Hardware Description Language (HDL) - Introduction

Hardware Description Language Hardware Description Language - Introduction  HDL is a language that describes the hardware of digital systems in a textual form.  It resembles a programming language, but is specifically oriented to describing hardware structures and behaviors.  The main dif...

Hardware Description Language Hardware Description Language - Introduction  HDL is a language that describes the hardware of digital systems in a textual form.  It resembles a programming language, but is specifically oriented to describing hardware structures and behaviors.  The main difference with the traditional programming languages is HDL’s representation of extensive parallel operations whereas traditional ones represents mostly serial operations.  The most common use of a HDL is to provide an alternative to schematics. HDL – Introduction (2)  When a language is used for the above purpose (i.e. to provide an alternative to schematics), it is referred to as a structural description in which the language describes an interconnection of components.  Such a structural description can be used as input to logic simulation just as a schematic is used.  Models for each of the primitive components are required.  If an HDL is used, then these models can also be written in the HDL providing a more uniform, portable representation for simulation input.  HDL – Introduction (3)  HDL can be used to represent logic diagrams, Boolean expressions, and other more complex digital circuits.  Thus, in top down design, a very high-level description of a entire system can be precisely specified using an HDL.  This high-level description can then be refined and partitioned into lower-level descriptions as a part of the design process. HDL – Introduction (4)  As a documentation language, HDL is used to represent and document digital systems in a form that can be read by both humans and computers and is suitable as an exchange language between designers.  The language content can be stored and retrieved easily and processed by computer software in an efficient manner.  There are two applications of HDL processing: Simulation and Synthesis Logic Simulation  A simulator interprets the HDL description and produces a readable output, such as a timing diagram, that predicts how the hardware will behave before its is actually fabricated.  Simulation allows the detection of functional errors in a design without having to physically create the circuit. Logic Simulation (2)  The stimulus that tests the functionality of the design is called a test bench.  To simulate a digital system  Design is first described in HDL  Verified by simulating the design and checking it with a test bench which is also written in HDL.  Logic Simulation  Logic simulation is a fast, accurate method of analyzing a circuit to see its waveforms Types of HDL  There are two standard HDL’s that are supported by IEEE.  VHDL (Very-High-Speed Integrated Circuits Hardware Description Language) - Sometimes referred to as VHSIC HDL, this was developed from an initiative by US. Dept. of Defense.  Verilog HDL – developed by Cadence Data systems and later transferred to a consortium called Open Verilog International (OVI). Verilog  Verilog HDL has a syntax that describes precisely the legal constructs that can be used in the language.  It uses about 100 keywords pre-defined, lowercase, identifiers that define the language constructs.  Example of keywords: module, endmodule, input, output wire, and, or, not , etc.,  Any text between two slashes (//) and the end of line is interpreted as a comment.  Blank spaces are ignored and names are case sensitive.  Verilog - Module  A module is the building block in Verilog.  It is declared by the keyword module and is always terminated by the keyword endmodule.  Each statement is terminated with a semicolon, but there is no semi-colon after endmodule. Verilog – Module (2) HDL Example module smpl_circuit(A,B,C,x,y); input A,B,C; output x,y; wire e; and g1(e,A,B); not g2(y,C); or g3(x,e,y); endmodule  Verilog – Gate Delays  Sometimes it is necessary to specify the amount of delay from the input to the output of gates.  