SPI Transfer Bit Counter
Track how many sample edges have occurred in a configurable transfer.
Task
Load bit_count=0 on start. Increment on sample_edge. Pulse done when the final bit index WORD_BITS-1 is sampled.
Examples
- Implement the required protocol behavior and preserve it across the stated timing/control conditions.
Constraints
- Use synthesizable SystemVerilog and synchronous state/control logic unless the task explicitly requires edge detection or open-drain behavior.
What this problem is testing
FPGA problems test whether logically correct RTL will behave predictably in hardware, including clock-domain boundaries, reset release, noisy inputs, inferred resources, and timing.
Interview habit: before coding, state reset behavior, the state that must be stored, and the conditions that update each output.
Verification plan
- Separate functional checks from implementation concerns such as CDC and timing.
- Test asynchronous or noisy inputs with realistic transitions when relevant.
- Confirm reset assertion and release do not create unintended pulses or state.
- Review whether the RTL infers the intended FPGA resource.
Edge cases worth testing
- First and last sampled bit.
- Chip-select boundaries and the configured sampling edge.
- Maximum value and wraparound.
- Enable low while the counter would otherwise advance.
- Reset coincident with another control.
Common mistakes
- Treating an asynchronous input as synchronous.
- Using a simple synchronizer for a pulse or multi-bit bus when a different CDC structure is needed.
- Generating clocks in ordinary RTL when a clock enable would be safer.
- Ignoring implementation timing because behavioral simulation passes.
Interview follow-up questions
- What hardware-specific issue could make a simulation-only solution unreliable?
- What timing or CDC check would you review before sign-off?
Concepts to review
SPI, counter, word length
Open SPI Transfer Bit Counter in the HDLForge interactive editor.
Related problems
- Parameterized Counter — Easy
- Programmable Clock Divider — Medium
- Button Debouncer — Easy
- Traffic Light FSM — Easy
- Binary to Gray Counter — Medium
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