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Raspberry Pi fast deep data capture question 
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Joined: Mon May 13, 2013 5:00 pm
Posts: 21
Real Name: John Hodges
Does anyone know if RaspberryPi has been or could be used as a fast and deep data capture device through its GPIO port? For example, for logic analysis with dumb triggering. codeandlife.com reports generating a 21 MHz squarewave on a Gpio pin using Native C. Does that suggest one could log a byte of data to RAM every 23 nSec? Could available RAM be filled in a single burst, or would there be OS-caused hiccups? Thanks!


Mon May 13, 2013 5:37 pm
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Joined: Wed Mar 07, 2012 10:40 am
Posts: 123
Real Name: Robert
OS caused hiccups are going to completely prevent this type of sampling. If you write your own OS you might be able to set up an uniterrupted capture where everything else in the RPi is shut down while you do this, but it would be very tough to set up a precise sampling rate. I get this overwhelming sense of wrong tool for the job--you would be making a major effort to get this to work. Get a decent FPGA board and this will be simple (and vastly faster, see my post on my Gigahertz FPGA analyzer). It's one of the best tools I've ever made--it's about the fifth version of analyzers that I have designed and built.

Nevertheless, I've said things like that before and have just annoyed determined people that have their reasons. If you don't need a regular sampling rate (eg, want data logging with time stamps), this might be a more workable platform. If the DDR is running, say, at 100Mhz, 32 bit width, 8 cycle burst, that should sustain a pure device max of around 200-300Mbytes per second. You would have to program trigger detection and sampling, which I think is going to limit you to maybe the 20Mhz you mention (50ns per sample), which makes for a logic analyzer of very limited use. In addition, you won't have any sampler synchronization, which means you potentially will only get real sampling at half that rate.

If you connect a de0-nano FPGA board to your RPi you should easily make an analyzer that can hit 500Mhz and free up the RPi to run Linux and run Python or whatever to quickly display and control your analyzer functions. I laid out a little adapter board that should hook up a RPi to a de0-nano, haven't tried it yet...

Bob

Bob


Tue May 14, 2013 9:11 am
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Joined: Mon May 13, 2013 5:00 pm
Posts: 21
Real Name: John Hodges
thanks Bob, most helpful. For all my ideas so far, RPi is either too much or too little. I'm searching for details of your FPGA analyzer. I'll send a private message once I'm allowed. -John Hodges


Tue May 14, 2013 5:30 pm
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Joined: Mon Dec 12, 2011 12:26 pm
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Real Name: Sol
jhodges wrote:
I'll send a private message once I'm allowed. -John Hodges


you should be able to pm now. :)


Tue May 21, 2013 9:41 am
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Joined: Mon May 13, 2013 5:00 pm
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Real Name: John Hodges
I bought an RPi to prep for the upcoming classes, and would like to try first boot during a Hack Night, figuring there'd be someone around to kibitz. I presume video is necessary, but my HDMI monitors are all 22+", cumbersome to transport.

Does anyone know if any of the monitors that live in the OLI maker space can accept HDMI, and could
borrowed for a few minutes?


Fri Jul 26, 2013 6:51 pm
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Joined: Wed Jun 06, 2012 9:30 pm
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Real Name: Randy
Not that we found. I am planning at being at hack night and you can use my atrix dock for video to get started. I will try to remember to bring it down.


Sun Jul 28, 2013 2:57 pm
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Joined: Mon May 13, 2013 5:00 pm
Posts: 21
Real Name: John Hodges
Here's a mockup of a RaspPi and USB hub mounted on the rear of a $42 ebay 1280x1024 17" monitor. The 1-meter $2.7 hdmi-dviD cable is the shortest I've found yet; a 0.3m hdmi-hdmi cable + adapter costs $6.5. I have a pair of prettier $1 1-amp switching supplies coming. Connectors for the homebuilt 20awg power cable <50c from ebay-China; I'll leave some free samples at the OLI space. 
Other costs: RasPi $35, sdhc $5, hub $5, (future) power $2, keyboard $5, mouse ?3, wireless $9. Total $118.

I can't find a perfect-length USB cable for the hub. (I did find one used 12", but no easy reorder). I have extra connectors, I wonder if shortening a USB cable takes special knowledge?

I'd replace the perforated aluminum with something cheaper & smaller once the layout is stable.


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Tue Aug 27, 2013 1:11 pm
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Joined: Mon May 13, 2013 5:00 pm
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Real Name: John Hodges
I built this to make it easier to use a Pi for the upcoming C class, but now find in some ways it's a nicer form-factor for a livingroom browsing computer than a laptop. I've seen Apple's in this shape, but not for $100.

I think I can power a second headless RasPi from this hub. With a crossover RJ45 cable, it'd make a portable setup for practicing networking of two similar computers at Hack Night.


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Tue Aug 27, 2013 1:15 pm
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Joined: Wed Jun 06, 2012 9:30 pm
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Real Name: Randy
Looks good John! Exactly along the lines of the Pi mission. 2 pi's and a monitor, one headless, great webserver demo, network practice platform!


Tue Aug 27, 2013 2:03 pm
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Joined: Mon May 13, 2013 5:00 pm
Posts: 21
Real Name: John Hodges
Caution! The $5 USB powered hub I bought for RasPi had it's wallwart-input barrel connector wired directly to the 5V pin of it's TypeB USB-input connector. So instead of powering RasPi and it's USB stuff separately, I was shorting the outputs of two switching supplies. I feel lucky nothing fried, or even overheated; it just caused boot to fail half the time. I cured it by cutting a trace on the hub's circuit board (lucky again, as it was .010 from another trace.) then got several good boots in a row by cycling AC power. 

The vendor was "yallstore" on ebay, but several ebay vendors show an apparently identical photo, with five USB jacks standing perpendicular to the circuit board. Can anyone think of an application for a powered hub that would want this 5V connection? I suspect an autorouter saw a "5V" pinname on the TypeB and connected it. Caveat Emptor!.  


Fri Aug 30, 2013 1:19 pm
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