Dangerous Prototypes Open Source Hardware

Triggering erratic?

June 23, 2010, 07:32:45 am

I've updated to the 2.1 firmware and bitstream and mostly things seem to be working.  I've done some simple tests with a pwm signal from a bus pirate and all the inputs seems to capture the correct waveform and timing.  However, I'm noticing that the triggering isn't very reliable.  I'm trying to trigger on a leading edge (using the simple trigger with 50/50 display) and I'd say about 1/2 to 2/3 of the time the trigger works and the rest of the time, the signal seems to be captured at a random time.  I don't see any other problems with the capture.  I've tried on both Windows and OS X using the java client included with the 2.1 release.  I've tried both the 8bit and 16 bit wide bit streams at various capture rates  and various input signal frequencies and see the same problem with triggering in all cases.  Is it just me, or do others see problems with triggering also?
-Scott

Re: Triggering erratic?

Reply #1 – June 23, 2010, 09:55:46 am

Hi,
I tried this on Linux, with the client from the 2.1 package and another one with some Linux mods.

I can confirm that on my two boards the Trigger does not work correctly on  falling edges.

I have the 8 channel bitstream on one of the boards and the 16 channel bitstream on the other one.
Both are running Firmware 0.6.
The error is independent of the sampling rate or buffersize settings. 

The trigger works fine for rising edges (simple and complex trigger settings).
When set to falling edges the OLS starts returning data at random. Could be that it returns immediatly, can't tell. At least it not caused by the Trigger setting.

I checked the uploaded commands that the client prints to the commandline, they are OK.

Eberhard

Re: Triggering erratic?

Reply #2 – June 23, 2010, 12:13:42 pm

Thanks for the investigation, it sounds like a v2.1 bug. I'll start a new thread as a quick reference for bugs, but it's probably good to add it to the tracker:
http://www.gadgetfactory.net/gf/project ... c/tracker/

Re: Triggering erratic?

Reply #3 – June 23, 2010, 03:13:24 pm

Hi Ian,
[quote author="ian"]
Thanks for the investigation, it sounds like a v2.1 bug. ..................
[/quote]
Just came in from a long walk with my dog which always restarts my brain...
While throwing sticks, it came to me that I have obviously not understood the way the trigger is supposed to work.

I reported that the trigger for a falling edge does not work. But the trigger does not fire on level-changes it simply checks the level as soon as the capturing starts.
So to detect a falling level transition in my signal I simply had to setup a 3-stage trigger:

• wait for low-signal
• wait for high-signal
• fire trigger when level goes back to low

This wasn't clear to me until now.
So this is not a bug.

Eberhard

Re: Triggering erratic?

Reply #4 – June 23, 2010, 06:13:07 pm

I can't seem to recreate the problem, triggering on a high to low transition seems to work fine for me. In the attached picture I triggered on channel 2 going low.

Any ideas on how to recreate?

Jack.

Re: Triggering erratic?

Reply #5 – June 23, 2010, 07:27:13 pm

[quote author="wayoda"]
Just came in from a long walk with my dog which always restarts my brain...
While throwing sticks, it came to me that I have obviously not understood the way the trigger is supposed to work.

I reported that the trigger for a falling edge does not work. But the trigger does not fire on level-changes it simply checks the level as soon as the capturing starts.
So to detect a falling level transition in my signal I simply had to setup a 3-stage trigger:

• wait for low-signal
• wait for high-signal
• fire trigger when level goes back to low

This wasn't clear to me until now.
So this is not a bug.

Eberhard
[/quote]
I think I need to get a dog or at least try taking a walk and throwing a stick.  I was thinking about the trigger in the way you would if you used an oscilloscope, when you generally set the trigger level to the leading or trailing edge of a signal.  But as Eberhard notes, that isn't what is going on with the OLS.  What I was seeing is triggering on the level.  So if I set a simple trigger on a high on pin 1, and feed a 50% pwm signal to pin 1, then the capture starts right away if the signal happens to be high when the run begins.  If the signal happens to be low, then the OLS triggers on the next low to high transition and in those cases, it looks like the trigger occurs on the leading edge.  So 50% of the time, you get a capture that triggers in a random part of the high cycle of the signal and the rest of the time you get a capture which triggers on the low to high transition.  You never will get a capture which triggers randomly in the low part of the pwm cycle.  This is exactly what I was seeing. 
So there is no problem, just a mistaken understanding of what was supposed to happen.
If I set up a complex trigger which first waits for a low and then starts capturing on a high level, then the captures reliably trigger on the leading edge of the signal.
-Scott