🚦 What is an Immediate Assertion?#
Immediate assertions are assertions that are used inside procedural blocks (initial, always, always_ff, always_comb, etc.) and are evaluated immediately. They are used for combinational checks and to catch errors right away.
Example:
always_ff @(posedge clk) begin
assert (data_valid) else $error("Data not valid!");
end
This construct checks data_valid on every clock edge and prints an error message if it fails.
⏳ What is a Deferred Immediate Assertion?#
Deferred immediate assertions are also used inside procedural blocks but defer their check to the next delta cycle. This means the check is not done immediately but after the block finishes. This is useful for ensuring data consistency, especially with non-blocking assignments.
Example:
always_ff @(posedge clk) begin
x <= y; // non-blocking assignment
assert final(x == y); // deferred immediate assertion
// assert #0 (x == y); // deferred immediate assertion
end
Here, the final keyword marks it as a deferred immediate assertion, and the assertion is evaluated after the block completes (at the delta cycle).
Immediate vs. Deferred Immediate#
| Feature | Immediate | Deferred Immediate |
|---|---|---|
| Execution time | Inside the block | At the end of the block |
| Code compatibility | Blocking assignment | Non-blocking assignment |
| Use case | Quick check | Data consistency check |