Possibility of analog output on Joulescope, for use with logic analyzer

Technical Discussion
1

Hi there Joulescope team,

I would like to drop the question whether it would be possible/feasible/sense-making to add some kind of analog output on the Joulescope, linearly related to the current value. So, instead of just spitting out the digital values of the current measured, also produce a (timed) analog output representative of the current measurement.

The scope behind this is: I work in the IoT industry, we’re developing low-consumption SOCs. When debugging HW or FW we use logic analyzers to track the digital signals (and some analog voltages) and the JS in order to track the current trace. The obvious problem is that, this way, we can’t have the same-time reference and same display representation.

So, if you could have a real-time voltage analog output representation of the current measured we could just hook it up on one of the Saleae channels and have everything in one plot. That would be a dream…

Not too optimistic on the feasibility of this though, I admit…

Many thanks
Thanos

2

Hi @thanos_vgenis , and thanks for the description and context.

I think that there are a few ways to accomplish this:

  1. Add a digital-to-analog converter (DAC) option. There are several approaches:
    1a. For low-latency (~10 µs), the samples would need to come directly from the Joulescope sensor-side FPGA. Requires new Joulescope hardware.
    1b. For higher latency (~1 ms), it could be on the host-side. Requires new Joulescope hardware.
    1c. For even higher latency (~100 ms), it could be a separate USB device. Could work with the JS220. The latency could probably be controlled to be nearly constant, and could easily be “removed” in the host Saleae software if they were to add a “delay compensation” feature. I don’t see this option in the Logic software today, and I am not sure if this is possible with an “Extension”.

  2. Integrate Joulescope support into the Saleae Logic software. The Joulescope host-side software is open source. For this, Saleae could integrate the joulescope_driver (C source code) into their software, and then added controls for Joulescope support. Joulescope samples arrive with UTC timestamps. With our next release, these should be accurate to better than 100 µs, typical.

  3. Record data from the Joulescope and the logic analyzer. Include a digital input timing signal in both the Joulescope and logic analyzer recording. Post-process the recordings to align them.
    3a. We could try to use the Saleae Logic software to view the unified file. The Joulescope JLS file format is open source with python bindings. I found this on the Saleae .sal file format, which is not open source. Some hacking required to add the Joulescope current waveform to the Saleae capture file.
    3b. Merge both files into a format support by Sigrok and use Sigrok to view the data.

What do you think?

3

Hi there Matt,

thanks for the extensive reply.

I’d go for solutions 1a or 2.

Solution ‘3’ can be cumbersome, some log files are very long, triggering is an issue…we’re trying to simplify rather than complicate.

1a is just amazing, it’s all on you, your effort, your feature. I think there is a strong business case here, but you know best.

2: I talked to Saleae already, hopefully they’ll be positive about this but I can’t know. 100 us latency would be suboptimal, but I’d take it if 1a wasn’t on the table.

Many thanks for all.

BR,
Thanos

4

Hi @thanos,

I have added (1a) to our feature tracker. We are just getting started on developing our third generation products, which are still a long time away from launch. However, I don’t think that (1a) will make it into the JS220 successor due to price. We are considering a higher priced instrument that could potentially include this feature.

Thanks for talking to Saleae. We are happy to support Joulescope integration into their Logic software for (2), but Saleae needs to take on this development.

The only solutions possible today are (1c) and (3b). For (1c), you could probably use a Raspberry Pi with one of the audio DAC hats. Audio is lower bandwidth than JS220 Joulescope data, but it may be fast enough for your application.

Thanks again for taking the time to describe this feature and how it would be useful to you!

5

Hi Matt,

large companies (like the one I am with) will have no second thoughts paying more for this kind of extra features. Right now we use ‘makeshift’ approaches for this kind of tasks and/or the Keysight Power analyzer with some channels tracking current and some others in voltage mode. Very slow and suboptimal.

Looking forward!

Thanks for this,
BR,
Thanos

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