On Thu, Sep 29, 2011 at 2:30 PM, Herbert Graf wrote: > Fine, but how many logic probes will you have? 8? 16? Even with 16 > you've got 15k samples, still MORE then enough for pretty much all debug > work you'll have. > I'm capturing bulk data being written to devices on various busses at the same time to analyze the timing between them and to reverse engineer the data. For any other purpose, I'd agree that 15k samples is usually enough, but when the data itself is of interest, I need to be able to capture it. > Interesting, I'm not familiar with the PIC's USB peripheral, does it > require that much hand holding? You can't just set up a bunch of stuff > and tell it to "go"? > As of now, I'm filling a buffer and calling transmit(). I'm pretty sure tha= t without DMA, the transmit() is blocking. Also, there's issues with polling and interrupt modes and USB tasks that have to be serviced periodically tha= t take up CPU time. I measured throughput with and without USB with an oscilloscope and it's a 10x difference. Of course, I'm using the stock Microchip USB libraries, so I'm assuming they're not super duper optimized. > If true you're right, and I'd say a PIC isn't the correct tool to use in > this case. > > Have you considered using an FPGA? I've done this sort of thing before, > if you are familiar with FPGAs this sort of project becomes rather > trivial. > Yes, I have. I have the Nexys 2 board with USB 2.0 high speed as well as 32MB of RAM which is perfect for this application. Drawback: I'm not good with FPGAs yet. Not enough to trust myself to do this. I still need to lear= n more with them. Very true, again, considering your requirements, and how "messy" USB can > be (I'm worried you're "on the fly" solution will never work due to > latency on the PC side servicing the USB port), I think you should > consider a change of target. > > FWIW I built a very basic serial port based logic analyzer many years > ago: > > http://repatch.dyndns.org/pic_stuff/logan/index.html > > (it's actually quite embarrassing looking at that project now, the code > is a such a mess, and the design is really crude). > > In that I decided on a RAM chip for data storage, a CPLD to address and > clock it, and a 16F877 PIC to do all the other good stuff. It was a > really fun project and I actually used it's for real work a few times. > > Looks good. I'm going to try my SPI SD card method first. I have a good feeling about this. SD says max 50 MHz via SPI which is more than enough fo= r me. --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .