multiple value changes for the same signal at the same timestamp

[raczben]

I tried to parse this file:
https://raw.githubusercontent.com/Akashay-Singla/RISC-V/refs/heads/main/Pipeline/datapath_log.vcd

The pipeline_datapath.Alu_output signal (with id A) is defined at the 53th line.

This signal is changed multiple times at 1300 time, in the 669th line and the 677th line. In the parsed data there are two changes but both with the value of the first change, aka.: b0000000000000000000000000000000000000000000000000000000000000100.

The desired behavior is to get both changes in the same time-slot with the two different values.

System info:

• Python 3.12.3
• pywellen==0.20.2
• Ubuntu 24.04.3 LTS

[ekiwi]

The desired behavior is to get both changes in the same time-slot with the two different values.

Generally, that would be the expected behavior. Can you post a snippet of code illustrating how you are retrieving the values? There might be a bug in wellen or it may be due to a clunky API.

Thanks for all your reports!

[raczben]

Thanks for fixing. :)

Here is my code:

#!/usr/bin/env python3


import pywellen as pw
import json
import sys

def extract_all_vars(top_scopes, hier):
    vars = []
    for sc in top_scopes:
        for var in sc.vars(hier):
            vars.append(var)
        vars.extend(extract_all_vars(sc.scopes(hier), hier))
    return vars

file = "temp.vcd"
# parsing...
vcd = pw.Waveform(file)

jsstruct = {}
signals = []

for var in extract_all_vars(vcd.hierarchy.top_scopes(), vcd.hierarchy):
    # create a json structure:
    fullname = var.full_name(vcd.hierarchy)
    signals.append({
        "hierarchy": fullname.split(".")[:-1],
        "name": var.name(vcd.hierarchy),
        "width": var.bitwidth(),
        "type": var.var_type().lower(),
        "direction": var.direction(),
        "length": var.length(),
        "enum_type": var.enum_type(vcd.hierarchy),
        "vhdl_type_name": var.vhdl_type_name(vcd.hierarchy),
        "is_real": var.is_real(),
        "is_string": var.is_string(),
        "is_bit_vector": var.is_bit_vector(),
        "is_1bit": var.is_1bit(),
    })
    # get the value changes for the signal
    ch = vcd.get_signal_from_path(fullname).all_changes()
    wave = []
    # print (list(ch)[0:3])
    for time, value in ch:
        # convert value to string if it's an int
        if isinstance(value, int):
            # convert to binary string with leading zeros according to the width of the signal
            value = format(value, f'0{var.bitwidth()}b')
        wave.append({"time": time, "bin": value})
    signals[-1]["wave"] = wave

jsstruct["signals"] = signals

#save the json structure to a file
jsstruct["signals"].sort(key=lambda x: (x["hierarchy"], x["name"]))
with open(".wellenout.json", "w") as f:
    json.dump(jsstruct, f, indent=2, sort_keys=True)

[ekiwi]

Thanks! I did not write the pywellen code myself, so I am not super familiar with it.

It looks like pywellen implements its own custom SignalChangeIter class, which -- most likely -- has a bug when it comes to handling delta changes correctly.
More precisely, the hard coded 0 as parameter to get_value_at means that you only ever get the first change:

wellen/pywellen/src/lib.rs

Line 437 in 4c40f7e

.map(|data_offset| self.signal.get_value_at(&data_offset, 0));

Are you interested in making a PR to fix this?
Here is the corresponding code for the Rust implementation:

wellen/wellen/src/signals.rs

Line 341 in 4c40f7e

impl<'a> Iterator for SignalChangeIterator<'a> {