Combinatorial Verilog

http://chiphack.org/talks/combinatorial-verilog/html/index.html#

Al Wood

Wires and net declarations

wire a, b;
wire y;
wire [1:0] a2, b2;

Inputs and outputs to and from modules are automatically declared as wires, unless they are subsequently declared as registers (next talk).

module mymod (
   input  [1:0] y2;
   output [2:0] y3;
  )

Logic bitwise primitives

Negation

assign y = ~a;

AND, OR and exclusive-OR gates

assign y = a & b;
assign y = a | b;
assign y = a ^ b;

Reduction

wire y;
wire [1:0] a2;
assign y = | a2;

is equivalent to:

wire y;
wire [1:0] a2;
assign y = a2[1] | a2[0];

Concatenation and Replication

wire y;
wire [1:0] y2;
wire [2:0] y3;

assign y2 = {a,b};            // creates a 2-bit signal of a with b
assign y2 = {a,1'b0};         // a with 1 bit binary 0 (constant)
assign y3 = {a,b,1'b1};       // a with b with binary 1 (constant)
assign y3 = {a,2'b10};        // a with 2 binary bits 1, 0
assign y3 = {a,a2};           // a with a2 (a2 is 2 bits)
assign y3 = {a,a2[0],1'b1};   // a with single bit from a2 with 1
assign {y2,y} = {y3[1:0],a};  // multiple assignment: creates y2 as
                              // 2 bits from y3 and y as a
assign y3 = {a,2{1'b1}};      // a with 2 lots of binary 1

Note use of // to introduce a comment.

Shifting

Both arithmetic and logic shifts are provided. The following table illustrates the difference.

a           a >> 2      a >>> 2     a << 2      a <<< 3
01001111    00010011    00010011    00111100    00111100
11001111    00110011    11110011    00111100    00111100

Examples:

assign y2 = a2 >> 1;    // Logical 0's shifted in
assign y2 = a2 >>> 1;   // Arithemtic MSB sign bit shifted in
assign y2 = a2 << 1;    // Logical shift left same result as
assign y2 = a2 <<< 1;   // Arithmetic shift left

Rotation

Rotate right 1 bit:

assign y4 = {y3[0],y3[2:1]};

Rotate right 2 bit:

assign y4 = {y3[1:0],y3[2]};

Conditional expressions

A tertiary operator:

assign max = (a > b) ? a : b;

Operator precedence

() [] :: .
+ - ! ~ & ~& | ~| ^ ~^ ^~ ++ -- (unary)
**
* / %
+ - (binary)
<< >> <<< >>>
< <= > >= inside dist
== != === !== =?= !?=
& (binary)
^ ~^ ^~ (binary)
| (binary)
&&
||
? : (conditional operator)
>
= += -= *= /= %= &= ^= |= <<= >>= <<<= >>>= := :/ <=
{} {{}}

Combinatorial always blocks

For use when simple expressions get too complicated

always @(*)
   a = b;

always @(*)
   begin
      a = b;
      y = a | b;
    end

SystemVerilog offers a more explicit format

always_comb
   a = b;

If/Else (1)

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

always_comb
   if(a < b)
      min = a;
   else
      min = b;

If/Else (2)

More generally:

always_comb
   if(boolean)
      begin     // need begin...end if >1 line of code within block
                // if code
      end
   else
      begin
                // else code
      end

Conditional example: 4-bit decoder

Given a 2 bit value input, set that bit in the output, but only if the enable input is asserted (high). Truth table:

en      a1      a2      y
0       -       -       0000
1       0       0       0001
1       0       1       0010
1       1       0       0100
1       1       1       1000

Conditional example: implementation

module decoder (
   input [1:0]  a,
   input        en,
   output [3:0] y
  )

   always_comb
      if (~en)
         y = 4'b0000;  // 4-bit wide, binary representation: 0000
      else if(a == 2'b00)
         y = 4'b0001;
      else if(a == 2'b01)
         y = 4'b0010;
      else if(a == 2'b10)
         y = 4'b0100;
      else
         y = 4'b1000;

endmodule

Conditional example: improved

module decoder (
   input [1:0]  a,
   input        en,
   output [3:0] y
  )

   always_comb
      case ({en,a})
         3'b000, 3'b001,3'b010,3'b011: y = 4'b0000;
         3'b100: y = 4'b0001;
         3'b101: y = 4'b0010;
         3'b110: y = 4'b0100;
         3'b111: y = 4'b1000;
      endcase    // {en,a}

endmodule

Uncertainty: X and Z values

X is the “don’t care” value. Specifiying this can make for more efficient synthesis, since the tools can choose whichever value is most efficient.

Z is the “high impedence” value, typically used for connections that can be both inputs and outputs, typically under the control of an enable signal.

assign y = (oen) ? a : 1'bz;

Decoder example using X

module decoder (
   input  [1:0] a,
   input        en,
   output [3:0] y
  )

   always_comb
      casex ({en,a})
         3'b0xx: y = 4'b0000;
         3'b100: y = 4'b0001;
         3'b101: y = 4'b0010;
         3'b110: y = 4'b0100;
         3'b111: y = 4'b1000;
      endcase    // {en,a}

endmodule

There is also casez.

Multiple assignment

always_comb
    if (en) y = 1'b0;

always_comb
    y = a & b;

Won’t synthesize because y is the output of two circuits which is contraditory. It should be written as:

always_comb
 if (en)
    y = 1'b0;
 else
    y = a & b;

Exercise 2

Start with button_led.v in the basic_verilog/button_led directory. Complete it so that the LED which is lit up depends on whether the button is pressed.

Build it with:

make button_led

Then experiment using both buttons, so that each combination of buttons lights a different LED. Use a case statement for this.