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Brief History

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  1. Brief History • Early ‘80s, US Dept. of Defense project • VHSIC Hardware Description Language • VHSIC = Very High Speed Integrated Circuit • For documentation & specification of ASICs • 1987, IEEE standard adopted • 1992, standard updated • ‘90s to date, highly popular in industry • Main HDL competitor: Verilog, “simpler to learn” • Commonly taught in CEng programs

  2. Uses Evolved • Documentation & specification • Circuit simulation • Circuit synthesis

  3. Why a “language”? • VHDL largely overlooked while schematic capture reigned • Moore’s Law put schematic capture out of business • Text-based modeling attractive → suitable input for CAD tools • Modeling, simulation, synthesis

  4. Is it “like C”? • Yes • There are statements, block structure, variables, constants, operators, even “;” • No • SW programmers follow sequential “Von Neumann” computation model; but… • HDL statements translate into logic gates, not instructions • HW is always there, always “on”, operating concurrently → surprises SW people (example later)

  5. Three ways to describe circuit • Structural • Instantiate specific library components and “wire” them together • Dataflow (RTL) • Instantiate register components from library • Code combo logic with high-level operations; let VHDL compiler synthesize logic gates • Behavioural • Code algorithm steps; let VHDL compiler infer components, datapath, and controller

  6. Basic Ingredients • Structure • Entity/architecture • Behaviour • Process • Timing • Data • Signal • Variable • Logic values

  7. x Basic Logic Gate F y architecture behav1 of AND_ent is begin process(x, y) begin -- compare to truth table if ((x='1') and (y='1')) then F <= '1'; else F <= '0'; end if; end process; end behav1; What did we observe? library ieee; use ieee.std_logic_1164.all; entity AND_ent is port(x: in std_logic; y: in std_logic; F: out std_logic ); end AND_ent; architecture behav2 of AND_ent is begin F <= x and y; end behav2; Comment Operation defined on std_logic Signal assignment (a “wire”)

  8. Library: Abstract Data Types IEEE std_logic_1164 values U Uninitialized X Unknown 0 Zero 1 One Z High Impedance (tristate not driving output) W Weak Unknown L Weak Zero (low) H Weak One (high) - Don’t Care Why all these??

  9. Entity/Architecture Pair • Entity → circuit’s external interface • Ports  function parameters • Strongly typed: std_logic • Mode: in, out, inout (tristate), buffer (“out” that’s readable inside architecture) • Architecture → internal implementation • Multiple implementations allowed • Code block: thingbegin…endthing ;

  10. Process • Wrapper for “sequential” statements • ( sensitivity list ) • Tells simulator to re-simulate the process when any member of list changes value • Sequential-type statements can only appear inside a process: <= If-Then-Elsif-Else Case-When • Concurrent-type statements can appear anywhere • Processes together become concurrent blocks of logic

  11. Result of Description • Like OO class definition, entity has to be instantiated in another architecture • One behaviour chosen upon instantiation • Libraries full of entity definitions • If simulation is goal: • Both behaviours are identical • If synthesis is goal (actual AND gate): • Should anticipate what circuit compiler will create • Different from SW where normally trust compiler

  12. Combinational Logic Design input1 input2 output Component Library “work” entity OR_GATE is port(X: in std_logic; Y: in std_logic; F2: out std_logic ); end OR_GATE; entity AND_GATE is port(A: in std_logic; B: in std_logic; F1: out std_logic ); end AND_GATE; input3 use work.all; entity comb_ckt is port(input1: in std_logic; input2: in std_logic; input3: in std_logic; output: out std_logic ); end comb_ckt; architecture follows →

  13. A input1 F1 input2 wire B X F2 output input3 Y Combo Logic architecture struct of comb_ckt is component AND_GATE is port(A:in std_logic; B: in std_logic; F1:out std_logic ); end component; component OR_GATE is port(X: in std_logic; Y: in std_logic; F2: out std_logic ); end component; signal wire: std_logic;-- signal just like wire begin -- use sign "=>" to clarify the pin mapping Gate1: AND_GATE port map (A=>input1, B=>input2, F1=>wire); Gate2: OR_GATE port map (X=>wire, Y=>input3, F2=>output); end struct; Label for statement

  14. a X b Y s c Example of Programmer’s “Surprise” entity … port( a, b, c: in bit; X, Y: out bit ); … signal s: bit:=‘0’; Y <= s and a; X <= a or c; s <= b or c; • What does Y output for (1,0,1) input? • As (bad) software, clearly 0 • As hardware, describes logic circuit structure • “b or c” isn’t “done after” Y<= assignment • result → 1

