12Implementation

Design a three-state detector from its contract

Problem

Contract. two_ones samples the 1-bit input din at every rising edge of clock. Its output seen is 1 while the last two values sampled since reset were both 1, and 0 otherwise. A run of three or more 1s keeps seen high; a single 0 drops it. seen is a Moore output: it depends on the state only, so it changes only at rising edges and never follows din inside a cycle. Reset is synchronous and active-high: after a reset edge no 1 has been seen yet and seen is 0.

Starting point. The starter has the ports and a placeholder that drives seen to 0. Everything else is yours: state names, state register, next-state logic and output logic. The public testbench below the marker is read-only.

Step 1: write the state table before any code

The machine needs to remember how many 1s in a row it has seen, counting no further than two. Name one state per count, for example ZERO, ONE and TWO, and fill in one row per state and input value:

Then use it to predict the seen column of the public run: pattern 0110_1110_1011, each line printed just after the edge that sampled that line's din.

Step 2: translate the table

Follow the three parts of the chapter: a state register written only in one always @(posedge clock) block with the reset branch first, a next-state always @* case with a default, and output logic that reads the state alone. Three states need two state bits; the fourth code is covered by the default.

Step 3: run, compare, and look at the waveform

Compare the printed seen column with your prediction line by line. In the waveform, check that seen changes only at rising edges, even where din changes in the middle of a cycle.

Completion check

• For every input pattern, seen after each rising edge is 1 exactly when the last two sampled values were 1.
• seen does not change between rising edges, whatever din does.
• Reset in the middle of a run of 1s starts the count again.
• Any correct encoding passes: binary, one-hot, or a saturating counter. Grading reports the first failing cycle after reset, whether it was checked before or after the edge, the expected and the actual seen.

Progressive hints

Transfer experiment

Change the contract to "three 1s in a row". How many states do you need, how many state bits, and which rows of the table change? Predict the new column for the public pattern before you run.