VIEWS: 7 PAGES: 96 POSTED ON: 9/20/2012 Public Domain
Topics of the Lecture • Packages • USE clause • Aliases • Data alias • Non-data alias • Resolved signals Packages Packages • Method for Grouping Related Declarations Which Serve a Common Purpose – Set of subprograms to operate on particular data type – Set of declarations for particular model – Separate interface from implementation – Reusable Packages – Unclutter rest of model – Allows declaration of “global” signals, e.g., clocks. • Not a generally good since behavior can change through means other than signals declared in entity interface Packages • Design Unit Similar to Entity Declarations and Architecture Bodies – Can be put in library and made accessible to other units – Access to items declared in the package is through using its Selected Name library_name.package_name.item_name – Aliases can be used to allow shorter names for accessing declared items Packages • Two Components to Packages –Package declaration –Package body Package Declaration • Subprograms Using Header, Implementation Is Hidden – “information hiding” • Constants, Do Not Need to Be Initialized in Declaration – “information hiding” Package Declaration • Types, Must Be Completely Specified – Can have variable size arrays • Signals Must Be Completely Specified Package Declaration Syntax package identifier is { package_declarative_item } end [ package ] [ identifier ] ; Package Declaration Example* package dp32_types is constant unit_delay : Time := 1 ns ; type bool_to_bit_table is array ( boolean ) of bit ; . . . *Ashenden VHDL cookbook Package Declaration Example* function bits_to_int ( bits : in bit_vector ) return integer ; function bits_to_natural ( bits : in bit_vector ) return natural ; procedure int_to_bits ( int : in integer ; bits : out bit_vector ) ; end dp32_types ; *Ashenden VHDL cookbook Package Body • Not Necessary If Package Declaration Does Not Declare Subprograms • May Contain Additional Declarations Which Are Local to the Package Body – Cannot declare signals in body Package Body • Declared Subprograms Must Include the Full Declaration As Used in Package Declaration – Numeric literals can be written differently if same value – Simple name may be replaced by a selected name provided it refers to same item Package Body Syntax package body identifier is { package_body_declarative_item } end [ package body ] [ identifier ] ; Package Body Example* package body dp32_types is constant bool_to_bit : bool_to_bit_table := ( false => '0' , true => '1' ) ; function resolve_bit_32 ( driver : in bit_32_array ) return bit_32 is constant float_value : bit_32 := X"0000_0000" ; variable result : bit_32 := float_value ; *Ashenden VHDL cookbook Package Body Example* begin for i in driver'range loop result := result or driver ( i ) ; end loop ; return result ; end resolve_bit_32 ; *Ashenden VHDL cookbook Library Clause • Makes Items in a Library Available to a VHDL Model • To Access Items in a Library Need to Use Their selected_name library identifier { , . . . } ; Use Clause • Tedious to Always Use an Item’s Selected Name • All Items Declared in a Package or Library Can Be Made “Visible” Through a Use Clause Use Clause • Can Be Used in Any Declarative Section • Keyword “All” Imports All Identifiers Use Clause Syntax use selected_name { , . . . } selected_name <= name . ( identifier | character_literal | operator_symbol | all ) Use Clause Example* use work.dp32_types.all ; entity dp32 is generic ( Tpd : Time := unit_delay ) ; port ( d_bus : inout bus_bit_32 bus ; a_bus : out bit_32 ; read, write, fetch : out bit ; ready, phi1, phi2, reset : in bit ) ; end dp32 ; *Ashenden VHDL cookbook Aliases A35ases Aliases • Alternative Identifier for an Item • Improves Readability • Allows One to Differentiate Among Identically Named Items in Different Packages • Can Refer to a Single Element or Part of a Composite Data Type, e.g., alias interrupt_level is PSW(30 downto 26); Aliases • Operations on Aliases Operate on Actual Items Except for the Following Attributes – x’simple_name – x’path_name – x’instance_name Aliases • Cannot Declare Aliases for –Labels –Loop parameters –Generate parameters (replicates items) Data Alias Syntax alias identifier [ : subtype_indication ] is name ; Data Alias • Subtype_indication Allows for the Type to Be Changed – If scalar original • Direction cannot change • Bounds cannot change • Unconstrained type allowed Data Alias • Subtype_indication Allows for the Type to Be Changed – If array or array slice • Direction can differ • Bounds can differ • Base type must remain unchanged Non-Data Alias Syntax alias ( identifier | character_literal | operator_symbol ) is name [ signature ] ; Non-Data Alias • Alias for Enumeration Type Does Not Require Definition of Aliases for Enumeration Literals of Original • Alias for Physical Type Retains Units Without Redefinition Non-Data Alias Syntax • Optional Signature – Only for subprograms and enumeration literals – Overloading of identifiers may require means of differentiating among alternatives • return type does this – Outer [ ] are required Non-Data Alias Syntax signature <= [ [ type_mark { , . . . } ] [ return type_mark ] ] – e.g., alias high is std.standard.’1’ [ return bit ] Resolved Signals • VHDL Requires a Function to Specify the Values Which Result From Tying Multiple Outputs Together • Resolved Signal Includes Resolution Function – Inclusion of function indicates it is a resolved signal Resolved Signals • Resolution Function Must Be Written for an Indeterminate Number of