Cheapest logic analyzer / datalogger with external clock and large sample depth - Page 1
Forum section: electronics / testgear

author: zappedy
author_rank: Regular Contributor
timestamp: July 26, 2016, 02:15:14 am
content: In my other thread I asked about how to analyze the asynchronous bus of a 5 volts 68000 running at 8Mhz. Ideally everything would be logged during the entire boot up process which is around 10 seconds, that is a huge number of samples!However, the analyzer will not be used for debugging hardware problems, but only as a high speed data logger. Perhaps this means it can be simpler / cheaper.I'm on a shoe string budget. I've looked at the offers on eBay but most of the cheap ones do not even have an external clock input. Getting 3 of those cheap 24Mhz 8-ch USB devices would be very cheap though, and perhaps it is enough?Can any of the cheap ones reliably stream data to a computer at 8Msamp/s in 24 bits over USB, to mitigate the low sample depth?I'm in Europe and used pro equipment is hard to find and expensive, and overseas shipping is astronomical.

author: gblades
author_rank: Contributor
timestamp: July 26, 2016, 02:13:15 pm
content: Looking at the official one the copies are based onhttps://www.adafruit.com/product/378to get all 8 channels being recorded at 24Mhz may require a bit of tweaking of the computer settings. Running 3 might well be a problem. You might be able to get it working by attaching each to their own usb controller but is is by no means guaranteed.You are asking the USB interface to transfer data at 24MB/s and the typical real world maximum speed is 33MB/s. Each device on the same controller shares the total speed so you can see that you will definitely need a controller per device and I don't know how well the software works at capturing all that data.

author: zappedy
author_rank: Regular Contributor
timestamp: July 27, 2016, 12:28:39 am
content: Ok, then it would have to be multiple laptops, one for each analyzer.What if you desolder the crystals and run the 24Mhz logic analyzer and the 16Mhz device-under-analysis from the same 48 MHz clock. Would that mitigate the lack of external sync?Edit: using external clock on the 24MHz Fx2 based logic analyzers from evilbay requires firmware re-build, but it is open source (Sigrok / fx2lafw)

author: zappedy
author_rank: Regular Contributor
timestamp: July 27, 2016, 07:13:20 am
content: I did some further investigation. Those super cheap 8- and 16-channel analyzers are really just Fx2lp eval boards, but some run from an external, and thus easily desolderable, 24MHz crystal. You can supply your own square wave to the crystal input.Perhaps two or more of those boards can be synchronized by running them from a single oscillator, for 32- or more channels, for $5 per board.A problem might be the 20x PLL that is used internally, both to generate 48MHz for the 8051 and for 480MHz for USB High Speed. Since a PLL is an analog circuit, would this mean that there is no way two boards will end up synchronized?The fx2lafw firmware is available as open source for those devices. Perhaps you could toggle the clock generator post-scaler back and forth between 1x and 2x for a while to nudge one of the 8051's into sync with the other? The 48MHz 8051 clocks are available on pins, so a diff signal could be generated.Please don't laugh if this is stupid, I'm a software guy so I don't know much about electronics.

author: zappedy
author_rank: Regular Contributor
timestamp: July 27, 2016, 03:25:10 pm
content: Another look at the datasheet indicates that simply taking the device in and out of sleep will restart the PLL (with a 200 us stabilization time), so just doing this a couple of times on one device until internal oscillator synchronization is achieved should be enough.