jnc at mercury.lcs.mit.edu
Thu Mar 30 15:13:13 CDT 2017
> From: Allison
> FYI this is the same problem designers hit with DRAMS back 40 years ago.
This didn't ring (pun not intended) a bell for me; can you say a bit more?
> From: Chuck Guzis
> I'll offer a suggestion that if your SD card *must* be a significant
> distance from its host
Like I said, this is a pre-prototype; on the production units, there will be
_no_ cable. The SD socket will be about 1-2" from the FPGA.
> From: Dwight Kelvey
> this behavior on my PDP-8/e where a 7474 flip flop chip was bad. The
> input looked great and the output was "half baked"
There's no chip at all on the driving end of the line (just that 470K
resistor); we see this with the SD card _unplugged_. And we see the exact
same thing on several lines.
I'm still not clear, from the discussion, how exactly that nice 'square-wave'
interference is happening - could it be capacitative crosstalk? (I'd have
thought capacitative cross-talk would be inverted - driving a positive voltage
on one 'side' of the 'capacitor' would, I would think, induce an oppposing
voltage on the other. But I'm clearly no EE! :-)
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