On Thu, 12 Feb 1998 17:25:34 -0500, Sean Breheny wrote: >At 04:00 PM 2/12/98 -0600, you wrote: >>Has anyone built a PIC-based digital O-scope / logic analyzer ? >> > > >Hi Dan, > > I am working on essentially the same project that you are. I have a >prototype finished and I am in the process of getting it to work. Because I >am currently a college student with very little free time, I have a Software Engineering career and two kids using up all of *my* free time ;-) >this is taking >longer than one might expect. My design is quite simple and consists of a >40 MS/s ADC buffered by a high speed op-amp. The output of the ADC is >captured by an SRAM (128kbit x 8). The clock is provided by a little can >type oscillator, and the address is derived from a pair of 8-bit >synchronous fast counters (74F579). A PIC controls when sampling is >started/stopped and also allows a PC to read out the contents of the SRAM >and clear the counter. Yes we *are* thinking along the very same lines. Which ADC & SRAM devices are you using and how much did you pay for them? >Communication is via bi-directional EPP parallel port. I was planning on RS232, but that of course, is way too slow for continuous readout on the PC! I was thinking more along the line of triggered sample, download to PC, scroll trace across screen with scroll bars to examine. Rather more like a logic analyzer than an O-scope. I would also like at least 2 channels. > I have had the prototype together for about a week and have had only about >2 days to actually work on it. My PIC programer still resides on my Radio Shack Experimentor Socket I haven't made the time to build a permanent version of it yet or to it into an enclosure. >It is giving erratic output, which I think >is due to some type of synchronization problem where either the SRAM is >being programmed while the output of the ADC is undefined or the parallel >port is reading the SRAM when it is not ready. > Are you trying to read the SRAM into the PC at the same time the samples are being clocked in? If so, how are you managing contention to the SRAM address and data busses? >I will continue to work on it and keep the piclist updated. I would >encourage you, Dan, to also try your design. I am willing to help with >component choice/ etc. I appreciate it. I am a long way from actually prototyping or even designing mine yet because I still have to finish making a modified version of the serial YAPP programmer and conclude my IR decoding experiments. I will keep a copy of your message around and keep in contact with you. As always my mind is way ahead of what I am actually working on at the moment.... Never enough time to see *all* of my ideas through to their conclusion. >As for trigger, I was going to just simply >continuously sample (well, until 64k or whatever) was full, and then only >display past a trigger point (similar to what commercial DSO do). This >method also allows pre-trigger viewing. A "soft" trigger of sorts huh? >The only problem is that I can only >sample in chunks of (maximum) 64k (16 bits of address) and the PC takes at >least 32 milliseconds to read this out. This would give a maximum sampling >duty cycle of 5% at 40MS/s or 40% at 5MS/s, the minimum rate for this ADC. >This is a problem which you might try to overcome. Hmmm, I as hoping to choose from sampling rates covering a range from audio frequencies all of the way up to the maximum sampling rate. I am not looking for any continuous sampling. I just want to trigger on an edge / level and look at the next 64k samples, at whatever time base, from the trigger. >Special triggering circuits might allow special functions, though, such as >repetitive sampling to get very high effective sample rates on repetitive >waveforms. > >Good luck, > Good luck to you! I am afraid that I am far away from starting the project. I am more in the "what if.." phase right know. >Sean > >+--------------------------------+ >| Sean Breheny | >| Amateur Radio Callsign: KA3YXM | >| Electrical Engineering Student | >+--------------------------------+ >Fight injustice, please look at >http://homepages.enterprise.net/toolan/joanandrews/ > >Personal page: http://www.people.cornell.edu/pages/shb7 >mailto:shb7@cornell.edu >Phone(USA): (607) 253-0315 > Dan