On Friday 09 December 2016 14:51:15 Peter C. Wallace wrote: > >> Also regards grounding, you need to watch out for ground loops that > >> let high frequency noise currents flow between the 7I90 and the > >> CPU. > > > > Humm, would one of those clamp-on ferrite chokes help? Packrat that > > I am, I'm pretty sure I could find one. > > A ferrite bead on the connections to the RPI and 7I90 will help, if > you put it on the cable between them you will make things worse > (raising ground impedance) > > >> The most critical signal is the clock from the SPI master. > >> Depending on the drive strength it may require a series termination > >> resistor (the 7I90 SPI output are series terminated so need no > >> termination ) > > > > Thank you Peter, this points me in the right direction. And I'll > > term the signals _from_ the r-pi at the r-pi. Would it be too much > > loss if I used 120 ohm r's? I am pretty sure I can find enough of > > those. Obviously I haven't a clue what the source impedance of a pi > > gpio pin is or what its swing is. The r-pi of course is running on 5 > > volts but I'd expect thats cut way down for power savings. > > I would suspect that the output impedance of the RPIs signals is not > lower than 25-50 Ohms so 120 is probably too high. > Which is probably why the only mention I have seen was for 80 ohms, since if its 50, plus 80=130, supposedly spot on for ribbon cable.
> Also note that when using series termination you must look at the > signals at the destination (the signals will look funky at > intermediate points) > I had forgotten that pesky detail, thanks a bunch. > Also you need either a passive probe (easy to make with a 450 Ohm > resistor into a 50 Ohm coax/50 Ohm scope input) or a FET probe, a > normal 10M probe is pretty much useless for SI checking (way way too > much input capacitance) Yup. Since both of my scopes are high impedance, I'll have to make the whole thing, terminating in a 50 ohm load at the scopes input. Been known to use a small 51 ohm commercial load on the other side of a bnc tee for that. But in my world of video, RG-58 isn't on the shelf. 7559 from Susan Clark (Clark Wire & Cable) in the windy city is my goto video cable. Well (110+ db) shielded, limp as well cooked spaghetti. I changed out about 4 miles of stuff with West Penn lables on the spools I inherited in 1984, that my predecessor built the place with. You could see the crosstalk on the air! Horrible stuff that West Penn was. Thanks Peter. Cheers, Gene Heskett -- "There are four boxes to be used in defense of liberty: soap, ballot, jury, and ammo. Please use in that order." -Ed Howdershelt (Author) Genes Web page <http://geneslinuxbox.net:6309/gene> ------------------------------------------------------------------------------ Developer Access Program for Intel Xeon Phi Processors Access to Intel Xeon Phi processor-based developer platforms. With one year of Intel Parallel Studio XE. Training and support from Colfax. Order your platform today.http://sdm.link/xeonphi _______________________________________________ Emc-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-users
