29 Mar
2017
29 Mar
'17
10:08 a.m.
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.
Noel
29 Mar
29 Mar
1:17 p.m.
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.
--Chuck
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. 
1:25 p.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. 
3:59 p.m.
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.
4:18 p.m.
On Mar 29, 2017, at 1:25 PM, Paul Koning via cctalk
<cctalk at classiccmp.org> wrote:
...
SD cards are not SPI, they are a variation of MMC.
Sorry about that, it turns out I was working from obsolete memories. That used to be
true, isn't any longer.
In any case, you're dealing with pretty fast signals, treating them as transmission
lines is going to be important.
paul
3:53 p.m.
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.
2672
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