Signals Since It Is Not Known When Declared How Many Signals Will Be Connected to It. • The Value of a Signal at a Transaction Is Determined by the Resolution Function Operating on the Multiply Connected Signals. Resolved Signal Syntax subtype_indication <= [ resolution_function_name ] type_mark [ range ( range_attribute_name | simple_expression ( to | downto ) simple_expression) | ( discrete_range { , . . . } ) ] ; Resolved Signal Example* package MVL4 is type MVL4_ulogic is ( ‘X’, ‘0’, ‘1’, ‘Z’ ); type MVL4_ulogic_vector is array ( natural range <> ) of MVL4_ulogic ; function resolve_MVL4 ( contribution : MVL4_ulogic_vector ) return MVL4_ulogic ; *Ashenden Resolved Signal Example* subtype MVL4_logic is resolve_MVL4 MVL4_ulogic ; end package MVL4 ; *Ashenden Resolved Signal Example* package body MVL4 is type table is array ( MVL4_ulogic , MVL4_ulogic ) of MVL4_ulogic ; Resolved Signal Example* constant resolution_table : table := -- ‘X’ ‘0’ ‘1’ ‘Z’ -- ------------------------- ( ( ‘X’ ‘X’ ‘X’ ‘X’ ), -- ‘X’ ( ‘X’ ‘0’ ‘X’ ‘0’ ), -- ‘0’ ( ‘X’ ‘X’ ‘1’ ‘1’ ), -- ‘1’ ( ‘X’ ‘0’ ‘1’ ‘Z’ ) ) ;-- ‘Z’ Resolved Signal Example* function resolve_MVL4 ( contribution : MVL4_ulogic_vector ) return MVL4_ulogic is variable result : MVL4_ulogic := ‘Z’; Resolved Signal Example* begin for index in contribution’range loop result := resolution_table ( result, contribution ( index ) ) ; end loop ; return result ; end function resolve_MVL4 ; end package body MVL4 ; • Advanced Modeling in VHDL • Standard Logic System • Package Body Example • Use of IEEE 1164 Package Advanced Modelling Techniques in VHDL • VHDL defines only basic features to model digital devices, such as, simple BIT type, two types of a delay, etc. • more complex data types for signals must be defined for realistic simulation. Advanced Modelling Techniques in VHDL • more complex data types for signals must be defined for realistic simulation. • Recall our old Example of Nand_2: Why we need more complex logic definition than BIT? – we want to get more accurate simulation results, – we use different technology and we want to simulate the same design in different technologies, – real world is more complex than '0' and '1', – more complex delays than standard VHDL models • inputs, output delays, • rising edge, falling edge delays, • min, max and typical delays A Case Example • Std_logic_1164 defined by IEEE as a portable constructs • Requirements: – support for more realistic signal modeling, – technology independence, allowing extensions for the future and providing support for all current technologies, – provide consistent features which facilitate logic level modeling, – hide the complexity of the package from the designer as much as possible, make models readable, – provide for the timing accuracy continuum, giving the modeler flexibility to choose the appropriate level of model accuracy. • Intended use of the packages: -- access standard logic facilities use ieee.Std_logic_1164.all; -- VHDL entity declaration -- Standard Logic System • modeling signals with '0' and '1' simplifies the real circuit, because it only consider signal voltage without signals current, – many simulators introduce, so called, signal strength related to the signal's current, – in std_logic_1164 we will introduce the following strengths of signals: • unknown, • forced, • weak, • high impedance, • don’t care. Standard Logic System Standard Logic System Rising and Falling Edges Package Body Package Body (cont’d) Package Body (cont’d) Package Body (cont’d) Use of IEEE 1164 Package • An Example: VHDL Standard Operators • Boolean Operators for bit and bit array types – not, and, nand, or, nor, xor, xnor most types, but not “expected” • Comparison Operators behavior for non numeric types – =, /=, >=, >, <=, < • Shifting for bit array types – sll, srl, sla, sra, rol, ror • Arithmetic for numeric types – +, -, + (unary), - (unary), *, /, mod, rem, **, abs for array types • Concatenation – & Operator Precedence • The following is the order of Operator Precedence in Extended Backus operator precedence Naur Form (EBNF) 1. Miscellaneous: **, abs, not expression ::= relation {and relation} | relation {or relation} | relation {xor relation} | relation {xnor relation} 2. Multiplying: *, /, mod, rem | relation [nand relation] | relation [nor relation] 3. Sign: +, - relation ::= shift_expression [(= | /= | < | <= | > | >=) shift_expression ] 4. Adding: +, -, & shift_expression ::= [(sll | srl | sla | sra | rol | ror) 5. Shifting: sll, srl, sla, sra, rol, simple_expression] ror simple_expression ::= [+ | -] term {(+ | - | &) term} 6. Relational: =, /=, <, <=, >, >= term ::= factor {(* | / | mod | rem) factor} 7. Logical: and, or, nand, nor, xor, factor ::= primary [** primary] | abs primary | not primary xnor primary ::= (expression) | literal | function_call | ... Operator Precedence (2) expression ::= relation {and relation} | relation {or relation} | relation {xor relation} | relation {xnor relation} | relation [nand relation] | relation [nor relation] --ILLEGAL!, can't mix logical operators result <= a and b or c; --! •Logical operators are the lowest precedence. --should be... result <= (a and b) or c; Logical operators are evaluated after the rest of the expression (the relations). --ILLEGAL!, can't chain nand or nor --(nand and nor are not associative) •Logical operators have the same precedence. This result <= a nand b nand c; --! result <= a nor b nor c; --! is unusual in that some languages equate and with the * operator and or with the + operator. --maybe