Love my Joulescope, but I've encountered some anomalies today

I’ve been using Joulescope for a few days now and I love it. However, I have experienced a couple of problems that are probably software related, and I’m sure can be corrected in an update.

  1. I have a short pulse of energy once per second when I wake up from sleep. This pulse is showing as different amplitudes at different manual current ranges. At 1.8mA, it is about 1.2mA. At 18 mA, it is about 4 mA. When in auto scale, it is about 12mA. I would expect it to be much closer at different ranges. I’m not sure what value to believe.

  2. Sometimes the Y-axis scales to what is on the screen when I pan, and sometimes it doesn’t. It does not rescale when I zoom…

  3. Today I had a few instances where the USB disconnected and reconnected while I was in live mode.

Hi @wired and welcome to the forum! Glad to hear that you are putting your Joulescope to work!

This question is interesting, and the answer is not entirely straightforward. Joulescope is a shunt ammeter, meaning that it inserts a resistance between your power source and your target. Your target has capacitance, so this forms an RC low-pass filter. Your target board capacitance is mostly constant at a given voltage & temperature. However, more sensitive Joulescope current ranges use larger resistors. The larger the resistor, the more low pass “smoothing” you will see of short spikes. However, the total charge and energy should be the same between different ranges, but that also depends slightly on the linearity of your target device over voltage.

Joulescope autoscaling does introduce a “glitch” of about 2 samples in duration. If you are looking at events only 2 samples long, Joulescope will capture them but with lower accuracy. Sometime in the future, we can modify the processing to get more accurate results around autoranging events. For fast glitches (on the order of samples, < 2 µs), selecting a fixed range on your Joulescope will give you the best accuracy of the glitch shape. Note that Joulescope only advertises 250 kHz (4 µs period) bandwidth.

We will investigate. What operating system and version are you using?

We have a pending firmware update that will improve USB connection negotiation, but your reported issue sounds different. We have run Joulescope on all platforms for multiple days, but that does not come close to the amount of variability that we are now seeing with the deployed Joulescopes in the field. We’ll keep an eye out. If you happen to notice any pattern or have any thoughts on what could be causing this with your computer, please post & let us know!

Follow up question: Did you have the max/min lines turned on or off when you observed this behavior? Note that the default is on, but you can turn them off in the preferences menu.

I’ve seen the same, and I think on the X axis as well when zooming by dragging the X zoom control. Doing something else later (sorry, I haven’t taken careful notes yet, but will) finally causes the display to update to the requested settings.

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They were on. I took a look at the fill option, but decided I liked max/min better, and went back to that.

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FYI, I’m running Win7 Pro SP1.

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FYI, I had Joulescope running for most of the day yesterday. I had about 4 instances where the USB disconnected and then immediately reconnected. I have not been able to determine what the cause might be. In one case, I left my office and when I came back it had reset. I know this is the case because when there is a power loss to my device it goes into a shutdown mode, and that is what the Joulescope was showing. It also happened a few times while I was sitting at my desk. In none of the cases was I taking any action on the computer when the disconnect occurred.

The upcoming 0.5.0 release may fix this issue. We worked with NXP to identify a silicon errata that supposedly had a workaround, but the workaround was not correctly applied. The new Joulescope UI 0.5.0 (coming in the next few days) contains the firmware with the fix. While this errata should have only affect USB high-speed negotiation, I have noted one USB setup (I am sure that there are more) where it caused reliability issues. You can find the full, highly detailed discussion on the NXP forum.

Once we release the 0.5.0, could you see if the behavior changes? Hopefully it will fix this issue. If not, I would love if you could post the Joulescope log so that we can troubleshoot this issue.

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