In Verilog, the delay is specified in terms of time units and the symbol #.  The association of a time unit with physical time is made using timescale compiler directive.  Compiler directive starts with the “backquote (`)” symbol. `timescale 1ns/100ps  The first number specifies the unit of measurement for time delays.  The second number specifies the precision for which the delays are rounded off, in this case to 0.1ns. Verilog – Module (4) //Description of circuit with delay module circuit_with_delay (A,B,C,x,y); input A,B,C; output x,y; wire e; and #(30) g1(e,A,B); or #(20) g3(x,e,y); not #(10) g2(y,C); endmodule Verilog – Module (5)  In order to simulate a circuit with HDL, it is necessary to apply inputs to the circuit for the simulator to generate an output response.  An HDL description that provides the stimulus to a design is called a test bench.  The initial statement specifies inputs between the keyword begin and end.  Initially ABC=000 (A,B and C are each set to 1’b0 (one binary digit with a value 0).  $finish is a system task.  Verilog – Module (6) module circuit_with_delay //Stimulus for simple circuit (A,B,C,x,y); module stimcrct; input A,B,C; reg A,B,C; output x,y; wire x,y; wire e; circuit_with_delay cwd(A,B,C,x,y); and #(30) g1(e,A,B); initial or #(20) g3(x,e,y); not #(10) g2(y,C); begin endmodule A = 1'b0; B = 1'b0; C = 1'b0; #100 A = 1'b1; B = 1'b1; C = 1'b1; #100 $finish; end endmodule  Verilog – Module (6) In the above example, cwd is declared as one instance circuit_with_delay. (similar in concept to objectclass relationship) Verilog – Module (7) Bitwise operators  Bitwise NOT : ~  Bitwise AND: &  Bitwise OR: |  Bitwise XOR: ^  Bitwise XNOR: ~^ or ^~ Verilog – Module (8) Boolean Expressions:  These are specified in Verilog HDL with a continuous assignment statement consisting of the keyword assign followed by a Boolean Expression.  The earlier circuit can be specified using the statement: assign x = (A&B)|~C) E.g. x = A + BC + B’D y = B’C + BC’D’ Verilog – Module (9) //Circuit specified with Boolean equations module circuit_bln (x,y,A,B,C,D); input A,B,C,D; output x,y; assign x = A | (B & C) | (~B & C); assign y = (~B & C) | (B & ~C & ~D); endmodule  Verilog – Module (10) User Defined Primitives (UDP):  The logic gates used in HDL descriptions with keywords and, or,etc., are defined by the system and are referred to as system primitives.  The user can create additional primitives by defining them in tabular form.  These type of circuits are referred to as user- defined primitives. Verilog – Module (12) UDP features ….  UDP’s do not use the keyword module. Instead they are declared with the keyword primitive.  There can be only one output and it must be listed first in the port list and declared with an output keyword.  There can be any number of inputs. The order in which they are listed in the input declaration must conform to the order in which they are given values in the table that follows.  The truth table is enclosed within the keywords table and endtable.  The values of the inputs are listed with a colon (:). The output is always the last entry in a row followed by a semicolon (;).  It ends with the keyword endprimitive.  Verilog – Module (13) //User defined primitive(UDP) primitive crctp (x,A,B,C); output x; input A,B,C; //Truth table for x(A,B,C) = Minterms (0,2,4,6,7) table // A B C : x (Note that this is only a comment) 0 0 0 : 1; 0 0 1 : 0; 0 1 0 : 1; 0 1 1 : 0; 1 0 0 : 1; // Instantiate primitive 1 0 1 : 0; module declare_crctp; 1 1 0 : 1; 1 1 1 : 1; reg x,y,z; endtable endprimitive wire w; crctp (w,x,y,z); endmodule  Verilog – Module (14)  A module can be described in any one (or a combination) of the following modeling techniques.  Gate-level modeling using instantiation of primitive gates and user defined modules.  This describes the circuit by specifying the gates and how they are connected with each other.  Dataflow modeling using continuous assignment statements with the keyword assign.  This is mostly used for describing combinational circuits.  Behavioral modeling using procedural assignment statements with keyword always.  This is used to describe digital systems at a higher level of abstraction.  