VHDL Reference Tables

Appendix B VHDL Reference Tables This appendix focuses on tables of information that are useful when writ- ing VHDL descriptions. Most of the information in the tables is available in the text of the book, however, these tables consolidate the information into one area for easy reference. Table B-1 lists all of the different kinds of statements alphabetically and includes an example usage. Table B-1 Statement or Clause Example(s) Access Type TYPE access_type IS ACCESS type_to_be_accessed; Aggregate record_type := (first, second, third); Alias ALIAS opcode : BIT_VECTOR (0 TO 3) IS INSTRUCTION(10 TO 13); Architecture ARCHITECTURE architecture_name OF entity name IS -- declare some signals here BEGIN -- put some concurrent statements here END architecture_name; Array Type TYPE array_type IS ARRAY (0 TO 7) OF BIT; Assert ASSERT x > 10 REPORT “x is too small” SEVERITY ERROR; Attribute Declaration ATTRIBUTE attribute_name : attribute_type; Attribute Specification ATTRIBUTE attribute_name OF entity_name : entity_class IS value; Block Statement block_name : BLOCK -- declare some stuff here BEGIN -- put some concurrent statements here END BLOCK block_name; Case Statement CASE some_expression IS WHEN some_value => -- do_some_stuff WHEN some_other_value => -- do_some_other_stuff WHEN OTHERS => -- do_some_default_stuff END CASE; Appendix B: VHDL Reference Tables 436 Table B-1 Continued. Statement or Clause Example(s) Component Declaration COMPONENT component_name PORT (port1_name : port1_type; port2_name : port2_type; port3_name : port3_type); END COMPONENT; Component Instantiation instance_name : component_name PORT MAP (first_port, second_port, third_port); instance_name : component_name PORT MAP (formal1 => actual1, formal2 => actual2); Conditional Signal Assignment target <= first_value WHEN (x = y) ELSE second_value WHEN a >= b ELSE third_value; Configuration Declaration CONFIGURATION configuration_name OF entity_name IS FOR architecture_name FOR instance_name : entity_name USE ENTITY library_name.entity_name (architecture_name); END FOR; FOR instance_name : entity_name USE CONFIGURATION library_name.configuration_name; END FOR; END FOR; END configuration_name; Constant Declaration CONSTANT constant_name : constant_type := value; Entity Declaration ENTITY entity_name IS PORT (port1 : port1_type; port2 : port2_type); END entity_name; Exit Statement EXIT; EXIT WHEN a <= b; EXIT loop_label WHEN x = z; File Type Declaration TYPE file_type_name IS FILE OF data_type; File Object Declaration FILE file_object_name : file_type_name IS IN “/absolute/path/name”; For Loop FOR loop_variable IN start TO end LOOP -- do_some_stuff END LOOP; 437 Appendix B: VHDL Reference Tables Table B-1 Statement or Clause Example(s) Function Declaration FUNCTION function_name (parameter1 : parameter1_type; parameter2 : parameter2_type) RETURN return_type; Function Body FUNCTION function_name (parameter1 : parameter1_type; parameter2 : parameter2_type) RETURN return_type IS BEGIN -- do some stuff END function_name; Generate Statement generate_label : FOR gen_var IN start TO end GENERATE label : component_name PORT MAP ( .); END GENERATE; Generic Declaration GENERIC (generic1_name : generic1_type; generic2_name : generic2_type); Generic Map GENERIC MAP (generic1_name => value1, value2); Guarded Signal Assignment g1 : BLOCK (clk = ‘1’ AND clk’EVENT) BEGIN q <= GUARDED d AFTER 5 NS; END BLOCK; IF Statement IF x <= y THEN -- some statements END IF; IF z > w THEN -- some statements ELSIF q < r THEN -- some more statements END IF; IF a = b THEN -- some statements ELSIF c = d THEN -- some more statements ELSE -- even more statements END IF; Incomplete Type TYPE type_name; Appendix B: VHDL Reference Tables 438 Table B-1 Continued. Statement or Clause Example(s) Library Declaration