Async FIFO Binary Pointers
Track independent write and read positions, including the wrap bit used by full detection.
Task
Implement `(ADDR_W+1)`-bit write and read binary pointers. Increment `wbin` only on an accepted write and `rbin` only on an accepted read.
Examples
- w_en && !full → wbin increments
- r_en && !empty → rbin increments
Constraints
- Write pointer is clocked only by wclk.
- Read pointer is clocked only by rclk.
- The extra MSB tracks wraparound.
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.
- A transition close to the destination clock edge.
- A pulse shorter than a destination clock period.
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, pointer, CDC
Open Async FIFO Binary Pointers 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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