29 Mar
2017
29 Mar
'17
10:25 a.m.
On Mar 29, 2017, at 1:17 PM, Chuck Guzis via cctalk
<cctalk at classiccmp.org> wrote:
On 03/29/2017 07:08 AM, Noel Chiappa via cctalk wrote:
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.
SD cards are not SPI, they are a variation of MMC. As with other interconnects, it's
good to consider the connections as transmission lines, and look at what termination is
expected. 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.
paul
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.