On 05/16/2017 09:56 PM, Gene Heskett wrote: > Is this driver your work, Bertho?
No, not at all. But, I have dealt with high frequency stuff before. The biggest problem is that some parts are in the realm of magic ;-) The RPI hardware is not made for such high frequencies. Pushing that out of a china-model 100mil header is, generally, crazy and you have impedance mismatches everywhere in the path. > My point is adding that 7 pf+whatever the tip clipon is, 3 or so more, to > the pi src side of the termination resistor of 82 ohms, of this clock > signal, makes it work, everytime. There are probably several effects at work here. You are changing the impedance which probably changes the transmissionline properties as to feed back into other parts of the circuit (including into the Broadcom chip). That it works is amazing all by itself. I've had lots of problems with 20MHz on that header already. BTW, I think that not only the probe tip has influence, but also where your probe's ground-lead is placed (and not that fluffy long aligator-clip lead). > I think I will solder a tightly > twisted pair there for a loading cap, but that seems like such a kludge > that I'd really like to be able to test and adjust for maximum data > integrity. At some point I'll kill it again, but I might be able to > arrive at a "this size of cap is best" by using half of what it takes to > kill it again. That, of course, will work for this specific setup. However, changing the RPI or changing any cable may throw you off again. -- Greetings Bertho (disclaimers are disclaimed) ------------------------------------------------------------------------------ Check out the vibrant tech community on one of the world's most engaging tech sites, Slashdot.org! http://sdm.link/slashdot _______________________________________________ Emc-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-users
