What approach would you use to formulate an SVA in System Verilog that guarantees a FIFO is cleared out before a read command is issued?

Question Analysis

The question is asking for a strategy to create a SystemVerilog Assertion (SVA) that ensures a First-In-First-Out (FIFO) buffer is empty before a read command is executed. SystemVerilog Assertions are used to validate the behavior of digital designs by specifying properties that must hold true. In this context, the candidate needs to focus on:

• Understanding FIFO behavior: A FIFO is a data structure where the first element added is the first one to be removed. It is crucial to ensure that no read operations are performed when the FIFO is empty to prevent errors.

• Using SystemVerilog Assertions: SVAs are used to specify and check the properties of the design in simulation. The candidate should be able to write assertions that can check the state of the FIFO and coordinate it with the read operations.

• Guaranteeing preconditions: The main task is to ensure that a read command is only issued when the FIFO is cleared, implying it has no pending data.

Answer

To formulate an SVA that guarantees a FIFO is cleared before a read command is issued, you can follow these steps:

1. Identify Conditions: You need to identify when the FIFO is cleared and when a read command is issued. Typically, a FIFO is cleared when its count is zero.

2. Create Assertion: Use SystemVerilog constructs to assert that the read command is only issued if the FIFO is empty.

Here's a simple example:

property fifo_clear_before_read;
  @(posedge clk)
  disable iff (reset)
  fifo_empty |-> !read_command;
endproperty

assert property (fifo_clear_before_read)
  else $error("Read command issued before FIFO is cleared.");

Explanation:

• property fifo_clear_before_read; defines the assertion.
• @(posedge clk) specifies that the assertion is checked on the rising edge of the clock.
• disable iff (reset) ensures that the assertion is not evaluated when the system is in reset.
• fifo_empty |-> !read_command; is the core of the assertion. This checks that whenever the FIFO is empty (fifo_empty is true), no read command (!read_command) is issued.
• assert property (fifo_clear_before_read) applies the assertion, and $error reports a violation if the property fails.

By using this assertion, you ensure that a read command is only executed when the FIFO is cleared, preventing erroneous operations.