On Saturday 27 May 2017 11:26:18 Peter C. Wallace wrote: > On Sat, 27 May 2017, Gene Heskett wrote: > > Date: Sat, 27 May 2017 08:41:26 -0400 > > From: Gene Heskett <ghesk...@shentel.net> > > Reply-To: "Enhanced Machine Controller (EMC)" > > <emc-users@lists.sourceforge.net> > > To: emc-users@lists.sourceforge.net > > Subject: Re: [Emc-users] switching to a slower spi driver to see if > > it works, [...] > You might try lowering the series termination resistor value since > this looks like a possible SI issue (and the clock signal will be very > sensitive to SI issues).
SI? Acronym for what? > Also standard 10M capacitive divider probles are pretty useless for > high speed signal waverform checking, they have way too much > capacitance and typically poor step response. I've no clue how accurately shaped the 1 KHz test signal is, but expanded to see at 2ns per division, it actually looks pretty good. However, the clock signal as I see it on this gigahertz sampler, is not too terrible removed from a sine-squared wave, so I'm not convinced I am looking at the truth. At the pi, the swing is 3.35 volts, beyond the term r its under 2.9 volts for swing, like the 7i90 is pulling it up pretty strongly. > > I would suggest tacking a 4.99K resistor on the end of a bit of 50 Ohm > COAX to make a 100-1 passive divider probe with ~1 GHz band with and > less than 1 PF input capacitance (assuming you have a 50 Ohm Scope > input option) This Chinese scope doesn't have a 50 ohm input. Std 1 meg. And probably about 50 pf. But its a thought, I'll see what I can cobble up right at the input bnc connector. And all the coax I can lay my hands on is 75 (73 actually) ohm. Video stuff. In the meantime, I found some 6mm dia. 30 pf max trimmers, 20 for about 7 dollars, s/b here around the 5th. On fleabay of course. That might get me going as they can be pasted on the back face of the 40 pin pattern. Getting at the smd pad to parallel another resistor will be fun, the header sockets are sitting on them. And even with hot air, I'm not sure I could R&R the header sockets without damaging the board in addition to trashing the headers. At a 1.25" cable length, I'd think I ought to be able to just bridge the pads. Theres not enough drive to damage anything. 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> ------------------------------------------------------------------------------ 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 Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