  15. a state_reg reset x clock comb_logic Sequential Design entity seq_design is port(a:in std_logic; clock:in std_logic; reset:in std_logic; x:out std_logic ); end seq_design; architecture FSM of seq_design is -- define the states of FSM model type state_type is (S0, S1, S2, S3); signal next_state, current_state: state_type; begin -- concurrent process#1: -- advance state on clock going high state_reg: process(clock, reset) begin if (reset='1') then current_state <= S0; elsif (clock'event and clock='1') then current_state <= next_state; end if; end process; more → Enumerated data type Becomes state register

  16. a state_reg reset x clock comb_logic Sequential Design -- concurrent process#2: -- compute output and next state comb_logic: process(current_state, a) begin -- use case statement to show the -- state transition case current_state is when S0 => x <= '0'; if a='0' then next_state <= S0; elsif a ='1' then next_state <= S1; end if; when S1 => x <= '0'; … when S2 => x <= '0'; … when S3 => x <= '1'; … when others => x <= '0'; next_state <= S0; end case; end process; end FSM;

  17. Many More Features • Logic vectors & arrays type mem is array(0 to 127) of std_logic_vector(15 downto 0); • Define registers, buses, memories • Variables c := a xor b; • Temporary value, not intended to generate hardware • Timing statements wait for 10 ns; • Used for simulation • “Test bench” code connected to system-under-test can check if timing constraints met/violated

  18. Still More Features • Libraries library ieee; • Default library = “work” • Built-in packages in “std” • Package use ieee.std_logic_1164.all; • Binds constants, types, operations into set of “abstract data types” • Configuration for Gate1: AND_ent use entity work.AND_Ent(behav2); • Selects alternative architectures when instantiating entity • Generic like C++ template parameterization

  19. VHDL • An acronym for Very high speed integrated circuit Hardware Description Language • VHDL enables hardware modelling from the gate to system level • Allows various design methodologies • Provides technology independence • VHDL has been standardised: • VHDL 87 • VHDL 93

  20. System Design; definition and use of its parts • A system communicates via an interface • Interface is “entity” in VHDL • Cannot have any VHDL system without an entity • Example: • entity my_entity is • ……………….. • endentity my_entity;

  21. Architecture • The body of the system accomplishes some tasks on the data • eg data transformation • In VHDL the body is called “architecture” • Example: • architecture my_architecture of my_entity is • ………………. • begin • ……………… • end architecture my_architecture ;

  22. Types of Architecture • Behavioural (Functional) • The system in terms of its functionality • It does not contain any information about the internal system structure • It describes the expected behaviour: What a system will do • Response of outputs to inputs • No clue as to HOW but describes WHAT a system has to do

  23. Types of Architecture • Structural • HOW a system is composed • what components should be used • describes internal structure of system • how they should be connected • akin to a textual version of a schematic diagram

  24. A behavioural architecture if CLK'event and CLK='1' then --CLK rising edge DOUT <= DIN;

  25. A structural architecture A Full Adder: SUM<=A xor B xor C; CARRY<= A and B or ((A or B) and C);

  26. A Structural Architecture • --generate construct • gen: for i in 0 to n-1 generate • --component instantiation • ins: full_adder port map (a(i), b(i), carry(i), sum(i), carry(i+1)); • end generate;

  27. One Entity - Many Architectures • More than one way to create a design • Similarly, more than one architecture for a single entity (cf. More than one schematic to fulfil same specification) • Same interface • BUT only one entity for any architecture

  28. Package • External source of description • Allow you to define items outside of VHDL standards • Must be declared in advance using “library” and “use” keywords, usually before entity

  29. Few Packages • Standard (defined by the std library) • Std_logic_Textio (defined by the IEEE library) • Std_logic_1164 (defined by the IEEE library) • Plus vendor-specific

  30. Communications • Signals: inside device or between devices • Single or multiple wire - (bus) or (vector) • Example • bit for single signals • bit_vector for multiple signals • in both cases, each signal line can be either ‘1’ or ‘0’ • For bit_vector the width of the vector must be specified using two key words: downto and to • Order important for vector • bit_vector(7 downto 0) ascending order of bits • bit_vector (0 to 7) descending order of bits

  31. external signals • External signals connect the system to the outside: they form the system’s interface • Each external signal is specified as port inside its entity • Each external needs a unique name, a type and a direction: • input - in • output - out • bi-directional - inout

  32. Port Syntax • port_name: port_direction port_type ; • example: port( result: inout bit_vector (0 to 7) ); • Multiple signals of different type separated by semicolons • Same type separated by commas • optional initial value, preceded by :=