Gate-Level Modeling  Here a circuit is specified by its logic gates and their interconnections.  It provides a textual description of a schematic diagram.  Verilog recognizes 12 basic gates as predefined primitives.  4 primitive gates of 3-state type.  Other 8 are: and, nand, or, nor, xor, xnor, not, buf  When the gates are simulated, the system assigns a four-valued logic set to each gate – 0,1,unknown (x) and high impedance (z)  Gate-level modeling (2)  When a primitive gate is incorporated into a module, we say it is instantiated in the module.  In general, component instantiations are statements that reference lower-level components in the design, essentially creating unique copies (or instances) of those components in the higher-level module.  Thus, a module that uses a gate in its description is said to instantiate the gate.  Gate-level Modeling (3)  Modeling with vector data (multiple bit widths):  A vector is specified within square brackets and two numbers separated with a colon. e.g. output[0:3] D; - This declares an output vector D with 4 bits, 0 through 3. wire[7:0] SUM; – This declares a wire vector SUM with 8 bits numbered 7 through 0. The first number listed is the most significant bit of the vector.  Gate-level Modeling  Two or more modules can be combined to build a hierarchical description of a design.  There are two basic types of design methodologies.  Top down: In top-down design, the top level block is defined and then sub-blocks necessary to build the top level block are identified.  Bottom up: Here the building blocks are first identified and then combine to build the top level block.  In a top-down design, a 4-bit binary adder is defined as top-level block with 4 full adder blocks. Then we describe two half-adders that are required to create the full adder.  In a bottom-up design, the half-adder is defined, then the full adder is constructed and the 4-bit adder is built from the full adders. Gate-level Modeling  A bottom-up hierarchical description of a 4-bit adder is described in Verilog as  Half adder: defined by instantiating primitive gates.  Then define the full adder by instantiating two half-adders.  Finally the third module describes 4-bit adder by instantiating 4 full adders.  Note: In Verilog, one module definition cannot be placed within another module description. 4-bit Half Adder 4-bit Full Adder  4-bit Full Adder //Gate-level hierarchical description of 4-bit adder module halfadder (S,C,x,y); input x,y; output S,C; //Instantiate primitive gates xor (S,x,y); and (C,x,y); endmodule module fulladder (S,C,x,y,z); input x,y,z; output S,C; wire S1,D1,D2; //Outputs of first XOR and two AND gates //Instantiate the half adders halfadder HA1(S1,D1,x,y), HA2(S,D2,S1,z); or g1(C,D2,D1); endmodule  4-bit Full Adder module _4bit_adder (S,C4,A,B,C0); input [3:0] A,B; input C0; output [3:0] S; output C4; wire C1,C2,C3; //Intermediate carries //Instantiate the full adder fulladder FA0 (S,C1,A,B,C0), FA1 (S,C2,A,B,C1), FA2 (S,C3,A,B,C2), FA3 (S,C4,A,B,C3); endmodule 2 to 4 Decoder  2 to 4 Decoder //Gate-level description of a 2-to-4-line decoder module decoder_gl (A,B,E,D); input A,B,E; output[0:3]D; wire Anot,Bnot,Enot; not n1 (Anot,A), n2 (Bnot,B), n3 (Enot,E); nand n4 (D,Anot,Bnot,Enot), n5 (D,Anot,B,Enot), n6 (D,A,Bnot,Enot), n7 (D,A,B,Enot); endmodule 2-to-4 Line Decoder  2-to-4 Line Decoder //2 to 4 line decoder module decoder_2_to_4_st_v(E_n, A0, A1, D0_n, D1_n, D2_n, D3_n); input E_n, A0, A1; output D0_n, D1_n, D2_n, D3_n; wire A0_n, A1_n, E; not g0(A0_n, A0), g1(A1_n, A1), g2(E,E_n); nand g3(D0_n,A0_n,A1_n,E), g4(D1_n,A0,A1_n,E), g5(D2_n,A0_n,A1,E), g6(D3_n,A0,A1,E); endmodule Dataflow Modeling  Dataflow modeling uses a number of operators that act on operands to produce desired results.  Verilog HDL provides about 30 operator types.  Dataflow modeling uses continuous assignments and the keyword assign.  A continuous assignment is a statement that assigns a value to a net.  The value assigned to the net is specified by an expression that uses operands and operators.  