Cross-talk square-wave?

[David Bridgham]

> 270k seems like a rather strange value, it certainly can't be a termination and it isn't a plausible pulldown either.  The SD spec should explain what is expected; I knew it at one time but forgot by now.

I'll agree that 270k is a strange value.  The idea is that the SD card
contains an internal 50k pull-up on that line (dat[3]) so if you put a
270k pull-down on the board then you can use it for card detection. 
It's a little funky and I wish I'd just gone with an SD socket that had
a card-detect switch but this is supposed to work too.

The 1V square pulse, whereever it's coming from, is just enough that I'm
detecting the card presence when there's no card plugged in yet.   I can
work around this in various ways but I want to understand it well enough
that I'm sure I'm not ignoring a problem that's going to bite when we go
to a production board.  In other words, if it's simply a result of the
silly ribbon cable, then I'm happy for now with a work-around.