result <= (a nand b) nand c; •Since logical operators have the same precedence, --or it is impossible to mix logical operators. Because result <= a nand (b nand c); logical operators cannot be mixed, parentheses must used a lot. •nand and nor cannot be chained (note the use of the [] rather than {} brackets in the EBNF syntax). This is because nand and nor are not associative. Operator Precedence (3) --ILLEGAL! for similar reason as logical if a < b = false then --! --should be if (a < b) = false then --this is fine expression ::= relation {and relation} | relation {or relation} if a = '1' and b = '0' then --and is the same as | relation {xor relation} | relation {xnor relation} if (a = '1') and (b = '0') then | relation [nand relation] | relation [nor relation] --ILLEGAL! cannot chain shift operators result <= a sll 2 srl 2; --! relation ::= shift_expression [(= | /= | < | <= | > | >=) --this is okay shift_expression ] if a sll 2 > 5 then --is equivalent to if (a sll 2) > 5 then shift_expression ::= [(sll | srl | sla | sra | rol | ror) simple_expression] --ILLEGAL, sign can’t appear in second term result <= a + -b; --! --but this is legal simple_expression ::= [+op | -op] term {(+ | - | &) term} result <= -a + b; --this... term ::= factor {(* | / | mod | rem) factor} result <= -a mod b; --is the same as result <= -(a mod b); factor ::= primary [** primary] | abs primary | not primary --ILLEGAL, not allowed result <= a ** b ** c; --! primary ::= (expression) | literal | function_call | ... result <= a ** (b ** c); --ILLEGAL result <= a ** -b --! --but, strangely enough, not illegal result <= a ** abs b; Standard, but not VHDL std types • Given VHDL doesn’t even contain the types that we most frequently use, how do we determine what operators are available for : – User defined types – Types from standardized packages (e.g. std_logic_vector..etc.) • This issue is made more complicated by the lack of a dominant standard synthesis package – VHDL loses some benefit if code is not completely portable among tools. Packages package test_parts constant cycleW is : integer := 3; • Packages are used to collect together constant addrW : integer := 22; constant dataW : integer := 16; a set of closely related sub- procedure generate_clock (Tperiod, Tpulse, Tphase : in time; programs, parts, or types. end package test_parts; signal clk : out std_logic); • For example, std_logic_1164 is a package body test_parts is package that defines four types procedure generate_clock ( Tperiod, Tpulse, Tphase : in time; (std_ulogic, std_logic, signal clk : out std_logic ) is begin std_ulogic_vector, std_logic_vector) wait for Tphase; loop as well as the operations that can clk <= '0', '1' after Tpulse; wait for Tperiod; end loop; be performed on them. end procedure generate_clock; • A package is comprised of a package end package body test_parts; header and a package body use work.my_components.all; architecture behavior of simplecircuit is library ieee; signal andout1, andout2 : std_logic; use ieee.std_logic_1164.all; begin use work.test_parts.all; U6: and2 port map (a1,b1,andout1); U7: and2 port map (a2,b2,andout2); . . sigout <= andout1 or andout2; . end behavior; Packages and Operator Overloading ------------------------------------------------------------------- • It is generally the case that the -- overloaded logical operators ------------------------------------------------------------------- operations that you want to FUNCTION "and" ( FUNCTION "nand" ( l l : : std_ulogic; r std_ulogic; r : : std_ulogic ) std_ulogic ) RETURN UX01; RETURN UX01; perform on your newly FUNCTION "or" FUNCTION "nor" ( ( l l : : std_ulogic; r std_ulogic; r : : std_ulogic ) std_ulogic ) RETURN UX01; RETURN UX01; FUNCTION "xor" ( l : std_ulogic; r : std_ulogic ) RETURN UX01; defined types (say -- function "xnor" function xnor ( ( l l : : std_ulogic; std_ulogic; r r : : std_ulogic std_ulogic ) ) return ux01; return ux01; std_logic_vector) are the same FUNCTION "not" ( l : std_ulogic ) RETURN UX01; ------------------------------------------------------------------- as the VHDL std operators. -- vectorized overloaded logical operators ------------------------------------------------------------------- • VHDL supports “operator FUNCTION "and" ( l, r : std_logic_vector ) RETURN std_logic_vector; FUNCTION "and" ( l, r : std_ulogic_vector ) RETURN std_ulogic_vector; overloading” FUNCTION "nand" ( l, r : std_logic_vector ) RETURN std_logic_vector; FUNCTION "nand" ( l, r : std_ulogic_vector ) RETURN std_ulogic_vector; – multiple versions of the FUNCTION "or" FUNCTION "or" ( l, r : std_logic_vector ) RETURN std_logic_vector; ( l, r : std_ulogic_vector ) RETURN std_ulogic_vector; operators can be available, FUNCTION "nor" ( l, r : std_logic_vector ) RETURN std_logic_vector; with the same name FUNCTION "nor" ( l, r : std_ulogic_vector ) RETURN std_ulogic_vector; – which version is “called” FUNCTION "xor" ( l, r : std_logic_vector ) RETURN std_logic_vector; FUNCTION "xor" ( l, r : std_ulogic_vector ) RETURN std_ulogic_vector; depends on the parameters and their types from std_logic_1164 package header bitwise boolean operators are overloaded for std_logic, so their behavior is now governed by the package body Implementation of AND in 1164 from std_logic_1164 package body • If you want to know how a CONSTANT and_table : stdlogic_table := ( -- ---------------------------------------------------- -- | U X 0 1 Z W L H - | | function works, check the -- ---------------------------------------------------- ( 'U', 'U', '0', 