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Picoscope i2c grabbing

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Picoscope i2c grabbing

Postby ost » Tue Dec 06, 2011 2:40 pm

Ive bought a 2204 to try out the i2c decoding feature and it gives me some headaces.

1. The sampling rate at 2ms/div is fine. At 5ms/div it gets too low. Is it possible to try a custom x axis at say 3 or 4ms/div?

2. The holdoff time is very large. Isnt it possible to get uninterrupted sampling at this rather low rate? I dont want to miss any of the data. The usb port shouldn't be the limiting factor.

3. I can select samplerate, but I cant find the REAL samplerate used. Can this be improved by for example reducing the # of wave buffers used? Playing around with preferences, all I got was a stopped scope displaying 0 of 0 waves in the top toolbar. I had to reset both preferences and startup settings to get going again.

Please advice
ost
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Re: Picoscope i2c grabbing

Postby Hitesh » Wed Dec 07, 2011 2:22 pm

Hi,

Thanks for your enquiry.

Within PicoScope the timebases are pre-defined so unfortunately it is not possible to change them.

If the hold-off time is too large, you could try changing to a longer timebase - if the default collection time (200ms/div) is used, then the device will switch to collecting data in streaming mode.

The actual sample rate can be found by clicking Views -> View Properties and viewing the Properties pane on the right side of the screen. The sample rate can be improved by adjusting the timebase and number of samples per buffer - adjusting the number of waveforms will not affect the rate as the sampling rate will determine the number of waveforms that can be stored in the buffer :D

Which version of PicoScope are you using?

Regards,
Hitesh

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Re: Picoscope i2c grabbing

Postby ost » Thu Dec 08, 2011 11:34 am

First, Im using:
PicoScope® 6 - PC Oscilloscope software version: 6.6.13.15
Model: PicoScope 2204
Hardware Version: 16
Driver Version: 2.0.8.101

Im afraid that 200ms/div doesnt give me the samplerate I need to analyze my i2c stream. Ideally I'd like a lossless analyser that could grab data continuously...

The serial decoding seems to only work at 500us, 1ms and 2ms/div for my stream. My i2c stream seems to have clk period of ~20us and greater (not fixed clkrate), and at 200us/div it cant decode it, even if the signal looks crisp on the curve.
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Re: Picoscope i2c grabbing

Postby Pico Stuart » Thu Dec 08, 2011 12:59 pm

Hi there,

Thankyou for your observations with respect to your i2c application. In order to assist you further please could you send over your data file. ps data of your i2c to suppport@picotech.com

I will then look through your data and endeavour to assist you further. In addition to this please find below a description about block and streaming mode in PicoScope 6.

The PicoScope 6 software and PicoScope drivers, supplied with all PicoScope oscilloscopes, have been improved to give faster data transfer over the USB interface. The improvements have focused on ‘streaming mode’, one of the two data transfer modes used by PicoScope devices (the other is ‘block mode’). When you use
PicoScope 6, you rarely need to know the difference between the two modes as the software switches automatically between them.

In streaming mode, PicoScope 6 tells the scope to capture data continuously and, at the same time, to transfer each sample to the PC with minimum delay. The amount of streamed data that PicoScope 6 can store is limited by design, to prevent the program from consuming too much of the PC’s resources.

PicoScope 6 limits the sampling rate to 1 MS/s in streaming mode, again to conserve the PC’s resources. If you request
a faster sampling rate, then the program switches to block mode. PicoScope 6 does this mode switching
automatically.

Oscilloscope series Maximum specifications
Record length Sampling rate
PicoScope 2000 2-channel 2 MS 1 MS/s


I will then look through your data and endeavour to assist you further.

Kindest regards,

Stuart
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Re: Picoscope i2c grabbing

Postby ost » Fri Dec 09, 2011 8:24 am

Wow, I didnt realize the 200ms/div actually could grab at 1Msps. That solved my biggest problem :D
I will post some files with the decoder problems.

Thanks!
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Re: Picoscope i2c grabbing

Postby ost » Fri Dec 09, 2011 8:31 am

Now, I should elaborate why I didnt think 200ms/div would sample my signal.

When I choose 100ms/div, the sample interval=163.8us, rate=6.104 kS/s and no.samples=6109, my signal looks like sawtooth indicating the samplerate is too low.
When I select 200ms/div the samplingrate drastically increases (against all logic) to sample interval=2us, rate=500 kS/s and no.samples=1,000,005.

Now this appears like strange behaviour and was totally unexpected, cause I assumed that continuous grab mode would drop to a rate where the usb could handle the bandwidth. Maybe you should consider having continuous mode as a separate sampling mode independant of timescale selected.
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Re: Picoscope i2c grabbing

Postby Pico Stuart » Fri Dec 09, 2011 12:21 pm

Hi there,

Thankyou for your observations with regard to PIcoScope 6. The 2000 series can switch into slow sampling mode at 100m/s. . Slow sampling Transition sets the collection time at which PicoScope will update the screen before the total collection time has expired.

You can change the collection time at which the transition happens by going to the Preferences menu.


Hope this helps, in the mean time I will look through .psdata file posted to support.

Kindest regards,

Stuart
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