svCheatSheet

Table of Contents

• Procedural Models
  • Functional Models
  • Behavioral Models
  • Process Statements
    • Initial Statements
    • Always_comb
    • Always_latch

Combinational Logic Modeling

• There are 3 kinds modeling for combinational logic:
  • Structural (gate primitive intrinsics)
  • Dataflow (continuous assignments)
  • Behavioral (procedural blocks)

Procedural Models

• Come in two flavors, functional or behavioral.
• execute sequentially like code.
• Describe synthesizeable functional blocks or testbenches
• Are described by process statements

Functional Models

• As name implies, think functions.
• Run in 0 sim time and must have no timing specifications.
• Only kind of procedural models that are synthesizeable.

Behavioral Models

• Allow for a sequence of scheduled (timing wise) actions
• Allow for timing specifications, ex:
  • #(time delay)
  • @(event control) such as a clock
  • wait(some time)

Process Statements

• Always concurrent
• Typically used ones are:
  • always_comb, combinational logic
  • always_ff, edge triggered logic
  • always_latch, level sensitive. The only time you want a latch is when you are intentional describing one.
  • initial, executed first at sim time 0. Used to initialize default values
  • final, executed as the last block of the simulation

Initial Statements

• Run once and only once at sim start. At sim time 0.
• However there is a caveat, so called initial forever statements:

    initial
    begin
        vIf.clk = 1'b0;
        forever #20 vIf.clk = ~vIf.clk;
    end

These blocks are used for simulation constructs such as clocks

Always_comb

• All the variables used on the RHS are automatically part of the block’s sensitivity list. Just as if you have done an always(*).
• Variables that are passed into functions that are called inside the block are also part of the sensitivity list.
• Variables that are written to inside any process block cannot be written to by another.
• always_comb blocks will always run at the start of a sim, sim time 0, after initial processes.
• Cannot contain time control statements such as #(time delay), @(event control), or wait statements

Always_latch

In the very rare instances you want a latch, use always_latch. It follows similar sensitivity list rules as always_comb.