Dataflow Modeling (2) //Dataflow description of a 2-to-4-line decoder module decoder_df (A,B,E,D); input A,B,E; output [0:3] D; assign D = ~(~A & ~B & ~E), D = ~(~A & B & ~E), D = ~(A & ~B & ~E), D = ~(A & B & ~E); endmodule A 2-to-1 line multiplexer with data inputs A and B, select input S, and output Y is described with the continuous assignment assign Y = (A & S) | (B & ~S) Dataflow Modeling (3) //Dataflow description of 4-bit adder module binary_adder (A,B,Cin,SUM,Cout); input [3:0] A,B; input Cin; output [3:0] SUM; output Cout; assign {Cout,SUM} = A + B + Cin; endmodule //Dataflow description of a 4-bit comparator. module magcomp (A,B,ALTB,AGTB,AEQB); input [3:0] A,B; output ALTB,AGTB,AEQB; assign ALTB = (A < B), AGTB = (A > B), AEQB = (A == B); endmodule  Dataflow Modeling (4)  The addition logic of 4 bit adder is described by a single statement using the operators of addition and concatenation.  The plus symbol (+) specifies the binary addition of the 4 bits of A with the 4 bits of B and the one bit of Cin.  The target output is the concatenation of the output carry Cout and the four bits of SUM.  Concatenation of operands is expressed within braces and a comma separating the operands. Thus, {Cout,SUM} represents the 5-bit result of the addition operation.  Dataflow Modeling (5)  Dataflow Modeling provides the means of describing combinational circuits by their function rather than by their gate structure.  Conditional operator (?:) condition ? true-expression : false-expression;  A 2-to-1 line multiplexer assign OUT = select ? A : B; //Dataflow description of 2-to-1-line mux module mux2x1_df (A,B,select,OUT); input A,B,select; output OUT; assign OUT = select ? A : B; endmodule Behavioral Modeling  Behavioral modeling represents digital circuits at a functional and algorithmic level.  It is used mostly to describe sequential circuits, but can also be used to describe combinational circuits.  Behavioral descriptions use the keyword always followed by a list of procedural assignment statements.  The target output of procedural assignment statements must be of the reg data type.  A reg data type retains its value until a new value is assigned.  Behavioral Modeling (2)  The procedural assignment statements inside the always block are executed every time there is a change in any of the variable listed after the @ symbol. (Note that there is no “;” at the end of always statement) //Behavioral description of 2-to-1-line multiplexer module mux2x1_bh(A,B,select,OUT); input A,B,select; output OUT; reg OUT; always @(select or A or B) if (select == 1) OUT = A; else OUT = B; endmodule Behavioral Modeling (3) 4-to-1 line multiplexer  Behavioral Modeling (4) //Behavioral description of 4-to-1 line mux module mux4x1_bh (i0,i1,i2,i3,select,y); input i0,i1,i2,i3; input [1:0] select; output y; reg y; always @(i0 or i1 or i2 or i3 or select) case (select) 2'b00: y = i0; 2'b01: y = i1; 2'b10: y = i2; 2'b11: y = i3; endcase endmodule Behavioral Modeling (5)  In 4-to-1 line multiplexer, the select input is defined as a 2-bit vector and output y is declared as a reg data.  The always block has a sequential block enclosed between the keywords case and endcase.  The block is executed whenever any of the inputs listed after the @ symbol changes in value. Descriptions of Circuits  Structural Description – This is directly equivalent to the schematic of a circuit and is specifically oriented to describing hardware structures using the components of a circuit.  Dataflow Description – This describes a circuit in terms of function rather than structure and is made up of concurrent assignment statements or their equivalent. Concurrent assignments statements are executed concurrently, i.e. in parallel whenever one of the values on the right hand side of the statement changes. Descriptions of Circuits (2)  Hierarchical Description – Descriptions that represent circuits using hierarchy have multiple entities, one for each element of the Hierarchy.  Behavioral Description – This refers to a description of a circuit at a level higher than the logic level. This type of description is also referred to as the register transfers level.  