IT was not terminated, 270K is way to high a value (likely 1000*). Its low enough to assert a weak pull-up or down to a CMOS input but not terminate a line. Typical transmission line for that is in the 120-250ohm range and the input to a SD/MMC is a roughly 10pf cap to ground and DC open (equivalent).
Also SD have two modes, one being SPI similar and the other a faster 4bit varient. I know that as I’ve used SD as “disk” for CP/M systems with 8085 and Z80. FYI this is the same problem designers hit with DRAMS back 40 years ago. Allison On Mar 29, 2017, at 4:06 PM, W2HX <w...@w2hx.com> wrote: > There are few things that come to mind here. The op seemed to indicate the > lines are terminated. If they are not terminated in the characteristic > impedance of the source and the transmission line, it is very unlikely he > would be seeing nice square waves at either end. The reflections would > distort the square wave. Given the reported squareness and that the op > indicates terminated line, I do not think impedance mismatch is the issue > here. > > I also agree that an induced current in an adjacent line would not be square. > So I agree with the op's thoughts that this signal is getting on this line in > some other fashion, I don't believe this is an issue of cross talk. However, > some pictures of some waveforms would be interesting to see > > Eugene W2HX > > > -----Original Message----- > From: cctalk [mailto:cctalk-boun...@classiccmp.org] On Behalf Of Parent > Allison via cctalk > Sent: Wednesday, March 29, 2017 11:54 AM > To: Noel Chiappa; General Discussion: On-Topic and Off-Topic Posts > Subject: Re: Cross-talk square-wave? > > > On Mar 29, 2017, at 9:40 AM, Noel Chiappa via cctalk <cctalk@classiccmp.org> > wrote: > >> Hi, a question about generic analog stuff. >> >> In the process of getting SD cards to work, Dave is seeing square-wave >> noise on a line. (1V of square wave, with pulses about 400ns long, >> running at >> 375kHz.) The line runs through a flat cable of modest length, along >> with other signal-carrying lines. (No, we were not smart, and didn't >> put ground lines between each pair of signal lines!) > > Oops! > >> >> Could cross-talk cause this kind of noise? We would have thought that >> you'd only get spikes, associated with the rising and trailing edges >> of a signal in a parallel wire, not a whole square-wave. During the >> constant-current period in the middle of the pulse, there shouldn't be >> any cross-talk? Is there some mechanism I/we don't understand that could do >> that? >> > Transmission line theory applies. Adjacent lines see the electric and > magnetic fields nominally seen along transmission lines. Some would say it > this way, you get induction from one line to another based on how those wires > are routed and terminated. > > Its only 375khz... No, its pulses with rise and fall times in the Nanosecond > region with bandwidth of hundreds of Mhz. > > >> (My guess is there's a leakage path in the circuitry on one end or the >> other, not cross-talk in the cable, but...) > > Nooooo. You have to treat those wires as transmission lines ( like coaxial > cable or parallel pair) for signals. Its not DC leakage. You send a pulse > (or a train of them) down a transmission line and if the line is not > terminated the pulse energy will be reflected rather than absorbed. Is there > is a signal line next to it it will see the resulting fields from the > currents flowing. > > Add to that your ground for the SD card is remote so there will be a current > flowing on that lead as well from circuit ground and the actual ground pin. > > This is why people do not remote SD cards (unless someone is forcing it). > Its input looks like a capacitor at the end of a transmission line and > incorrectly handled you get reflections and ringing. Just like backplanes > and all sorts of other media. > > > Allison > >> Thanks! >> >> Noel >