LIBRARY library_name; Loop Statement FOR loop_variable IN start TO end LOOP -- do lots of stuff END LOOP; WHILE x < y LOOP -- modify x and y and do other stuff END LOOP; Next Statement IF i < 0 THEN NEXT; END IF; Others Clause WHEN OTHERS => -- do some stuff Package Declaration PACKAGE package_name IS -- declare some stuff END PACKAGE; Package Body PACKAGE BODY package_name IS -- put subprogram bodies here END package_name; Physical Type TYPE physical_type_name IS RANGE start TO end UNITS unit1 ; unit2 = 10 unit1; unit3 = 10 unit2; END UNITS; Port Clause PORT ( port1_name : port1_type; port2_name : port2_type); Port Map Clause PORT MAP (port1_name => signal1, signal2); Procedure Declaration PROCEDURE procedure_name (parm1 : in parm1_type; parm2 : out parm2_type; parm3 : inout parm3_type); Procedure Body PROCEDURE procedure_name (parm1 : in parm1_type; parm2 : out parm1_type; parm3 : inout parm3_type) IS BEGIN -- do some stuff END procedure_name; 439 Appendix B: VHDL Reference Tables Table B-1 Statement or Clause Example(s) Process Statement PROCESS (signal1, signal2, signal3) -- declare some stuff BEGIN -- do some stuff END PROCESS; Record Type TYPE record_type IS RECORD field1 : field1_type; field2 : field2_type; END RECORD; Report Clause ASSERT x = 10 REPORT “some string”; Return Statement RETURN; RETURN (x + 10); Selected Signal Assignment WITH z SELECT x <= 1 AFTER 5 NS WHEN 0, 2 AFTER 5 NS WHEN 1, 3 AFTER 5 NS WHEN OTHERS; Severity Clause ASSERT x > 5 REPORT “some string” SEVERITY ERROR; Signal Assignment a <= b AFTER 20 NS; Signal Declaration SIGNAL x : xtype; Subtype Declaration SUBTYPE bit8 IS INTEGER RANGE 0 TO 255; Transport Signal Assignment x <= TRANSPORT y AFTER 50 NS; Type Declaration TYPE color is (red, yellow, blue, green, orange); TYPE small_int is 0 to 65535; Use Clause USE WORK.my_package.all; Variable Declaration VARIABLE variable_name : variable_type;Wait Statement WAIT ON a, b, c; WAIT UNTIL clock’EVENT AND clock = ‘1’; WAIT FOR 100 NS; WAIT ON a, b UNTIL b > 10 FOR 50 NS; While Loop WHILE x > 15 LOOP -- do some stuff END LOOP; Table B-2 lists all of the predefined attributes that retrieve infor- mation about VHDL type data. The descriptions are necessarily terse to fit into the table cells; see Chapter 6, “Predefined Attributes” for more detailed information. Appendix B: VHDL Reference Tables 440 Table B-2 Attribute Explanation Examples T’BASE Returns the base type of NATURAL’BASE returns datatype it is attached to INTEGER T’LEFT Returns left value specified in INTEGER’LEFT is -2147483647 type declaration BIT’LEFT is ‘0’ T’RIGHT Returns right value specified INTEGER’RIGHT is 2147483647 in type declaration BIT’RIGHT is ‘1’ T’HIGH Returns largest value specified TYPE bit8 is 255 downto 0 in declaration bit8’HIGH is 255 T’LOW Returns smallest value TYPE bit8 is 255 downto 0 specified in declaration bit8’LOW is 0 T’POS(X) Returns position number of TYPE color IS (red, green, argument in type (first blue, orange); position is 0) color’POS(green) is 1 T’VAL(X) Returns value in type at TYPE color IS (red, green, specified position number blue, orange); color’VAL(2) is blue T’SUCC(X) Returns the successor to the TYPE color IS (red, value passed in green, blue, orange); color’SUCC(green) is blue T’PRED(X) Returns the predecessor to TYPE color IS (red, green, the value passed in blue, orange); color’PRED(blue) is green T’LEFTOF(X) Returns the value to the left TYPE color IS (red, green, of the value passed in blue, orange); color’LEFTOF(green) is red T’RIGHTOF(X) Returns the value to the right TYPE color IS (red, green, of the value passed in blue, orange); color’RIGHTOF(blue) is orange Table B-3 lists all predefined attributes that return information about array datatypes. The N parameter for all attributes specifies to which par- ticular range the attribute is being applied. This only makes sense for multidimensional arrays. For single-dimensional arrays, the parameter can be ignored. For more detailed information, see Chapter 6, “Predefined Attributes.” 