[SI-LIST] Re: Strange resets happening in one of our circuit boards
- From: "Andrew Ingraham" <a.ingraham@xxxxxxxx>
- To: "SI-List" <si-list@xxxxxxxxxxxxx>
- Date: Fri, 2 Jun 2006 11:41:56 -0400
I too have seen similar effects caused by chairs. I spent at least one late
night investigating a circuit board and rolling various chairs around the
lab floor. The chair was very definitely generating static discharges
somewhere (probably in the rollers of the chair) and this was radiating and
being picked up by (a) the scope probe leads, (b) my circuit board, and (c)
any cables and leads attached to the circuit under test. As I recall, it
didn't matter if I wore a wrist strap, while sitting in the chair. The
actual arcing was inaudible and invisible to me.
If I remember right (it's been some years now), there was some indication
that much of the induced noise was common-mode and being converted to
differential later. I wonder if a common-mode choke around your JTAG port
might help....
I don't remember now whether those chairs had the grounding chains on their
bottoms. Some of our lab chairs had about a foot of loose chain dangling
underneath that is supposed to lay on the floor, and our labs were supposed
to have been built with flooring material that is slightly conductive.
(Google "conductive linoleum" and you'll come up with several hits.) While
this would prevent the entire chair frame from becoming charged, it probably
does nothing to prevent static from accumulating and arcing within the
wheels themselves unless they, too, are made with conductive material. I've
just come to accept the fact that all chairs (and many other things too, I'm
sure) generate occasional random EMI and I'll see it in my lab measurements
from time to time.
Was it a capacitor across the pull-up resistor that "fixed" your problem, or
was it a capacitor from the reset line to ground? If you had noise from the
power supply, a capacitor across the pull-up resistor would not have helped.
It would be interesting to see if the problem continues with the unit in its
metal housing but with the JTAG attached.
Regards,
Andy
> We see +5/-3V worth of noise on the signal just by rolling my chair back
> and forth in front of the emulator.
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