Extra Unwanted Sessions

I am using a 16-bit channel Saleae Logic Pro over a USB 3.0 link. Whenever I power cycle my board (not the Logic Pro but my PCBA) the Logic 2 software creates a new session and changes focus to that session. I then have to close down that session to get back to my original session.

I’m not sure what causes these newly create and unwanted sessions to pop up. I’m also not sure how power cycling my board would trigger the Logic 2 to know that a session needs to be created. Is it looking at the grounds? My grounds are isolated from my PC power supply.

Is there a way to stop this from happening? Thanks!

@michael.manna This is pretty strange behavior. Our logic analyzer and software shouldn’t be creating new tabs simply based on what’s happening at the input channels. My hunch is that your power cycling method is somehow causing Logic Pro 16 to disconnect and reconnect, which would in fact create a new capture session.

Can you let me know more details about how everything is powered and hooked up? What signals are attached to your Logic Pro 16? Is it possible the signal inputs spike to a level above +/-25V (absolute max voltage tolerance for our inputs)? How are you power cycling your DUT, and could your Logic Pro 16 be connected (via power) to the same source? These are some common scenarios that might cause this to happen.

Thanks for replying and helping! I’ve tried this on 2 Logic Pro devices: and 8 and 16. Both have the same problem. The Logic Pro is powered via the USB cable, from my PC. I’m analyzing a 3.3V video parallel bus. The signals are not exceeding 3.3V. (I confirmed with an o-scope during the PCBA boot process.) The PCBA has its own power source: an AC/DC converter with an isolated ground.

Is there any other hidden or non-obvious method to create sessions? For example, a certain pin that triggers a new sesson?

Again, thanks for the quick reply. I appreciate the help!


Is there any other hidden or non-obvious method to create sessions? For example, a certain pin that triggers a new sesson?

No, signals on the input pins should not create new sessions.

How is your DUT powered and what is the process for power cycling it? Also, just to test the theory above, can you try removing one channel at a time, power cycle your DUT, and repeat? Is a certain channel signal in fact causing the Logic to create a new session? Then what happens when you completely remove all Logic channel inputs from your DUT?

I’d still go with Tim’s thought that you are causing the logic analyzer to reset. Here’s the thing, you said that the grounds are isolated from the PC power supply. But, they’re not if you’ve connected a Logic to them. In that case it’s a ground path from the DUT to the PC. After all, the logic analyzer must reference the ground in order to detect the signal. So, it must be connected to your DUT ground pins. It is also connected to your PC and thus the Logic is always grounded to the PC itself. So, just by the way it is constructed, you are shorting the two grounds together. This is no different from a standard non-differential oscilloscope. It will assume the ground potential of your DUT. Thus, when you boot up your DUT it is entirely possible that there is a large ground potential that is shorted through the logic analyzer and that moment where the two grounds are brought to the same potential might cause a USB reset.

Thanks for the feedback. Sorry that I haven’t written back earlier. I think you guys have the situation correct. I am also debugging a small eval board, on the same ground as the PC. It does not cause the Logic Pro to create new sessions after a boot.

I can’t change the fact that my PCBA has an isolated ground, so I will have to learn to deal with it. : )

Can you connect common on your PCBA to the computer ground? That may reduce the current spike through the USB ground from the Logic Pro to the computer and remove the problem.

Our setup is fairly complicated. The hardware is a mixture of one board running off an isolated power supply and a COTS EVK from an IC vendor. The two boards are not intended to be used together, but we must to diagnose failures with the IC. So, I just dealt with the extra sessions that the Logic Pro created. It was the least of my challenges. I have solved the issue with the IC, thanks to data that I gathered from the Logic Pro. We can close this issue if you’d like. Thanks for the support.