它是用一種“同時性”的訊號指定敘述(concurrent signal assignment statement) 來 描述 一個VHDL 檔案中只能有一個entity 以及一個architecture。 例如：.

VHDL - Flaxer Eli Data-Flow Modeling Ch 6 - 1 Chapter 6 Data-Flow Modeling VHDL VHDL - Flaxer Eli Data-Flow Modeling Ch 6 - 2 Outline zConcurrent Signal Assignment zConditional Signal Assignment zSelected Signal Assignment zUnaffected value zBlock Statement zConcurrent Assertion Statement VHDL - Flaxer Eli Data-Flow Modeling Ch 6 - 3 Concurrent Statement zThe Data-Flow modeling is a

tion using only concurrent signal assignments. S'VHDL statement is mapped into a separate control place . Any statement placed in architecture body is concurrent. Only one type of conditional statements is allowed as concurrent which are shown here. 2.1 Conditional  (when-else).

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VHDL Statement Types Concurrent Statements: assignment (concurrent - behavioral): example: w <= a and b or c;-----when-else Conditional Signal Assignment: In my experience, many people use the when else to make a concurrent mux. In fact, this is so common it should become one of your most basic tools in coding VHDL. Keep in mind that most synthesis tools will look at the output signal and the input signals and see what clock domain they are on, such that timing constraints will apply. The value of the expression preceding a WHEN keyword is assigned to the target signal for the first Boolean expression that is evaluated as true. If none of the Boolean expressions are true, the expression following the last ELSE keyword is assigned to the target. The following Conditional Signal Assignment Statement has multiple alternatives. VHDL CONSTRUCTS C. E. Stroud, ECE Dept., Auburn Univ.

2018-02-21

library IEEE; use IEEE.STD_LOGIC_1164.ALL; entity mux_4to1_top is Port ( SEL : in STD_LOGIC_VECTOR (1 downto 0); -- … Essential VHDL for ASICs 1 Conditional Concurrent Signal Assignment The conditional concurrent signal assignment statement is modeled after the “if statement” in software programming languages. The general format for this statement is: target_signal <= value1 WHEN condition1 ELSE value2 WHEN condition2 ELSE value3 WHEN condition3 ELSE.. valueN; 5 Concurrent Constructions Concurrent VHDL statements are \executed" continuously, and corresponds to combinational logic.

port ( A, B, C : in std_logic; -- two dashes is a COMMENT in VHDL Y: out std_logic); end majority;-- this is the architecture declaration, uses only one concurrent statement. ARCHITECTURE concurrent of majority is begin Y <= (A and B) or (A and C) or (B and C); end concurrent; This is a style of one big expression

Official name for this VHDL when/else assignment is the conditional signal assignment b <= "1000" when a = "00" else "0100" when a = "01" else "0010" when a = "10" else "0001" when a = "11"; 7 Concurrent Statements A VHDL architecture contains a set of concurrent statements. Each concurrent statement defines one of the intercon-nected blocks or processes that describe the overall behav-ior or structure of a design. Concurrent statements in a design execute continuously, unlike sequential statements (see Firstly, a single line vhdl statement actually infers a process with all the signals on the rhs in the sensitivity list. Si you actually have 3 processes in parallel. --- Quote End --- Yes, this is what happens when we use concurrent statements. --- Quote Start --- Secondly, signals are only updated when a process suspends.

For the comparator I was In VHDL-93, any signal assigment statement may have an optinal label. VHDL -93 defines an unaffected keyword, which indicates a condition when a signal is not given a new assignment: label: signal = expression_1 when condition_1 else expression_2 when condition_2 else unaffected ; 6. SEQUENTIAL AND CONCURRENT STATEMENTS IN THE VHDL LANGUAGE A VHDL description has two domains: a sequential domain and a concurrent domain. The sequential domain is represented by a process or subprogram that contains sequential statements.
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Concurrent conditional statement. The concurrent conditional statement can be used in the architecture concurrent In my experience, many people use the when else to make a concurrent mux.

For the comparator I was In VHDL-93, any signal assigment statement may have an optinal label. VHDL -93 defines an unaffected keyword, which indicates a condition when a signal is not given a new assignment: label: signal = expression_1 when condition_1 else expression_2 when condition_2 else unaffected ; 6. SEQUENTIAL AND CONCURRENT STATEMENTS IN THE VHDL LANGUAGE A VHDL description has two domains: a sequential domain and a concurrent domain. The sequential domain is represented by a process or subprogram that contains sequential statements.
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2006-11-10

ELSE • Similar to an IF statement – example multiplexer: ARCHITECTURE example OF mux IS BEGIN q <= i0 WHEN a = ‘0’ AND b = ‘0’ ELSE It’s a more elegant alternative to an If-Then-Elsif-Else statement with multiple Elsif’s. Other programming languages have similar constructs, using keywords such as a switch, case, or select. Among other things, Case-When statements are commonly used for implementing multiplexers in VHDL. Continue reading, or watch the video to find out how!