Cross-talk square-wave?

[Chuck Guzis]

On 03/29/2017 07:08 AM, Noel Chiappa via cctalk wrote:
>> From: Dwight Kelvey
> 
>> Is there any load resistance at the end of the line?
> 
> Yes, 270K to ground (i.e. pretty large). How does that have an effect
> on whether cross-talk can create a square wave? Sorry, I'm not
> understanding.

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

SD cards can be very noisy devices--remember that they have "smarts"
inside, so they're not passive devices.

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.

--Chuck