  33. Example • Example: entity ROM_MEMORY is • port ( A_ROM :in bit_vector (3 downto 0); • CS_ROM : in bit; • D_ROM : outbit_vector(7 downto 0) ); • end entity ROM_MEMORY;

  34. Internal signals • Internal signals declared inside an architecture • The keyword signal is required to declare an internal signal • Internal signals do not require a direction • It is possible to specify an initial value for an internal signal

  35. Generics • Method of providing constant values for different parameters. • Must be in entity, before ports • Need the keyword ‘generic’, name, type, value, optional comment • example • generic (BusWidth : integer:= 2; • MaxDelay : time:= 100 ns ) ; • Can be used anywhere a constant is needed

  36. Generics • Can be used anywhere a constant is needed • Parameters are typically specified by generics: • The size of some objects such as arrays or buses • Timing parameters • Useful in structural and behavioral models

  37. Standard Data Types • Enumeration types ( Boolean, bit, character) • Integer type • Real type • Physical type (time) • Standard array types ( string, bit_vector)

  38. Enumeration types • Bit • 0,1 • not same as Boolean in VHDL • Boolean • true, false • Character • all characters in ISO 8859-1 (West European) • eg ‘0’, ‘1’, ‘X’

  39. Integer and real types • Integer • implementation dependent • must include -2147483647 to +2147483647 • Real • implementation dependent • must include -1.0E308 to +1.0E308

  40. Physical Types • They specify the object values and the units they are expressed in. • VHDL defines only one: time • Note primary and secondary units

  41. Predefined arrays • Arrays are collections of elements of the same type • The number of elements is specified by a range • Predefined VHDL arrays are: • bit_vector (first element is 0) • and string (first element is 1) • single element in single quotes ‘’, multiple element double quotes ”” • one-dimensional

  42. User-defined Types • Use the keywords type or subtype • Example: • type short isrange -128 to 127; • subtype natural isintegerrange 0 to 147483647; • ( subtype of the integer type) • type FSMstates is( idle, decode, execute ); • ( a user defined enumerated type) • type TYPE_NAME is array (0 to 31) of BIT; • ( user defined array of element of type bit)

  43. Signal Assignment • Uses <= • Target on left • eg a <= b or c • arrow denotes flow of information • not, and , or, nand, nor, xor, xnor • not has highest precedence • all others equal and are evaluated left to right

  44. Example 2-Input AND Entity And2 is port (x,y : in bit; z : out bit); end entity And2; Architecture ex1 of And2 is begin z <= x and y; end architecture ex1;

  45. Logical operators • not, and , or, nand, nor, xor, xnor • They accept operands of type BIT, Boolean and bit_vector. • If the operands are of type bit_vector, they must be of the same size • a sequence of logical operators requires parentheses. • Example: • a and b or c and d; -- wrong • (a and b) or (c and d); --correct

  46. Arithmetic Operators • Addition ‘+’, subtraction ’-’,multiplication ‘*’, division ‘/ ‘, modulus ‘mod’, remainder ‘rem’, exponentiation ‘**’ and absolute value ‘abs’ • Predefined for • integer • real ( except ‘mod’ and ‘rem’) • time Not defined for bit_vector ‘+’ and ‘- ‘ can also be used as unary operators ( sign operators)

  47. Arithmetic Operators • Operands of same type (except exponentiation : exponent always integer) • an operand of type time can be multiplied or divided by an integer (result of type time) • order of precedence: • exponentiation ‘**’ • multiplication ‘*’, division ‘/’ • unary sign operators ‘+’ and ‘-’ • addition ‘+’ and subtraction ‘-’ • Example: 3+4*5=23, (3+4)*5=35

  48. Relational operators • The operators equal ‘=‘, not equal ‘/=‘, less than ‘<‘, greater than ‘>’, less than or equal ‘<=‘, greater than or equal ‘=>’, compare objects of the same type • Result always BOOLEAN (true or false) • Bit_vectors operands do not need to be of the same length: both operands are left justified

  49. Shift Operators • Applied only on the one-dimensional array • with the elements of the type BIT or BOOLEAN • srl, sll, ror, rol, sla, sra • ‘0’ shifted in for type bit, false for type Boolean • Example: z_bus <=a_bus sll 2;

  50. Concatenation • Allows creation of a new array from several others • ‘&’ is the concatenation operator • Length of new array is sum of length of both operands • Example: signal a, b: bit_vector (3 downto 0); signal z: bit_vector (7 downto 0); z <= a & b;