamentals from an eminently practical point of view. Unlike other more "classic" digital circuits courses, our interest focuses more on the system than on the electronics that support it. This approach will allow us to lay the foundation for the design of complex digital systems. You will learn a set of design methodologies and will use a set of (educational-oriented) computer-aided-design tools (CAD) that will allow you not only to design small and medium size circuits, but also to access to higher level courses covering so exciting topics as application specific integrated circuits (ASICs) design or computer architecture, to give just two examples. Course topics are complemented with the design of a simple processor, introduced as a transversal example of a complex digital system. This example will let you understand and feel comfortable with some fundamental computer architecture terms as the instruction set, microprograms and microinstructions. After completing this course you will be able to: * Design medium complexity digital systems. * Understand the description of digital systems using high-level languages such as VHDL. * Understand how computers operate at their most basic level (machine language).

À partir de la leçon

Sequential circuits (I)

<b><font size=4 color=#B22222><b>Click on "v More" to read the purpose of this module</b></font> </b><br/><br/>This is the first module dedicated to Sequential Circuits (Digital Systems with Memory).<br/> <b>To solve the quizzes you will need VerilUOC_Desktop</b>. Remember that the first week includes a complete description of VerilUOC_Desktop. In particular, VerilChart is presented in the second video.

P5.1 Combinational blocks12:35

P5.2 Sequential blocks12:20

Rencontrer les enseignants

Elena Valderrama
Professor
Departamento de Microelectrónica y Sistemas Electrónicos. UAB.
Jean-Pierre Deschamps
Professor
Lluis Terés
Professor
Centro Nacional de Microelectrónica del CSIC
Merce Rullan
ProfesoraTitular
Departamento de Microelectrónica y Sistemas Electrónicos. UAB.
Joaquín Saiz Alcaine
Ingeniero Informático
Departamento de Microelectrónica y Sistemas Electrónicos. UAB.
David Bañeres
Profesor Agregado
Estudios de Informática, Multimedia y Telecomunicación. UOC.
Juan Antonio Martínez
Collaborator
APSI - UAB

[MUSIC]

This lesson and the two following deal with the implementation of the

sequential blocks, namely the "output selection" block, the "register bank"

and the "go to" block. The first sequential block that we will implement

is the "output selection". This is its functional specification.

We have already commented before that OUT0,

OUT1, and so on, are registered outputs.

That means that if the value of an output port, say OUTi,

is not updated by a DATA_OUTPUT instruction or by an OUTPUT_VALUE

instruction, then OUTi keeps the same value as before.

So, this is a sequential circuit.

The behavior of this block depends on the instruction under execution.

So, we define two control signals: "out_en" for

output enable and "out_sel" for output selection.

out_en is equal to 1 if

the executed instruction is DATA_OUTPUT or

OUTPUT_VALUE, and is equal to 0 in the other cases.

And "out_sel" is equal to 1 if

the executed instruction is a DATA_OUTPUT,

and to 0 if the executed instruction is OUTPUT_VALUE.

Its value is left undefined in the other cases.

Then, with the definition of the control signals,

the functional specification is modified.

The inputs of the "output selection" block are two control signals,

out_en and out_sel;

i, an index included within the instruction.

A, a number included in the instruction, and

"reg", a value that comes from the register bank.

And the outputs are the eight output ports.

Thus the modified model or specification is this one:

when out_en and out_sel are equal to 11,

then the output number i is connected to the "reg" input.

When 10, the output number i is connected

to A, and in the other cases no operation is performed.

That means that the value of the output ports is unchanged.

Here is a straightforward implementation using

an address decoder that selects the output port that will be updated;

a set of AND gates that make active or

non active the address decoder outputs.

Then eight registers controlled by the signals

EN0, EN1, and so on.

And a 2-data-input multiplexer that

selects the data to be stored within the selected

output register: can be either A or

a value that comes from the register bank.

This circuit can be described in VHDL.

After making visible the IEEE package and

also our user-defined package, the entity is declared.

It gives the list of inputs and outputs.

Namely, inputs A and reg

that are vectors;

control signals out_enable and out_sel

that are bits, and index i,

a 3-bit vector that selects

one of the eight output ports;

and there are eight outputs,

the eight output ports that are vectors.

There is also a clock signal, not represented in this circuit,

but necessary to synchronize the working of the output registers.

Within the architecture, signal EN,

that is the set of enable signals,

signal DEC_OUT, that is the set of decoder outputs, and

signal to_ports, the output

of the multiplexer, are declared.

Then four processes describe the circuit.

The first process describes the address decoder:

if index i is equal to 000 (represents number 0)

then component 0 of DEC_OUT is equal to 1;

if index i represent number 1,

then component number 1 is equal to 1, and so on.

If i represent number 6, then component number 6 is equal to 1,

and in the last case, component number 7 is equal to 1.

The second process describes the set of AND gates.

It's a loop. For every index i.

the value of ENABLE(i), component number i of

vector ENABLE, is equal to the AND function

(the Boolean AND function) of component number i of

DEC_OUT and of input control signal out_en.

The third process describes this multiplexer.

If control signal out_sel is equal to 0, then to_ports

is connected to input A, and if it's equal to 1,

then to_ports is connected to input "reg".

The fourth process, describes the output registers.

In this process, the only one sensitive to

a non-represented clock signal clk, the condition

"if clk'event and clk = '1'" is used.

It is the way a positive edge of clk is represented in VHDL

and, by the way, this condition confirms that this is a sequential circuit.

Then, on a positive edge of clk,

when component 0 of EN is equal to 1

(this component), then output 0

is updated with the value of to_ports.

If EN(1), component 1 of EN, is equal to 1,

the OUT1 is updated with the to_ports value, and so on.

In all other cases, in particular when out_en = 0,

and thus all components of vector EN are equal to 0,

there is no operation, that means that

the value of the output ports remains unchanged.

Summary.

The sequential output selection block has

been implemented and the VHDL model has been generated.

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