'U', 'U', 'U', '0', 'U', 'U' ), -- | U | ( 'U', 'X', '0', 'X', 'X', 'X', '0', 'X', 'X' ), -- | X | source in the package body. ( '0', '0', '0', '0', '0', '0', '0', '0', '0' ), -- ( 'U', 'X', '0', '1', 'X', 'X', '0', '1', 'X' ), -- | | 0 1 | | • AND on two ( 'U', 'X', '0', 'X', 'X', 'X', '0', 'X', 'X' ), -- ( 'U', 'X', '0', 'X', 'X', 'X', '0', 'X', 'X' ), -- ( '0', '0', '0', '0', '0', '0', '0', '0', '0' ), -- | | | Z W L | | | std_logic_vectors works as we ( 'U', 'X', '0', '1', 'X', 'X', '0', '1', 'X' ), -- ( 'U', 'X', '0', 'X', 'X', 'X', '0', 'X', 'X' ) -- | | H - | | would expect, bitwise and’ing ); … of the two vectors. FUNCTION "and" ( l,r : std_logic_vector ) RETURN std_logic_vector IS Metalogical and hi-Z values -- pragma built_in SYN_AND -- pragma subpgm_id 198 --synopsys synthesis_off are handled according to the ALIAS lv : std_logic_vector ( 1 TO l'LENGTH ) IS l; ALIAS rv : std_logic_vector ( 1 TO r'LENGTH ) IS r; table. VARIABLE result : std_logic_vector ( 1 TO l'LENGTH ); --synopsys synthesis_on BEGIN • This is the simulator model, --synopsys synthesis_off IF ( l'LENGTH /= r'LENGTH ) THEN ASSERT FALSE understanding this lets you REPORT "arguments of overloaded 'and' operator are not of the same length" understand the behavior, not ELSE SEVERITY FAILURE; necessarily the synthesis FOR i IN result'RANGE LOOP result(i) := and_table (lv(i), rv(i)); END LOOP; implementation. Synthesis END IF; RETURN result; Implementation is typically --synopsys synthesis_on END "and"; the minimal combinatorial representation and is guaranteed by the tool vendor to match the simulation Comparison Operators for array types Since std_logic_1164 only overloads logical operators, we are left with the VHDL built-in comparisons, arithmetic..etc. These don’t always behave like you expect, because now std_logic_vector is just an array of enumerated types. Example circuit for a < b where A(2) A(1) A(0) a’range is 2 downto 0 and b’range is 3 downto 0. = = = '1' Less Than < < < B(3) B(2) B(1) B(0) • Arrays are compared Operator a'length < a'length = a'length > b'length b'length b'length from left to right, < 1 0 0 <= 1 1 0 regardless of their ranges > 0 0 1 or sizes! If a bit is not >= 0 1 1 found that satisfies the Comparison Operators for array types Examples : library ieee; a = “00000000” z = “1111” : altz = ‘1’, agtz = ‘0’ use ieee.std_logic_1164.all; a = “1111- - - -” z = “1111” : altz = ‘0’, agtz = ‘1’ a = “10111111” z = “1111” : altz = ‘1’, agtz = ‘0’ entity test is a = “00011111” z = “1111” : altz = ‘1’, agtz = ‘0’ port (a : in std_logic_vector (7 downto 0); z : in std_logic_vector (3 downto 0); altz, agtz : out std_logic); end entity test; architecture example of test is begin altz <= '1' when a < z else '0'; agtz <= '1' when a > z else '0'; end architecture example; • Arrays are compared from Operator a'length < a'length = a'length > b'length b'length b'length left to right, regardless of < 1 0 0 their ranges or sizes! If a bit is <= 1 1 0 not found that satisfies the > 0 0 1 >= 0 1 1 condition, the result is determined by the lengths. Numerical Packages std_logic_arith & numeric_std • Problems – comparison operators have strange behavior on our array types – no arithmetic operators available for arrays of std_logic (which we definitely need) – shifting operators not supported (87) for std_logic_vector, and these also have some non- conventional behavior • Solutions – Packages that define new types and overload appropriate operators type UNSIGNED is array (NATURAL range <>) of STD_LOGIC; type SIGNED is array (NATURAL range <>) of STD_LOGIC; – This gives us a type that represents the hardware we are building like a std_logic_vector, while still giving us the mathematical capabilities of a integer • There is a cost to using these packages, and that is simulation time – When using integers, the simulator can before the operation as written, when using std_logic_arith the operation is much more complicated – For example, an integer multiplication working on 32-bit operands is a single operation, whereas the same multiplication on 32 bit arrays takes more than a thousand operations – This is only a concern when it affects us, therefore it is rarely a concern! std_logic_arith & numeric_std • Since the arrays of std_logic now represent numeric values – Operators can be defined to support the arithmetic operations (which the integer type provided us)… – The comparison operations can be overloaded to be numerical comparisons – PLUS, we can relatively easily use the bitwise logical operations on our numbers (unlike integers)… – …as well as slicing and concatenation (which are also not offered by integers). These are VHDL standard array operators, and come for free. • Note that the shift operators are not supported <check this> std_logic_arith & numeric_std Convention Note : All of the functions in the standard packages are defined such that a vector is always interpreted where the left-most bit is the MSB (regardless of array range at definition) Reminder: although signed,unsigned,and std_logic_vector look the same, VHDL is strongly typed, so they cannot be interchanged without explicit conversion. What package to use? • Approved IEEE package : numeric_std • Old Defacto std : std_logic_arith – written by Synposys before there was a standard. – still widely in use mostly due to inertia • Since numeric_std is indeed a standard, it is guaranteed to be portable among synthesizers and simulators. • Though std_logic_arith is fairly uniform, there are various implementations • Recommendation : use approved IEEE package Operators defined in numeric_std • Many operators are defined in the numeric_std package for signed and unsigned: – Comparison: =, /=, <, <=, >, >= • return Boolean, and work exactly as you would expect, and based on the numeric value. Operators are overloaded for comparing each type with itself and with integers only. • function ">" (L, R: UNSIGNED) return BOOLEAN; • function ">" (L, R: SIGNED) return BOOLEAN; • function ">" (L: NATURAL; R: UNSIGNED) return BOOLEAN; • function ">" (L: INTEGER; R: SIGNED) return BOOLEAN; • function ">" (L: UNSIGNED; R: NATURAL) return BOOLEAN; • function ">" (L: SIGNED; R: INTEGER) return BOOLEAN; – Arithmetic: sign -, sign +, abs, +, -, * (sign – and abs are only defined for signed) • Again, these are overloaded for the same combinations. Note the lack of overloading for a combination of signed and unsigned operands. – Boolean Operators: not, and, or, nand, nor, xor • These are only supported for types signed and unsigned with themselves. (std_logic_1164 contains these for std_logic_vector) – Limited Arithmetic Operators : /, mod, rem • not universally synthesizeable, can be synthesized in XST (Xilinx Synthesis Technology) if left operand is a constant power of 2 Comparison Operators in numeric_std Example : library ieee; use ieee.numeric_std_all; use ieee.std_logic_1164.all; entity test is port (a : in unsigned (7 downto 0); a = “00000000” z = “1111” : altz = ‘1’, agtz = ‘0’ z : in unsigned (3 downto 0); a = “1111- - - -” z = “1111” : altz = ‘0’, agtz = ‘1’ altz, agtz : out std_logic); a = “10111111” z = “1111” : altz = ‘0’, agtz = ‘1’ end entity test; a = “00011111” z = “1111” : altz = ‘0’, agtz = ‘1’ architecture example of test is begin altz <= '1' when a < z else '0'; agtz <= '1' when a > z else '0'; end architecture example; • Numerical values are compared regardless of size Resize Functions (Numeric_Std) function RESIZE (ARG: SIGNED; NEW_SIZE: NATURAL) return SIGNED; function RESIZE (ARG: UNSIGNED; NEW_SIZE: NATURAL) return UNSIGNED; msb lsb library ieee; use ieee.std_logic_1164.all; use ieee.numeric_std.all; sign entity resizes is port (sa: in signed(7 downto 0); sb: in signed(15 downto 0); sa_large : out signed (15 downto 0); sb_small : out signed (7 downto 0); ua: in unsigned(7 downto 0); ub: in unsigned(15 downto 0); ua_large : out unsigned (15 downto 0); ub_small : out unsigned (7 downto 0) ); end entity resizes; architecture example of resizes is begin --signed length conversions sa_large <= resize(sa, sa_large'length); sb_small <= resize(sb, sb_small'length); --unsigned length conversions ua_large <= resize(ua, ua_large'length); ub_small <= resize(ub, ub'length); msb lsb end architecture example; -- conventional resizing could be done using slice -- sb_small <= sb(sb_small’range); -- sb_small <= sb(7 downto 0); numeric_std Arithmetic Operators (+/-) • The “+” and “-” preserve library ieee; use ieee.std_logic_arith.all; use ieee.std_logic_1164.all; the length of the data entity addition is port (a,b : in signed (7 downto 0); path z : out signed (7 downto 0)); end entity addition; – This means that addition architecture example of addition is begin and subtraction wrap --a + b could possibly wrap z <= a + b; end architecture addition; around on overflow – This can be avoided by resizing prior to the library ieee; operation use ieee.std_logic_arith.all; use ieee.std_logic_1164.all; • The “+” and “-” entity addition is port (a,b : in signed (7 downto 0); operators have been z : out signed (8 downto 0)); end entity addition; defined so that they can architecture example of addition is begin --a + b cannot wrap, but the length has be used on arguments --not been preserved z <= resize(a,z’length) which are different sizes. + resize(b, z’length); end architecture addition; – In the case when the Exa : “+” in numeric_std • “+” resizes input operands (and function "+" (L, R: UNSIGNED) return UNSIGNED is constant SIZE: natural := MAX(L'LENGTH, R'LENGTH); variable L01 : UNSIGNED(SIZE-1 downto 0); takes the strength out if it variable R01 : UNSIGNED(SIZE-1 downto 0); begin if ((L'LENGTH < 1) or (R'LENGTH < 1)) then return NAU; end if; L01 := TO_01(RESIZE(L, SIZE), 'X'); exists) if (L01(L01'LEFT)='X') then return L01; end if; R01 := TO_01(RESIZE(R, SIZE), 'X'); • calls local function if (R01(R01'LEFT)='X') then return R01; end if; return ADD_UNSIGNED(L01, R01, '0'); ADD_UNSIGNED, shown end "+"; below • ADD_UNSIGNED operates function ADD_UNSIGNED (L, R: UNSIGNED; C: std_logic) return UNSIGNED is constant L_LEFT: integer := L'LENGTH-1; alias XL: UNSIGNED(L_LEFT downto 0) just like you’d expect addition is L; alias XR: UNSIGNED(L_LEFT downto 0) is R; to 0); variable RESULT: UNSIGNED(L_LEFT downtooperate, LSB to MSB, variable CBIT: std_logic := C; begin keeping track of carry between for I in 0 to L_LEFT loop RESULT(I) := CBIT xor XL(I) xor bits. XR(I); CBIT := (CBIT and XL(I)) or (CBIT and XR(I)) or (XL(I) and XR(I)); end loop; return RESULT; end ADD_UNSIGNED; Arithmetic Ops • Seeing this model emphasizes the simulation time penalty paid by using ennumerrated array types for arithmetic. • Synthesis for this operation would simply be a standard adder implemented optimally (hopefully) for that particular technology “Wrapping” code R: required in a counter, UNSIGNED is • function ADD_UNSIGNED (L, notUNSIGNED; C: std_logic) return since as you can see, “111” + “001” = “000”, just like a hardware constant L_LEFT: integer := L'LENGTH-1; alias XL: UNSIGNED(L_LEFT downto 0) is L; adder alias XR: UNSIGNED(L_LEFT