Cross-talk square-wave?

David Bridgham dab at
Wed Mar 29 14:53:14 CDT 2017

> 1v across 270K represents 3.7 microamps, which isn't much, particularly
> at 25MHz. (I assume that you're using SPI to access the card, but the
> observation still holds).

Yup, I'm planning to use the SD card in SPI mode (at least for now). 
And this line is the CS/CD line, so it's not even running at 25MHz.  And
as Noel pointed out, this is happening without the SD card even being
plugged in.  I'm not surprised at cross-talk, I'm surprised that the
cross-talk appears so clean.  It's not spikes but fairly clean, 1V
square pulses.

> And if you're using SPI, have you installed
> pullups on unused pins?

Yup, I put in all the pull-ups.

> I'd go to interleaved ground cable or UTP for the device.     Also, make
> sure that your 3.3V supply is adequately decoupled--an SD card can draw
> somewhere n the neighborhood of 100ma when operating, if the datasheets
> are to be believed.   I use a separate 3.3V LDO regulator at the SD card
> socket.

Good point.  I really should have put a good decoupling cap nearby and
the production card should have its own regulator.

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