332:437 Lecture 10 Verilog Language Details Parameters Blocking Assignment Operator Delay time units Arrays of registers FIFO Example Flip-flop descriptions Verilog Operators Summary Material from The Verilog Hardware Description Language, by Thomas and Moorby, Kluwer Academic Publishers, VHDL for Programmable Logic, by Kevin Skahill, Addison Wesley Longman. 2/19/2019 Thomas: Digital Systems Design Lecture 10

Thomas: Digital Systems Design Lecture 10 Parameters Parameters – hold value that cannot change within design description #(parameter width = 8, delay=10) assign #(delay) xout <= xin1 ^ xin2; 2/19/2019 Thomas: Digital Systems Design Lecture 10

Blocking Assignment Operator Can be used everywhere, including in functions & tasks MAY NOT BE SYNTHESIZED INTO WIRES OR MEMORY ELEMENTS Blocking assignment –- immediate, not scheduled, holds only 1 value at a time = is the blocking assignment operator 2/19/2019 Thomas: Digital Systems Design Lecture 10

Thomas: Digital Systems Design Lecture 10 8-Bit AND Gate AND gate code that works properly Does not declare variable “i” – when “i” used in a loop, it gets implicitly declared module will_work (); wire temp, bit; always @(a_bus) begin temp = 1; for (i = 7; i >= 0; i = i - 1) temp = a_bus (i) and temp; end endmodule 2/19/2019 Thomas: Digital Systems Design Lecture 10

Thomas: Digital Systems Design Lecture 10 Time Units for #delay Specify by: `timescale <time_unit> / <time_precision> Example: `timescale 10 ns / 1 ns // Each time unit is // 10 ns, maintained to a precision of 1 ns Unit of Measurement Abbreviation seconds s milliseconds ms microseconds us nanoseconds ns picoseconds ps femtoseconds fs 2/19/2019 Thomas: Digital Systems Design Lecture 10

Composite Types -- Memories Arrays of registers reg [0:3] table8xr4 [0:7]; initial begin table8xr4 = {b’000_0, b’001_1, b’010_1, b’011_0, b’100_1, b’101_0, b’110_0, b’111_1}; end 2/19/2019 Thomas: Digital Systems Design Lecture 10

Thomas: Digital Systems Design Lecture 10 Arrays Can insert underscore between any two adjoining digits in the array values Hexadecimal and octal fill bit arrays with bits Three equivalent statements: a <= 2’x7A; // bit string hex “01111010” a <= 3’o172; // octal (base 8) a <= 8’b01111010; // binary x and z single digit values automatically fill out to entire width of number When fewer binary, octal, or hexadecimal digits are specified than the width of the number, the unspecified leftmost digits are set to 0 2/19/2019 Thomas: Digital Systems Design Lecture 10

Thomas: Digital Systems Design Lecture 10 Verilog Attributes Give some property of a signal full_case – all case items are specified explicitly or by default Causes case statement to be considered to be full, even though all cases are not specified Unspecified cases are treated as don’t cares for synthesis, and no latch is inferred parallel_case – Verilog case statement is allowed to have overlapping case items Statements for items are executed in the order specified Leads to complex logic – a priority encoder Parallel_case means that the synthesis tool assumes that there are no overlapping items in the case statement, and it will implement it as a sum-of-products expression with a MUX 2/19/2019 Thomas: Digital Systems Design Lecture 10

Verilog Attribute Example module synAttributes (output reg f, input a, b, c); always @(*) (* full_case, parallel_case *) case ({a, b, c}) 3’b001: f = 1’b1; 3’b010: f = 1’b1; 3’b011: f = 1’b1; 3’b100: f = 1’b1; 3’b110: f = 1’b0; 3’b111: f = 1’b1; endcase endmodule 2/19/2019 Thomas: Digital Systems Design Lecture 10

Thomas: Digital Systems Design Lecture 10 Verilog FIFO Example Concurrent statements – lie outside of process, so order of execution is unimportant Creates an aggregate of correct width to match fifo (i) width – Decimal 0 is a convenient shorthand for an arbitrarily wide vector of zeroes 2/19/2019 Thomas: Digital Systems Design Lecture 10

Thomas: Digital Systems Design Lecture 10 FIFO module fifo (clk, rst, oe, rd, wr, rdinc, wrinc, rdptrclr, wrptrclr, data_in, data_out); parameter wide = 31, deep = 20, counter = 5; input clk, rst, oe, rd, wr, rdinc, wrinc, rdptrclr, wrptrclr; input [wide:0] data_in, output [wide:0] data_out; reg [wide:0] data_outx; wire negrst; reg [wide:0] fifo [deep:0]; reg [counter:0] wrptr; reg [counter:0] rdptr; reg [counter:0] realrdptr; reg [counter:0] realrdptrb; reg [counter:0] i; 2/19/2019 Thomas: Digital Systems Design Lecture 10

Thomas: Digital Systems Design Lecture 10 FIFO (continued) // fifo register array always @(posedge rst or posedge clk) begin if (rst == 1'b1) begin for (i = 0; i < deep; i = i + 1) begin fifo [i] <= 'b0; end end else begin if (wr == 1'b1) fifo [wrptr] <= data_in; end end 2/19/2019 Thomas: Digital Systems Design Lecture 10

Thomas: Digital Systems Design Lecture 10 FIFO (continued) // read pointer always @(posedge rst or posedge clk) begin if (rst == 1'b1) rdptr <= 32'b0; else if (clk == 1) begin if (rdptrclr == 1'b1) rdptr <= 'b0; else if (rdinc == 1'b1) rdptr <= realrdptr + 1; end 2/19/2019 Thomas: Digital Systems Design Lecture 10

