Advanced FPGA Based System Design Lecture-9 & 10 VHDL Sequential Code

By: Dr Imtiaz Hussain [email protected]

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• • • • Introduction Process Signals & Variables IF

Contents

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Introduction

• VHDL Code is Inherently Concurrent.

• PROCESSES, FUNCTIONS, and PROCEDURES are the only sections of code that are executed sequentially.

• One important aspect of sequential code is that it is not limited to sequential logic.

• With it we can build sequential circuits as well as combinational circuits.

• Sequential code is also called behavioral code.

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Introduction

• The statements discussed in this lecture are all sequential, that is, allowed only inside ROCESSES, FUNCTIONS, or PROCEDURES.

• They are: IF, WAIT, CASE, and LOOP.

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Process

The statements discussed in this lecture are all sequential, that is, allowed only inside ROCESSES, FUNCTIONS, or PROCEDURES.

• They are: IF, WAIT, CASE, and LOOP.

• A PROCESS is a sequential section of VHDL code.

• It is characterized by the presence of IF, WAIT, CASE, or LOOP, and by a sensitivity list (except when WAIT is used).

• A PROCESS must be installed in the main code, and is executed every time a signal in the sensitivity list changes (or the condition related to WAIT is fulfilled).

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Process

• Its syntax is shown below.

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Process Example

• DFF with Asynchronous Reset 7

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Signals and Variables

• VHDL has two ways of passing non-static values around: by means of a SIGNAL or by means of a VARIABLE.

• A SIGNAL can be declared in a PACKAGE, ENTITY or ARCHITECTURE (in its declarative part).

• While a VARIABLE can only be declared inside a piece of sequential code (in a PROCESS, for example).

• Therefore the value of the SIGNAL can be global, the VARIABLE is always local.

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Signals and Variables

• The value of a VARIABLE can never be passed out of the PROCESS directly.

• If necessary, then it must be assigned to a SIGNAL.

• On the other hand, the update of a VARIABLE is immediate, that is, we can promptly count on its new value in the next line of code.

• That is not the case with a SIGNAL (when used in a PROCESS), for its new value is generally only guaranteed to be available after the conclusion of the present run of the PROCESS.

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Signals and Variables

• The assignment operator for a SIGNAL is ‘‘<=’’ (ex: sig <= 5).

• And for a VARIABLE it is ‘‘:=’’ (ex: var := 5).

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IF

As mentioned earlier, IF, WAIT, CASE, and LOOP are the statements intended for sequential code.

• Therefore, they can only be used inside a PROCESS, FUNCTION, or PROCEDURE.

• The natural tendency is for people to use IF more than any other statement.

• The syntax of IF is shown below.

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IF (Example)

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Example: One Digit Counter

• 1-digit decimal counter (0 to 9 to 0).

• • It contains a single-bit input (clk) and a 4-bit output (digit).

A variable, temp, was employed to create the four flip-flops necessary to store the 4-bit output signal.

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Example:

One Digit Counter with Asynchronous Reset I/P clk C O U N T E R rst digit(3:0) 16

Example: Shift Register

• • • Figure shows a 4-bit shift register.

The output bit (q) must be four positive clock edges behind the input bit (d).

It also contains an asynchronous reset, which must force all flip-flop outputs to ‘0’ when asserted.

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Example: Shift Register

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Example: Shift Register

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WAIT

• Wait Statement has three different formats. 20

WAIT UNTIL

• The WAIT UNTIL statement accepts only one signal, thus being more appropriate for synchronous code than asynchronous.

• Since the PROCESS has no sensitivity list in this case, WAIT UNTIL must be the first statement in the PROCESS.

• The PROCESS will be executed every time the condition is met.

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WAIT UNTIL Example

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WAIT ON

• WAIT ON, on the other hand, accepts multiple signals.

• The PROCESS is put on hold until any of the signals listed changes.

• In the example below, the PROCESS will continue execution whenever a change in rst or clk occurs.

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WAIT ON

• In the example below, the PROCESS will continue execution whenever a change in rst or clk occurs.

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WAIT FOR

• Finally, WAIT FOR is intended for simulation only (waveform generation for test benches).

• Example: WAIT FOR 5ns;.

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Example: DFF

• DFF with Asynchronous Reset using WAIT ON instead of IF.

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Example: One Digit Counter

• Use WAIT UNTIL to design a one digit counter clk C O U N T E R rst digit(3:0) 28

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END OF LECTURE-9-10

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