Advanced FPGA Based System Design Lecture-9 & 10 VHDL Sequential Code By: Dr Imtiaz Hussain 1

Contents Introduction Process Signals & Variables IF 2

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. 3

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. 4

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). 5

Process Its syntax is shown below. 6

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. 9

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. 10

Signals and Variables The assignment operator for a SIGNAL is ‘‘<=’’ (ex: sig <= 5). And for a VARIABLE it is ‘‘:=’’ (ex: var := 5). 11

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. 12

IF (Example) 13

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. 14

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Example: One Digit Counter with Asynchronous Reset I/P 16 clk digit(3:0) rst COUNTERCOUNTER

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. 17

Example: Shift Register 18

Example: Shift Register 19

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. 21

WAIT UNTIL Example 22

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. 23

WAIT ON In the example below, the PROCESS will continue execution whenever a change in rst or clk occurs. 24

WAIT FOR Finally, WAIT FOR is intended for simulation only (waveform generation for test benches). Example: WAIT FOR 5ns;. 25

Example: DFF DFF with Asynchronous Reset using WAIT ON instead of IF. 26

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Example: One Digit Counter Use WAIT UNTIL to design a one digit counter 28 clk digit(3:0) rst COUNTERCOUNTER

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END OF LECTURE-9-10 To download this lecture visit imtiazhussainkalwar.weebly.com 30