2-to-4 Line Decoder – Data flow description //2-to-4 Line Decoder: Dataflow module dec_2_to_4_df(E_n,A0,A1,D0_n,D1_n,D2_n,D3_n); input E_n, A0, A1; output D0_n,D1_n,D2_n,D3_n; assign D0_n=~(~E_n&~A1&~A0); assign D1_n=~(~E_n&~A1& A0); assign D2_n=~(~E_n& A1&~A0); assign D3_n=~(~E_n& A1& A0); endmodule 4-to-1 Multiplexer  4-to-1 Multiplexer //4-to-1 Mux: Structural Verilog module mux_4_to_1_st_v(S,D,Y); input [1:0]S; input [3:0]D; output Y; wire [1:0]not_s; wire [0:3]N; not g0(not_s,S),g1(not_s,S); and g2(N,not_s,not_s,D), g3(N,S,not_s,D), g4(N,not_s,S,D), g5(N,S,S,D); or g5(Y,N,N,N,N); endmodule  4-to-1 Multiplexer – Data Flow //4-to-1 Mux: Dataflow description module mux_4_to_1(S,D,Y); input [1:0]S; input [3:0]D; output Y; assign Y = (~S&~S&D)|(~S&S&D) |(S&~S&D)| (S&S&D); endmodule //4-to-1 Mux: Conditional Dataflow description module mux_4_to_1(S,D,Y); input [1:0]S; input [3:0]D; output Y; assign Y = (S==2’b00)?D : (S==2’b01)?D : (S==2’b10)?D : (S==2’b11)?D:1’bx;; endmodule  4-to-1 Multiplexer //4-to-1 Mux: Dataflow Verilog Description module mux_4_to_1(S,D,Y); input [1:0]S; input [3:0]D; output Y; assign Y=S?(S?D:D):(S?D:D); endmodule Adder 4-bit Adder 4-bit-Adder 4-bit Adder //4-bit adder : dataflow description module adder_4bit (A,B,C0,S,C4); input [3:0] A,B; input C0; output [3:0]S; output C4; assign {C4,S} = A + B + C0; endmodule Sequential System Design  Sequential System Design (2) 1. Obtain either the state diagram or the state table from the statement of the problem. 2. If only a state diagram is available from step 1, obtain state table. 3. Assign binary codes to the states. 4. Derive the flip-flop input equations from the next- state entries in the encoded state table. 5. Derive output equations from the output entries in the state table. 6. Simplify the flip-flop input and output equations. 7. Draw the logic diagram with D flip-flops and combinational gates, as specified by the flip-flop I/O equations. Behavioral Modeling in SSD  There are two kinds of behavioral statements in Verilog HDL: initial and always.  The initial behavior executes once beginning at time=0.  The always behavior executes repeatedly and re- executes until the simulation terminates.  A behavior is declared within a module by using the keywords initial or always, followed by a statement or a block of statements enclosed by the keywords begin and end. Behavioral Modeling in SSD (2)  An example of a free-running clock initial begin clock = 1’b0; repeat (30); #10 clock = ~clock; end initial begin clock = 1’b0; #300 $finish; end always #10 clock = ~clock  Behavioral Modeling in SSD (3)  The always statement can be controlled by delays that wait for a certain time or by certain conditions to become true or by events to occur.  This type of statement is of the form: always @ (event control expression) Procedural assignment statements  The event control expression specifies the condition that must occur to activate the execution of the procedural assignment statements.  The variables in the left-hand side of the procedural statements must be of the reg data type and must be declared as such.  Behavioral Modeling in SSD (4)  The statements within the block, after the event control expression, execute sequentially and the execution suspends after the last statement has executed.  Then the always statement waits again for an event to occur.  Two kind of events:  Level sensitive (E.g. in combinational circuits and in latches) always @(A or B or Reset) will cause the execution of the procedural statements in the always block if changes occur in A or B or Reset.  Edge-triggered (In synchronous sequential circuits, changes in flip-flops must occur only in response to a transition of a clock pulse. always @(posedge clock or negedge reset)will cause the execution of the procedural statements only if the clock goes through a positive transition or if the reset goes through a negative transition.  Behavioral Modeling in SSD (5)  A procedural assignment is an assignment within an initial or always statement.  There are two kinds of procedural assignments: blocking and non-blocking  Blocking assignments (executed sequentially in the order they are listed in a sequential block) B = A C = B + 1  Non-blocking assignments (evaluate the expressions on the right hand side, but do not make the assignment to the left hand side until all expressions are evaluated.  B