downto 0) is R; variable RESULT: UNSIGNED(L_LEFT downto 0); variable CBIT: std_logic := C; begin for I in 0 to L_LEFT loop RESULT(I) := CBIT xor XL(I) xor XR(I); CBIT := (CBIT and XL(I)) or (CBIT and XR(I)) or (XL(I) and XR(I)); end loop; return RESULT; end ADD_UNSIGNED; numeric_std Arithmetic Operators (sign +/-, abs) library ieee; Decimal Two's use ieee.numeric_std.all; Number Complement use ieee.std_logic_1164.all; 15 0,1111 • As noted, the sign +, sign entity negation is 14 0,1110 13 0,1101 -, abs, +, -, and * port (a : in signed (7 downto 0); aneg : out signed (7 downto 0); 12 0,1100 operators are available aabs : out signed (7 downto 0)); 11 0,1011 end entity negation; 10 0,1010 for signed vectors, while architecture example of negation is 9 0,1001 8 0,1000 the +, -, and * operators begin --if a is max neg number, then will return 7 0,0111 are available for unsigned --the same thing! 6 0,0110 aneg <= -a; 5 0,0101 vectors. aabs <= abs a; 4 0,0100 end architecture example; 3 0,0011 • The operators have been 2 0,0010 1 0,0001 defined so that the 0 0,0000 numbers wrap on library ieee; use ieee.numeric_std.all; -1 -2 1,1111 1,1110 overflow use ieee.std_logic_1164.all; -3 -4 1,1101 1,1100 – Therefore, adding 1 to entity negation is port (a : in signed (7 downto 0); -5 1,1011 -6 1,1010 the most positive aneg : out signed (7 downto 0); -7 1,1001 aabs : out signed (8 downto 0)); signed number returns end entity negation; -8 1,1000 -9 1,0111 the most negative architecture example of negation is -10 1,0110 signed number (“01..1” begin -11 1,0101 --this is a fix, but probably + ‘1’ => “11...1”)… -12 1,0100 --not neccesary -13 1,0011 aneg <= -resize(a, 9); – …while adding 1 to the aabs <= abs resize(a, 9); -14 1,0010 end architecture example; -15 1,0001 most positive unsigned -16 1,0000 number returns 0 numeric_std Arithmetic Operators (*) • The “*” operator is an exception, in that it does not preserve the data bus size library ieee; use ieee.numeric_std.all; • The result size is calculated by use ieee.std_logic_1164.all; adding the sizes of the arguments entity mult is port (a,b : in signed (7 downto 0); • Therefore, multiplication can z : out signed (7 downto 0)); end entity mult; never overflow, so resizing is not architecture example of mult is necessary before performing a signal product : signed(15 downto 0); begin multiplication operation --multiply, and then resize product <= a*b; • However, it often means that the z <= resize(product, z’length); result after the multiplication will --shorter method z <= resize(a*b, 8); need to be truncated by slicing or end architecture example; by resizing. numeric_std Arithmetic Operators (*) library ieee; use ieee.numeric_std.all; use ieee.std_logic_1164.all; entity mult is port (a,b : in signed (7 downto 0); z : out signed (7 downto 0)); end entity mult; architecture example of mult is signal product : signed(15 downto 0); begin --multiply, and then resize product <= a*b; z <= resize(product, z’length); --shorter method z <= resize(a*b, 8); end architecture example; Spartan II Virtex II # BELS : 99 # GND : 1 Cell Usage : # LUT2 : 16 # BELS : 1 # LUT4 : 16 # GND : 1 # MULT_AND : 16 # IO Buffers : 24 # MUXCY : 25 # IBUF : 16 # XORCY : 25 # OBUF : 8 # IO Buffers : 24 # Others : 1 # IBUF : 16 # MULT18X18 : 1 # OBUF : 8 ======================================== Type Conversion Functions (Numeric_Std) function TO_INTEGER (ARG: SIGNED) return INTEGER; function TO_INTEGER (ARG: UNSIGNED) return NATURAL; function TO_UNSIGNED (ARG, SIZE: NATURAL) return UNSIGNED; function TO_SIGNED (ARG: INTEGER; SIZE: NATURAL) return SIGNED; library ieee; library ieee; use ieee.std_logic_1164.all; use ieee.std_logic_1164.all; use ieee.numeric_std.all; use ieee.numeric_std.all; entity to_integer_demo is entity to_unsigned_demo is port (a : in unsigned(7 downto 0); port (value : in natural range 0 to 255; z : out natural range 0 to 255); result : out unsigned (7 downto 0)); end entity to_integer_demo; end entity to_unsigned_demo; architecture example of to_integer_demo is architecture behavior of to_unsigned_demo is begin begin z <= to_integer(a); result <= to_unsigned(value,8); end architecture example; end architecture example; -- good practice : -- result <= to_unsigned(value, result’length); Type Conversion Functions (Numeric_Std) Remember : we automatically have closely related type conversion functions: library ieee; use ieee.numeric_std.all; library ieee; use ieee.std_logic_1164.all; use ieee.numeric_std.all; use ieee.std_logic_1164.all; entity type_demo2 is port (a : in unsigned(6 downto 0); entity type_demo is z : out signed(7 downto 0)); port (a : in std_logic_vector(7 downto 0); end entity type_demo2; z : out unsigned(7 downto 0)); end entity type_demo; architecture example of type_demo2 is begin architecture example of type_demo is begin z <= ‘0’ & signed(a); z <= unsigned(a); end architecture example; end architecture example; -- we add an extra bit to preserve numeric -- representation -- NOTHING special is done here, we are simply -- telling the compiler to interpret the bits -- in the array as a different type Constants constant values of signed and unsigned (since they are array’s of std_logic) can be represented by string values : Assignments : z <= “00000000”; -- MSB on left… must be exactly right length z <= (others => ‘0’); -- avoid this using aggregate Bit String Literals : VHDL 87 – Available only to arrays of type bit. VHDL 93 – All character array types that contain ‘0’ and ‘1’ (e.g. std_logic, signed, unsigned…) x <= B”0000_0000_1111”; -- B = Binary, underscores optional x <= O”00_17”; -- O = Octal x <= X”00F”; Arithmetic with Constants mixing types by expressing the constant as an integer is often more clear library ieee; library ieee; use ieee.numeric_std.all; use ieee.numeric_std.all; use ieee.std_logic_1164.all; use ieee.std_logic_1164.all; entity increment is entity increment is port (a : in signed(7 downto 0); port (a : in signed(7 downto 0); z : out signed(7 downto 0)); z : out signed(7 downto 0)); end entity increment; end entity increment; architecture example of increment is architecture example of increment is begin begin z <= a + “1”; z <= a + 1; end architecture example; end architecture example; Example : Comparison Operators library ieee; use ieee.std_logic_1164.all; use ieee.numeric_std.all; entity compare is port (a : in signed (15 downto 0); negative : out std_logic); end entity compare; architecture example of compare is begin negative <= ‘1’ when a<0 else ‘0’; end architecture example; library ieee; use ieee.std_logic_1164.all; use ieee.numeric_std.all; Note the use of the to_signed entity max is port (a : in signed (15 downto 0); function when copying the integer b : in natural; c : out signed (15 downto 0)); to end entity max; signal c. Although the functions architecture example of max is are begin c <= a when (a > b) else to_signed(b, c'length); overloaded, signal assignments still end architecture example; are strongly typed. VHDL’93 built-in Shift Functions • a(4 downto 0) sll 2 == a(2 downto 0) & “00” • a(4 downto 0) srl 2 == “00” & a(4 downto 2) • a(4 downto 0) sla 2 == ? • a(4 downto 0) sra 2 == ? • a(4 downto 0) rol 2 == a(2 downto 0) & a(4 downto 3) • a(4 downto 0) ror 2 == a(1 downto 0) & a(4 downto 2) for bit vectors… numeric_std overloads sll, srl, rol, ror for signed and unsigned types also available (especially in ’87 synthesizers) as shift_left(), shift_right(), rotate_left(), and rotate_right() Shift Functions for numeric_std msb lsb 0 0 0 0 • Two shift operations are available for std_logic_arith, sll (shift_left) and srl unsigned) sll (signed andand Signed Unsigned (shift_right). shift-left (shl • The sll and srl functions each take two msb lsb arguments, the first begin the value, 0 0 0 0 signal, or variable to shift, and the second being an unsigned value specifying how far to shift the value srl (unsigned) Unsigned shift-right (shr) – Note that for synthesis the shift value must be a constant value because the synthesis msb lsb interpretation of a shift is a hardwired rearrangement of the bits of not anymore! the bus – z <= shift_left(a,4); – z <= a sll 4; Signed shift-right (shr) srl (signed) Barrel Shifter Synthesizing Unit <test>. library IEEE; Related source file is C:\… use IEEE.STD_LOGIC_1164.ALL; Found 8-bit shifter logical left for signal <sh>. use IEEE.numeric_std.all; Summary: inferred 1 Combinational logic shifter(s). entity test is Unit <test> synthesized. Port ( norm : in unsigned(7 downto 0); sh : out unsigned(7 downto 0); ===================================== HDL Synthesis Report num : in natural range 0 to 5); end test; Macro Statistics # Logic shifters :1 architecture Behavioral of test is 8-bit shifter logical left :1 begin ===================================== sh <= norm sll num; end Behavioral; Boolean Operators • Boolean operators (not, and nand, or, nor, xor, xnor) for std_logic_vectors: – The boolean operation is performed bit-by-bit (positionally) on the two vectors – Each vector must have the same ‘length (but not neccesarily the same ‘range) • In Numeric_Std supported for signed and unsigned – SHOULD be the same length a : signed(7 downto 0); 10011110 XST synthesis results b : signed(6 downto 0); 1111111 c : signed(15 downto 0)); 1111111110011110 c <= a and b; Simulation produces “Run-Time Error” STD_LOGIC_ARITH std_logic_unsigned std_logic_signed Boolean Operators • Boolean operators (not, and nand, or, nor, xor, xnor) for std_logic_vectors: – The boolean operation is performed bit-by-bit (positionally) on the two vectors – Each vector must have the same ‘length (but not neccesarily the same ‘range) • Surprisingly, the boolean operators are not supported for std_logic_arith – This can be circumvented by type converting signed or unsigned vectors to std_logic_vectors, and then using the boolean operators available (remember that type conversion are available for closely related types. – As an example, consider performing a masking operation on an signed value: • z <= a when signed(std_logic_vector(c) and “00001111”) /= “0” else b; – Note that the string “00001111” is implicity assumed to be a std_logic_vector, and that the “0” does not need to be the same length as the resultant signed value because the signed comparisons are defined for different length vectors. Type conversions All functions in the form “conv_type” where type Argument Second is the format to convert Function Name Type Argument? Result Type to. conv_integer integer no integer No second argument conv_integer signed no integer because the size of conv_integer unsigned no integer the integer type is conv_integer std_ulogic no integer fixed conv_unsigned integer yes unsigned conv_unsigned signed yes unsigned conv_unsigned unsigned yes unsigned Functions that conv_unsigned std_ulogic yes unsigned take and return conv_signed integer yes signed the same type conv_signed signed yes signed are used as conv_signed unsigned yes signed resize functions conv_signed std_ulogic yes signed conv_std_logic_vector integer yes std_logic_vector