An array declaration of a net or variable can be either scalar or vector. Any number of dimensions can be created by specifying an address range after the identifier name and is called a multi-dimensional array. Arrays are allowed in Verilog for reg, wire, integer and real data types.
reg y1 [11:0]; // y is an scalar reg array of depth=12, each 1-bit wide
wire [0:7] y2 [3:0] // y is an 8-bit vector net with a depth of 4
reg [7:0] y3 [0:1][0:3]; // y is a 2D array rows=2,cols=4 each 8-bit wide
An index for every dimension has to be specified to access a particular element of an array and can be an expression of other variables. An array can be formed for any of the different data-types supported in Verilog.
Note that a memory of n 1-bit reg is not the same as an n-bit vector reg.
Assignment
y1 = 0; // Illegal - All elements can't be assigned in a single go
y2[0] = 8'ha2; // Assign 0xa2 to index=0
y2[2] = 8'h1c; // Assign 0x1c to index=2
y3[1][2] = 8'hdd; // Assign 0xdd to rows=1 cols=2
y3[0][0] = 8'haa; // Assign 0xaa to rows=0 cols=0
Example
The code shown below simply shows how different arrays can be modeled, assigned and accessed. mem1 is an 8-bit vector, mem2 is an 8-bit array with a depth of 4 (specified by the range [0:3]) and mem3 is a 16-bit vector 2D array with 4 rows and 2 columns. These variables are assigned different values and printed.
module des ();
reg [7:0] mem1; // reg vector 8-bit wide
reg [7:0] mem2 [0:3]; // 8-bit wide vector array with depth=4
reg [15:0] mem3 [0:3][0:1]; // 16-bit wide vector 2D array with rows=4,cols=2
initial begin
int i;
mem1 = 8'ha9;
$display ("mem1 = 0x%0h", mem1);
mem2[0] = 8'haa;
mem2[1] = 8'hbb;
mem2[2] = 8'hcc;
mem2[3] = 8'hdd;
for(i = 0; i < 4; i = i+1) begin
$display("mem2[%0d] = 0x%0h", i, mem2[i]);
end
for(int i = 0; i < 4; i += 1) begin
for(int j = 0; j < 2; j += 1) begin
mem3[i][j] = i + j;
$display("mem3[%0d][%0d] = 0x%0h", i, j, mem3[i][j]);
end
end
end
endmodule
Simulation Log ncsim> run mem1 = 0xa9 mem2[0] = 0xaa mem2[1] = 0xbb mem2[2] = 0xcc mem2[3] = 0xdd mem3[0][0] = 0x0 mem3[0][1] = 0x1 mem3[1][0] = 0x1 mem3[1][1] = 0x2 mem3[2][0] = 0x2 mem3[2][1] = 0x3 mem3[3][0] = 0x3 mem3[3][1] = 0x4 ncsim: *W,RNQUIE: Simulation is complete.
Memories
Memories are digital storage elements that help store a data and information in digital circuits. RAMs and ROMs are good examples of such memory elements. Storage elements can be modeled using one-dimensional arrays of type reg and is called a memory. Each element in the memory may represent a word and is referenced using a single array index.
Register Vector
Verilog vectors are declared using a size range on the left side of the variable name and these get realized into flops that match the size of the variable. In the code shown below, the design module accepts clock, reset and some control signals to read and write into the block.
It contains a 16-bit storage element called register which simply gets updated during writes and returns the current value during reads. The register is written when sel and wr are high on the same clock edge. It returns the current data when sel is high and wr is low.
module des ( input clk,
input rstn,
input wr,
input sel,
input [15:0] wdata,
output [15:0] rdata);
reg [15:0] register;
always @ (posedge clk) begin
if (!rstn)
register <= 0;
else begin
if (sel & wr)
register <= wdata;
else
register <= register;
end
end
assign rdata = (sel & ~wr) ? register : 0;
endmodule
The hardware schematic shows that a 16-bit flop is updated when control logic for writes are active and the current value is returned when control logic is configured for reads.
Array
In this example, register is an array that has four locations with each having a width of 16-bits. The design module accepts an additional input signal which is called addr to access a particular index in the array.
module des ( input clk,
input rstn,
input [1:0] addr,
input wr,
input sel,
input [15:0] wdata,
output [15:0] rdata);
reg [15:0] register [0:3];
integer i;
always @ (posedge clk) begin
if (!rstn) begin
for (i = 0; i < 4; i = i+1) begin
register[i] <= 0;
end
end else begin
if (sel & wr)
register[addr] <= wdata;
else
register[addr] <= register[addr];
end
end
assign rdata = (sel & ~wr) ? register[addr] : 0;
endmodule
It can be seen in the hardware schematic that each index of the array is a 16-bit flop and the input address is used to access a particular set of flops.
The primary intent of data-types in the Verilog language is to represent data storage elements like bits in a flip-flop and transmission elements like wires that connect between logic gates and sequential structures.
What values do variables hold ?
Almost all data-types can only have one of the four different values as given below except for real and event data types.
| 0 | represents a logic zero, or a false condition |
| 1 | represents a logic one, or a true condition |
| x | represents an unknown logic value (can be zero or one) |
| z | represents a high-impedance state |
The following image shows how these values are represented in timing diagrams and simulation waveforms. Most simulators use this convention where red stands for X and orange in the middle stands for high-impedance or Z.
A structure can contain elements of different data types which can be referenced as a whole or individually by their names. This is quite different from arrays where the elements are of the same data-type.
// Normal arrays -> a collection of variables of same data type
int array [10]; // all elements are of int type
bit [7:0] mem [256]; // all elements are of bit type
// Structures -> a collection of variables of different data types
struct {
byte val1;
int val2;
string val3;
} struct_name;
Syntax
struct {
[list of variables]
} struct_name;
Unpacked Structures
A structure is unpacked by default and can be defined using the struct keyword and a list of member declarations can be provided within the curly brackets followed by the name of the structure.
