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# Cadence / DSD 4 / ver_l2

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Operators

** ... Power Operator (i.e. 2**8 returns 256)

= = = .. Inputs Equal including X and Z (simulation only)

! = = .. Inputs Not Equal including X and Z (simulation only)

<<< ... Left shift and maintain sign bit

>>> ... Right shift and maintain sign bit

Operands

• Numbers

• Wires and registers (wire, reg)

- Bit-selects

wire [7:0] a,b,c;

assign c[0] = a[0] & b[0];

• Part-selects

assign c[7:0] = a[7:0] & b[7:0]

• Function calls

Verilog allows you to call one function from inside an expression and use the return value from the called function as an operand.

assign error = legal(in1, in2);

Concatenation of Operands

wire [7:0] byte;

wire [3:0] nibble1, nibble2;

assign byte = {nibble1,nibble2};

Functional Descriptions

• Sequential Constructs

Sequential Statements

x = b;

if (y) x = x + a;

Equivalent Combinational Description

if (y) x = b + a; else x = b;

Function Declarations

A user defined function is a set of Verilog statements that can be called from elsewhere within the body of the code by an assignment. A funcion can have multiple inputs however can return only a single output. No timing information can be specified within a function.

• Input declarations

• Output from a function

• Register declarations

• Memory declarations

• Parameter declarations

• Integer declarations

function [<lower>:<upper>] <output_name> ;

input <name>;

begin

<statements>

end

endfunction

Example

function [9:0] gray_encode;

input [9:0] binary_input;

begin

gray_encode[9] = binary_input[9];

for (k=8; k>=0; k=k-1) begin

gray_encode[k] = binary_input[k+1] ^ binary_input[k];

end

end

endfunction

Integer variables are local or global variables that hold numeric values. The syntax for an integer declaration is

integer identifier_list;

You can declare integer variables locally at the function level or globally at the module level. The default size for integers is 32 bits.

Function Statements

• Procedural assignments

Procedural assignments are assignment statements used inside a function. The left side of a procedural assignment can contain only reg variables and integers.

sum = a + b;

control[5] = (instruction == 8’h2e);

{carry_in, a[7:0]} = 9’h 120;

• RTL assignments

RTL Nonblocking Assignments

module rtl (clk, data, regc, regd);

input data, clk;

output regc, regd;

reg regc, regd;

always @(posedge clk)

begin

regc <= data;

regd <= regc;

end

endmodule

Blocking Assignment

module rtl (clk, data, rega, regb);

input data, clk;

output rega, regb;

reg rega, regb;

always @(posedge clk)

begin

rega = data;

regb = rega;

end

endmodule

begin...end block statements

begin : block_name

reg local_variable_1;

integer local_variable_2;

parameter local_variable_3;

... statements ...

end

if...else statements

if ( e xpr )

begin ... statements ... end

else

begin ... statements ... end

case, casex, and casez statements

case ( expr )

case_item1: begin ... statements ... end

case_item2: begin ... statements ... end

default: begin ... statements ... end

endcase

Example

case (<input>)

2'b00 : <output> <= 4'b0001;

2'b01 : <output> <= 4'b0010;

2'b10 : <output> <= 4'b0100;

2'b11 : <output> <= 4'b1000;

default : <output> <= 4'b0000;

endcase

for loops

for (index = low_range; index < high_range; index = index + step)

for (index = high_range; index > low_range; index = index - step)

for (index = low_range; index <= high_range ;index = index + step)

for (index = high_range; index >= low_range ;index = index - step)

Ex: for ( i=0; i < 8; i=i+1 ) c[i] = a[i] & b[7-i];

while loops

while (<condition>) begin <statement>; end

forever loops

forever begin <statement>; end

disable statements

disable <loop_identifier>;

Task statements are similar to functions, but task can have any number of inputs, outputs and inouts as well as contain timing information.

input <input_name>;

<more_inputs>

output <output_name>;

<more_outputs>

begin <statements>; end

Only reg variables can receive output values from a task; wire variables cannot.

always Blocks

An always block can imply latches or flip-flops, or it can specify purely combinational logic. An always block can contain logic triggered in response to a change in a level or the rising or falling edge of a signal.

always @ ( event-expression [or event-expression*] )

begin ... statements ... end

• A change in a specified value

always @ ( identifier ) begin ... statements ... end

• The rising/ falling edge of a clock

always @ ( posedge event ) begin ... statements ... end

always @ ( negedge event ) begin ... statements ... end

Accumulator

parameter ACC_SIZE=<accumulator_width>;

reg [ACC_SIZE-1:0] <accumulate_out>;

always @ (posedge <clock> or posedge <reset>)

if (<reset>)

<accumulate_out> <= 0;

else if (<clock_enable>)

<accumulate_out> <= <accumulate_out> + <accumulate_in>;

assign <sum> = <a_input> + <b_input>;

Comparator

reg <output>;

always @(posedge <clock> or posedge <reset>)

if (<reset>)

<output> <= 1'b0;

else if (<input1> > <input2>)

<output> <= 1'b1;

else

<output> <= 1'b0;

Counter

reg [<upper>:0] <reg_name>;

always @(posedge <clock> or posedge <reset>)

if (<reset>)

<reg_name> <= 0;

else if (<clock_enable>)

if (<up_down>)

<reg_name> <= <reg_name> + 1;

else

<reg_name> <= <reg_name> - 1;

Decoder

reg [3:0] <output>;

<reg_or_wire> [1:0] <input>;

always @(posedge <clock> or posedge <reset>)

if (<reset>)

<output> <= 4'h0;

else

case (<input>)

2'b00 : <output> <= 4'b0001;

2'b01 : <output> <= 4'b0010;

2'b10 : <output> <= 4'b0100;

2'b11 : <output> <= 4'b1000;

default : <output> <= 4'b0000;

endcase

Encoder

reg [1:0] <output>;

<reg_or_wire> [3:0] <input>;

always @(posedge <clock> or posedge <reset>)

if (<reset>)

<output> <= 2'b00;

else

case (<input>)

4'b0001 : <output> <= 2'b00;

4'b0010 : <output> <= 2'b01;

4'b0100 : <output> <= 2'b10;

4'b1000 : <output> <= 2'b11;

default : <output> <= 2'b00;

endcase

Multiplexer

always @(<2-bit_select>, <input1>, <input2>, <input3>, <input4>)

case (<2-bit_select>)

2'b00: <output> = <input1>;

2'b01: <output> = <input2>;

2'b10: <output> = <input3>;

2'b11: <output> = <input4>;

endcase

Register

always @(posedge <clock> ) begin

if (!<reset>) begin

<reg> <= 1'b0;

end

else if (<clock_enable>) begin

<reg> <= <signal>;

end

Shift Register

parameter piso_shift = <shift_width>;

reg [piso_shift-2:0] <reg_name>;

reg <output>;

always @(posedge <clock> or negedge <reset>)

if (!<reset>) begin

<reg_name> <= 0;

<ouput> <= 1'b0;

end

<reg_name> <= <input>[piso_shift-1:1];

<ouput> <= <input>[0];

end

else if (<clock_enable>) begin

<reg_name> <= {1'b0, <reg_name>[piso_shift-1:1]};

<output> <= <reg_name>[0];

end

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