15 / 24 · Concept
Diagnose reset and counting errors
Check the initial value, reset and first edge in order.
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Learning goals
- Build an expected-state trace before observing results
- Distinguish reset, initialization and bit-order mistakes
The first edge anchors the trace
The provided lab starts at 11 and resets at the first rising edge. After reset is released, the predicted observed sequence is 00, 01, 10, 11, 00. Assuming it began at 00 can shift your edge count.
The run settings apply reset for the first cycle in this lab. Check that mechanism before adding rst to the input JSON. Keep the source of a signal clear instead of controlling it from two places.
| Symptom | First check |
|---|---|
| Never leaves 00 | Reset remains asserted? |
| Sequence starts one value late | Initial state and first reset edge |
| 01 and 10 appear swapped | Bit order in values and display |
| Stops at 11 | RTL contains a hold condition? |
Change one condition per comparison
Change initialization and reset duration in separate experiments. Changing both makes differences harder to explain. Keep the source, settings and expectations together for comparison.
If the output differs from your prediction, trace only as far as the first mismatch. Calculate the next value from count and rst at that edge before considering the rest of the waveform.
Try it yourself
If count is 00 after every rising edge, should you change the addition first?
Read the explanation
First check whether rst stays high and whether clock edges occur. With reset asserted, the correct code repeatedly stores 00.