Structure Example
module tb;
// Create a structure called "st_fruit"
// which to store the fruit's name, count and expiry date in days.
// Note: this structure declaration can also be placed outside the module
struct {
string fruit;
int count;
byte expiry;
} st_fruit;
initial begin
// st_fruit is a structure variable, so let's initialize it
st_fruit = '{"apple", 4, 15};
// Display the structure variable
$display ("st_fruit = %p", st_fruit);
// Change fruit to pineapple, and expiry to 7
st_fruit.fruit = "pineapple";
st_fruit.expiry = 7;
$display ("st_fruit = %p", st_fruit);
end
endmodule
Simulation Log
ncsim> run
st_fruit = '{fruit:"apple", count:4, expiry:'hf}
st_fruit = '{fruit:"pineapple", count:4, expiry:'h7}
ncsim: *W,RNQUIE: Simulation is complete.
What is the need to typedef a structure ?
Only one variable was created in the example above, but if there's a need to create multiple structure variables with the same constituents, it'll be better to create a user defined data type of the structure by typedef. Then st_fruit will become a data-type which can then be used to create variables of that type.
module tb;
// Create a structure called "st_fruit"
// which to store the fruit's name, count and expiry date in days.
// Note: this structure declaration can also be placed outside the module
typedef struct {
string fruit;
int count;
byte expiry;
} st_fruit;
initial begin
// st_fruit is a data type, so we need to declare a variable of this data type
st_fruit fruit1 = '{"apple", 4, 15};
st_fruit fruit2;
// Display the structure variable
$display ("fruit1 = %p fruit2 = %p", fruit1, fruit2);
// Assign one structure variable to another and print
// Note that contents of this variable is copied into the other
fruit2 = fruit1;
$display ("fruit1 = %p fruit2 = %p", fruit1, fruit2);
// Change fruit1 to see if fruit2 is affected
fruit1.fruit = "orange";
$display ("fruit1 = %p fruit2 = %p", fruit1, fruit2);
end
endmodule
Simulation Log
ncsim> run
fruit1 = '{fruit:"apple", count:4, expiry:'hf} fruit2 = '{fruit:"", count:0, expiry:'h0}
fruit1 = '{fruit:"apple", count:4, expiry:'hf} fruit2 = '{fruit:"apple", count:4, expiry:'hf}
fruit1 = '{fruit:"orange", count:4, expiry:'hf} fruit2 = '{fruit:"apple", count:4, expiry:'hf}
ncsim: *W,RNQUIE: Simulation is complete.
Packed Structures
A packed structure is a mechanism for subdividing a vector into fields that can be accessed as members and are packed together in memory without gaps. The first member in the structure is the most significant and subsequent members follow in decreasing order of significance.
A structure is declared packed using the packed keyword which by default is unsigned.
Example
// Create a "packed" structure data type which is similar to creating
// bit [7:0] ctrl_reg;
// ctrl_reg [0] represents en
// ctrl_reg [3:1] represents cfg
// ctrl_reg [7:4] represents mode
typedef struct packed {
bit [3:0] mode;
bit [2:0] cfg;
bit en;
} st_ctrl;
module tb;
st_ctrl ctrl_reg;
initial begin
// Initialize packed structure variable
ctrl_reg = '{4'ha, 3'h5, 1};
$display ("ctrl_reg = %p", ctrl_reg);
// Change packed structure member to something else
ctrl_reg.mode = 4'h3;
$display ("ctrl_reg = %p", ctrl_reg);
// Assign a packed value to the structure variable
ctrl_reg = 8'hfa;
$display ("ctrl_reg = %p", ctrl_reg);
end
endmodule
Simulation Log
ncsim> run
ctrl_reg = '{mode:'ha, cfg:'h5, en:'h1}
ctrl_reg = '{mode:'h3, cfg:'h5, en:'h1}
ctrl_reg = '{mode:'hf, cfg:'h5, en:'h0}
ncsim: *W,RNQUIE: Simulation is complete.
In complex testbenches some variable declarations might have a longer data-type specification or require to be used in multiple places in the testbench.
In such cases we can use a typedef to give a user-defined name to an existing data type. The new data-type can then be used throughout the code and hence avoids the need to edit in multiple places if required.
// Normal declaration may turn out to be quite long
unsigned shortint my_data;
enum {RED, YELLOW, GREEN} e_light;
bit [7:0] my_byte;
// Declare an alias for this long definition
typedef unsigned shortint u_shorti;
typedef enum {RED, YELLOW, GREEN} e_light;
typedef bit [7:0] ubyte;
// Use these new data-types to create variables
u_shorti my_data;
e_light light1;
ubyte my_byte;
Syntax
typedef data_type type_name [range];
Example
module tb;
typedef shortint unsigned u_shorti;
typedef enum {RED, YELLOW, GREEN} e_light;
typedef bit [7:0] ubyte;
initial begin
u_shorti data = 32'hface_cafe;
e_light light = GREEN;
ubyte cnt = 8'hFF;
$display ("light=%s data=0x%0h cnt=%0d", light.name(), data, cnt);
end
endmodule
Simulation Log ncsim> run light=GREEN data=0xcafe cnt=255 ncsim: *W,RNQUIE: Simulation is complete.
An event is a static object handle to synchronize between two or more concurrently active processes. One process will trigger the event, and another process waits for the event.
- Can be assigned or compared to other event variables
- Can be assigned to
null - When assigned to another event, both variables point to same synchronization object
- Can be passed to queues, functions and tasks
