Async FIFO Full Detection
Detect the wraparound relationship that means the write pointer is one complete buffer ahead of the read pointer.
Task
Assert `full` when the next write Gray pointer equals the synchronized read Gray pointer with its top two bits inverted.
Examples
- next write catches synchronized read pointer after one wrap → full=1
Constraints
- Use the next write pointer, not the current one.
- Invert the top two Gray bits for full comparison.
- Parameterize address width.
What this problem is testing
A strong answer translates the requirement into synthesizable state and combinational logic, states reset and update semantics precisely, and handles boundary conditions rather than only the nominal case.
Interview habit: before coding, state reset behavior, the state that must be stored, and the conditions that update each output.
Verification plan
- Check reset or initialization before normal operation.
- Exercise the nominal sequence cycle by cycle.
- Test boundaries such as zero, maximum count, wraparound, full/empty, or simultaneous controls when they apply.
- Prove the design holds state when an enable or qualifying condition is absent.
Edge cases worth testing
- Write when full and read when empty.
- Simultaneous read and write.
- Pointer wraparound and transitions into and out of full/empty.
Common mistakes
- Incomplete combinational assignments that infer a latch.
- Blocking/non-blocking assignments that do not match combinational versus sequential intent.
- Parameter-width and boundary errors.
- Passing the happy path while missing reset, overflow, simultaneous operations, or illegal requests.
Interview follow-up questions
- How would you parameterize this design without introducing width or boundary bugs?
- What changes if reset timing or polarity changes?
Concepts to review
FIFO, full, Gray-code
Open Async FIFO Full Detection in the HDLForge interactive editor.
Related problems
- 2:1 Multiplexer — Easy
- Parameterized Counter — Easy
- Priority Encoder — Medium
- Synchronous FIFO — Hard
- Shift Register — Easy
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