conv_std_logic_vector signed yes std_logic_vector conv_std_logic_vector unsigned yes std_logic_vector conv_std_logic_vector std_ulogic yes std_logic_vector Note that there are not conversion functions from Gives the size of the unsigned./signed to array to be created, must std_logic_vector be constant for synthesis std_logic_arith: Resize Functions library ieee; use ieee.std_logic_arith.all; use ieee.std_logic_1164.all; msb lsb entity resizes is port (sa: in signed(7 downto 0); sb: in signed(15 downto 0); sign sa_large : out signed (15 downto 0); sb_small : out signed (7 downto 0); ua: in unsigned(7 downto 0); ub: in unsigned(15 downto 0); ua_large : out unsigned (15 downto 0); ub_small : out unsigned (7 downto 0) ); end entity resizes; architecture example of resizes is begin --signed length conversions msb lsb sa_large <= conv_signed(sa, sa_large'length); sb_small <= conv_signed(sb, sb_small'length); --unsigned length conversions ua_large <= conv_unsigned(ua, ua_large'length); ub_small <= conv_unsigned(ub, ub'length); --would be the same as ua_large <= (ua’range => '0') & ua; ub_small <= ub(ub_small'range); end architecture example; For example: “1000000000000001” (-32767) => “00000001” (+1) Constant Values and Type Ambiguity • Constant values of signed and unsigned types can be represented by string values – Z <= “00000000”; – Z <= X”00”; – Z <= (others => ‘0’); library ieee; use ieee.std_logic_1164.all; – Z <= conv_unsigned(‘0’, 8); use ieee.std_logic_arith.all; • When adding a signal to a constant value, there is no need to match the size of the arguments, since the entity increment is port (a : unsigned (7 downto 0); functions can take operators of different lengths z : unsigned (7 downto 0)); • Due to the different permutations of additions possible end; for signed and unsigned vectors (because of the architecture behavior of increment is extensive overloading), type ambiguity can occur --ERROR, type abiguity, the compiler --is not able to resolve this • This is resolved by type qualification z <= a + "1"; – When there is ambiguity in the model (that is there are --we can perform a type qualification several different values that the analyzer could choose), --to solve the problem type qualification informs the analyzer which to choose z <= a + unsigned'("1"); – A type qualifier consists of the name of the expected type, --or we can add a '1', this is a tick (i.e. a single quote mark), and the expression being --intepreted as a std_ulogic '1', --which the “+” for which the unsigned resolved enclosed in parentheses. --and signed types are overloaded z <= a + '1'; end architecture increment; std_logic_signed and std_logic_unsigned library IEEE; use IEEE.std_logic_1164.all; use IEEE.std_logic_arith.all; package STD_LOGIC_UNSIGNED is function "+"(L: STD_LOGIC_VECTOR; R: STD_LOGIC_VECTOR) return STD_LOGIC_VECTOR; function "+"(L: STD_LOGIC_VECTOR; R: INTEGER) return STD_LOGIC_VECTOR; function "+"(L: INTEGER; R: STD_LOGIC_VECTOR) return STD_LOGIC_VECTOR; These functions define the function "+"(L: STD_LOGIC_VECTOR; R: STD_LOGIC) return STD_LOGIC_VECTOR; function "+"(L: STD_LOGIC; R: STD_LOGIC_VECTOR) return STD_LOGIC_VECTOR; unsigned operations for std_logic_vectors function "-"(L: STD_LOGIC_VECTOR; R: STD_LOGIC_VECTOR) return STD_LOGIC_VECTOR; function "-"(L: STD_LOGIC_VECTOR; R: INTEGER) return STD_LOGIC_VECTOR; function "-"(L: INTEGER; R: STD_LOGIC_VECTOR) return STD_LOGIC_VECTOR; function "-"(L: STD_LOGIC_VECTOR; R: STD_LOGIC) return STD_LOGIC_VECTOR; function "-"(L: STD_LOGIC; R: STD_LOGIC_VECTOR) return STD_LOGIC_VECTOR; All functions in std_logic_arith are . . overloaded such that they can be . used on std_logic_vectors, treated function "<"(L: STD_LOGIC_VECTOR; R: STD_LOGIC_VECTOR) return BOOLEAN; function "<"(L: STD_LOGIC_VECTOR; R: INTEGER) return BOOLEAN; as unsigned of course function "<"(L: INTEGER; R: STD_LOGIC_VECTOR) return BOOLEAN; . . . function CONV_INTEGER(ARG: STD_LOGIC_VECTOR) return INTEGER; . Note how the +, -, <, . operators can be . end STD_LOGIC_UNSIGNED; overloaded. package body STD_LOGIC_UNSIGNED is function "+"(L: STD_LOGIC_VECTOR; R: STD_LOGIC_VECTOR) return STD_LOGIC_VECTOR is -- pragma label_applies_to plus The conversion functions constant length: INTEGER := maximum(L'length, R'length); available for closely related variable result : STD_LOGIC_VECTOR (length-1 downto 0); begin types are used to convert to result := UNSIGNED(L) + UNSIGNED(R);-- pragma label plus return std_logic_vector(result); unsigned, then the unsigned end; operations are used. . . . Overview / Suggestions • Use Numeric_Std • Force yourself to use some of the nice features when you are dealing with numbers, code will be much more readable and compact – mixing types – mixing sizes – using shifting operators and type conversions – represent constants in a natural fashion (I.e. decimal integers or bitstring literals) Examples library ieee; use ieee.numeric_std.all; use ieee.std_logic_1164.all; signal clk : std_logic; signal data_in : unsigned(15 downto 0); entity test is signal shiftreg : unsigned(15 downto 0); port (a : in std_logic_vector (3 downto 0); signal load : std_logic; dec : out std_logic_vector (15 downto 0)); end entity test; … process (clk) architecture example of test is begin if rising_edge(clk) then begin if load = ‘1’ then shiftreg <= data_in; else shiftreg <= shiftreg sll 1; process(a) end if; begin end process; dec <= (others => '0'); dec(to_integer(unsigned(a))) <= '1'; end process; end architecture example; Sources • Krzysztof Kuchcinski • K. J. Hintz