441 Appendix B: VHDL Reference Tables All of the next examples apply to the following declaration: TYPE a_type IS ARRAY(0 TO 3, 7 DOWNTO 0) OF BIT; Table B-4 lists all predefined attributes that return information about signals or create new signals. For more detailed information, see Chapter 6, “Predefined Attributes.” Table B-5 lists all of the operators and their relative precedence. Table B-6 lists all of the different types of literals and a sample usage. In all cases, the _ character is ignored when interpreting the value of a literal. The base that the exponent in the based integer and based real examples is applied to is the base specified for interpreting the number. Bit string literals are used to specify values for types that resemble the BIT_VECTOR type. Table B-3 Attribute Explanation Example A’LEFT(N) Returns left array bound a_type’LEFT(1) is 0 of selected index range a_type’LEFT(2) is 7 A’RIGHT(N) Returns right array bound a_type’RIGHT(1) is 3 of selected index range a_type’RIGHT(2) is 0 A’HIGH(N) Returns largest array a_type’HIGH(1) is 3 bound value of selected a_type’HIGH(2) is 7 index range A’LOW(N) Returns smallest array a_type’LOW(1) is 0 bound value of selected a_type’LOW(2) is 0 index range A’RANGE(N) Returns selected index a_type’RANGE(1) is 0 range TO 3 a_type’RANGE(2) is 7 DOWNTO 0 A’REVERSE_RANGE(N) Returns selected index a_type’REVERSE_RANGE(1) range reversed is 3 DOWNTO 0 a_type’REVERSE_RANGE(2) is 0 TO 7 A’LENGTH(N) Returns size of selected a_type’LENGTH(1) is 4 index range a_type’LENGTH(2) is 8 Appendix B: VHDL Reference Tables 442 Table B-4 Attribute Explanation Example S’DELAYED(T) Creates a new signal delayed by T clock’DELAYED(10 ns) S’QUIET(T) Creates a new signal that is true reset’QUIET(5 ns) when signal S has had no transactions for time T; otherwise, false S’STABLE(T) Creates a new signal that is true clock’STABLE(1 ns) when signal S has had no events for time T; otherwise, false S’TRANSACTION Creates a signal of type BIT that load’TRANSACTION toggles for every transaction on signal S S’EVENT Returns true when an event has clock’EVENT occurred for signal S this delta S’ACTIVE Returns true when a transaction load’ACTIVE has occurred for signal S this delta S’LAST_EVENT Returns the elapsed time since the data’LAST_EVENT last event on signal S S’LAST_ACTIVE Returns the elapsed time since the clock’LAST_ACTIVE last transaction on signal S S’LAST_VALUE Returns the previously assigned data’LAST_VALUE value of signal S Table B-5 Precedence Operator Class Operator Highest Miscellaneous **, ABS, NOT Multiplying *, /, MOD, REM Sign +, - Adding +, -, & Relational =, /=, <, <=, >, >= Lowest Logical AND, OR, NAND, NOR, XOR 443 Appendix B: VHDL Reference Tables Table B-6 Literal Type Example Decimal Integer 52 0 3E3 -- equals 3000 1_000_000 -- equals 1 million Decimal Real 52.0 0.0 .178 1.222_333 Decimal Real with Exponent 1.2E+10 4.6E-9 Based Integer 16#FF# -- equals 255 8#777# -- equals 511 2#1101_0101# -- equals 213 16#FF#E1 -- equals 4080 Based Real 2#11.11# 16#AB.CD#E+2 8#77.66#E-10 Character 'a' '*' ' ' -- the space character String “this is a string” " " -- empty string “ABC” & “CDE” -- concatenation Bit String X”FFEF” O”770770” B”1111_0000_1111” This page intentionally left blank. . VHDL Reference Tables This appendix focuses on tables of information that are useful when writ- ing VHDL descriptions. Most of the information in the tables. do_some_other_stuff WHEN OTHERS => -- do_some_default_stuff END CASE; Appendix B: VHDL Reference Tables 436 Table B-1 Continued. Statement or Clause Example(s) Component