Thomas: Digital Systems Design Lecture 10 FIFO (continued) // force Synopsys to give us a rdptr flip-flop register // this had to be coded structurally, because Synopsys // design_analyzer refused to create a flip-flop for rdptr GTECH_FD2 (rdptr [0], clk, negrst, realrdptr [0], realrdptrb [0]), (rdptr [1], clk, negrst, realrdptr [1], realrdptrb [1]), (rdptr [2], clk, negrst, realrdptr [2], realrdptrb [2]), (rdptr [3], clk, negrst, realrdptr [3], realrdptrb [3]), (rdptr [4], clk, negrst, realrdptr [4], realrdptrb [4]), (rdptr [5], clk, negrst, realrdptr [5], realrdptrb [5]); 2/19/2019 Thomas: Digital Systems Design Lecture 10

Thomas: Digital Systems Design Lecture 10 FIFO (continued) // write pointer always @(posedge rst or posedge clk) begin if (rst == 1'b1) wrptr <= 32'b0; else if (clk == 1) begin if (wrptrclr == 1'b1) wrptr <= 'b0; else if (wrinc == 1'b1) wrptr <= wrptr + 1; end end 2/19/2019 Thomas: Digital Systems Design Lecture 10

Thomas: Digital Systems Design Lecture 10 FIFO (concluded) // three-state control of outputs always @(oe) begin if ((oe == 1'b1) && (rd == 1’b1)) data_outx <= fifo [wrptr]; else data_outx <= 'bz; end assign data_out = data_outx; endmodule 2/19/2019 Thomas: Digital Systems Design Lecture 10

Synthesized Priority Encoder (*parallel_case*) case({w, x, y, z}) 4’b1xxx: j = a; 4’bx1xx: j = b; 4’bxx1x: j = c; 4’bxxx1: j = d; default: j = 0; endcase 2/19/2019 Thomas: Digital Systems Design Lecture 10

Thomas: Digital Systems Design Lecture 10 Level Sensitive Latch always @(clk, d) begin if (clk == 1’b 1) q <= d; end 2/19/2019 Thomas: Digital Systems Design Lecture 10

Thomas: Digital Systems Design Lecture 10 T Flip-Flop module tff_logic (input t, input clk, output q); always @(posedge clk) begin if (t == 1’b1) q <= not (q); else q <= q; end endmodule 2/19/2019 Thomas: Digital Systems Design Lecture 10

8-bit Register Description module reg_logic (input [0:7] d, input clk, output [0:7] q); always @(posedge clk) begin q <= d; end endmodule 2/19/2019 Thomas: Digital Systems Design Lecture 10

Alternate Clocking Descriptions Give up now on writing a single process for design_analyzer with events on both the rising and falling clock edges It will never let you do it Instead, write this as two separate processes Moral: Only a subset of the legal Verilog code can be synthesized by design_analyzer 2/19/2019 Thomas: Digital Systems Design Lecture 10

8-bit Register, Asynchronous Reset, Synchronous Preset module reg_logic (input d [0:7], input reset, input init, input clk, output q [0:7]); always @(posedge clk, posedge reset) begin if (reset == 1’b1) q <= 8’b0; else if (init == 1’b1) q <= 8’b11111111; // decimal -1 q <= d; end endmodule 2/19/2019 Thomas: Digital Systems Design Lecture 10

Problems with Don’t Cares Synthesis treats x as a 1 that cannot occur Real hardware never has signals with x Comparing x to 0 or 1 with === operator always evaluates to false WHY? Because 0 or 1 does not EXACTLY match x 2/19/2019 Thomas: Digital Systems Design Lecture 10

Verilog Relational Operators Obvious: ==, !=, <, <=, >, >= Unknown (x) or high-impedance (z) values are treated as 0 Case equality: === Unknown (x) or high-impedance (z) values must match exactly Case inequality: !== 2/19/2019 Thomas: Digital Systems Design Lecture 10

Concatenation Operator Collects multiple signals into an n-bit value {a, b, c, d} Aggregates 4 signals a, b, c, d into a 4-bit value in the order specified 2/19/2019 Thomas: Digital Systems Design Lecture 10

Thomas: Digital Systems Design Lecture 10 Boolean Operators Logical negation ! Logical AND && Logical OR || Bitwise negation ~ Bitwise AND & Bitwise | Bitwise XOR ^ Equivalence ^~ or ~^ 2/19/2019 Thomas: Digital Systems Design Lecture 10

Advanced Boolean Operators Single bit AND of all operand bits & Single bit OR of all operand bits | Single bit NAND of all operand bits ~& Single bit NOR of all operand bits ~| Single bit XOR of all operand bits ^ Single bit XNOR of all operand bits ~^ Left Shift << Right shift >> Arithmetic shift left <<< Arithmetic shift right >>> Conditional as in the C language ?: Convert to signed $signed (m) Convert to unsigned $unsigned (m) 2/19/2019 Thomas: Digital Systems Design Lecture 10

Thomas: Digital Systems Design Lecture 10 Arithmetic Operators Adding + Subtracting - Multiplying * Dividing / Often refuses to synthesize hardware for this Power ** Modulus % 2/19/2019 Thomas: Digital Systems Design Lecture 10

Thomas: Digital Systems Design Lecture 10 Summary Parameters Blocking Assignment Operator Delay time units Arrays of registers FIFO Example Flip-flop descriptions Verilog Operators 2/19/2019 Thomas: Digital Systems Design Lecture 10