First passing simulation

Run the starter's test, read the output line by line, and open the waveform.

Student 2 min read · Updated 2026-10-05

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        Before you startRun the testRead the outputWhat a pass provesOpen the waveformIf it did not passNext
      

A new project passes its test without any change. This page shows what "passed" looks like, so you recognise it and know what it proves.

Before you start#

You need a project. If you do not have one, follow First project.

Run the test#

In the Workspace terminal:

cd ~/project/mychip
nelix test

Or press Simulate on its card on Overview. Both start the same run.

Read the output#

The last lines of a passing MAC-8 run look like this:

text

    95.00ns INFO     cocotb.top          All tests passed
    95.00ns INFO     cocotb.regression   test_top.test_mac_8bit passed
                                         ** TESTS=1 PASS=1 FAIL=0 SKIP=0 **

Part	Meaning
`95.00ns`	Simulated time when the line was printed. The test ran the design for 95 nanoseconds
`test_top.test_mac_8bit passed`	The test function named `test_mac_8bit`, in `tb/test_top.py`, finished with every check holding
`TESTS=1 PASS=1 FAIL=0`	One test ran, and it passed

The run also gets an id, such as TST-20261005-0011. Use it to find this run again on the Runs page.

What a pass proves#

It proves that the test and the design agree. For MAC-8 the test multiplies pairs of signed 8-bit numbers and checks the running total after each one, including the largest and smallest values and a reset.

It does not prove the design is right in cases the test never tries. That is why a test is worth reading. Open tb/test_top.py and look at what it checks.

Open the waveform#

On the Simulate card choose See the waveform, or run nelix waveform.

Follow the acc row from left to right. It starts at 0, becomes 0000c (12, which is 3 times 4), then grows with each product. The full table of values is in Waveforms, reports and artefacts.

The MAC-8 design, simulated with the values the starter test uses, shown in decimal. acc is 12 after 3 x 4, then 42, 28 and 43. It holds 43 while en is low, and returns to 0 when rst_n goes low. Recorded from a real simulation.

If it did not pass#

A fresh starter always passes. If yours did not, the most likely cause is that the file was edited. See When a run fails.

Next#

Change one line of rtl/top.sv, run the test again, and watch it fail. That is the fastest way to learn what the test protects.
Go on to Synthesize and build the chip.

SimulateRun the test against your design and read the result.
Waveforms, reports and artefactsOpen the waveform, the schematic, the layout and the chip report, and read a run's log.

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