FreeLists / si-list / [SI-LIST] Re: Strange resets happening in one of our circuit boards

[<stubenner@xxxxxxxx>]

Shiraz,

This doesn't sound very strange at all, and it probably is a good =
question
for Doug to field, but I'll throw in my two cents-worth.

You prematurely ruled-out ESD. It may be true that your problem is not a
direct discharge ("arc") to your circuit, however, any discharge also
launches a broadband EM signal as well - just like a lightning strike. =
This
signal, I'll assume it's a radiated-only issue at this point, can be =
coupled
into conductors connected to your circuit and you may see the sort of
problem that you are experiencing.

The chair ESD discharge issue pops up a lot. Years ago I had a similar
problem where the chair was discharging to a desk over 20 feet from some
electronic test equipment and it caused the equipment to malfunction.

Regards,
Stu Benner
EMConsulting
www.emcwizard.com

-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] =
On
Behalf Of Bashir, Shiraz (GE Healthcare)
Sent: Thursday, June 01, 2006 7:42 PM
To: SI-List
Subject: [SI-LIST] Strange resets happening in one of our circuit boards


Hi,
A unit with microprocessor based system circuit board is resetting when =
we
plug in emulator port in JTAG port. Using scope we see that CPU reset =
pin is
asserted, hence the reset. We see +5/-3V worth of noise on the signal =
just
by rolling my chair back and forth in front of the emulator.

This happens in following scenario:
        1. User is sitting on chair, user gets up (doesn't touches the
unit) and unit resets.
            We demonstrated this on three randomly selected units. Sounds
ridiculous but it is true

OR

        2. If we keep solder wires for scoping purposes.

It doesn't happen when Unit is enclosed in it's metal housing.

Investigated..
a. Reviewed schematics and CPU's reset line is pulled to 3.3V using 1K =
ohm
resistor. b. Physically checked that pull up resistor is in place and is
connected with BGA ball of reset pin. c. We checked power monitor chip =
which
can drive reset too..but no issue there. D. Reset line is not going =
anywhere
else other than JTAG port and Power Monitor Chip

Brainstormed..
a. Soldering issue due to BGA ? Ruled out as how can soldering issue be =
on
three randomly selected units. b. ESD- Ruled out as per our =
understanding
ESD (air discharge) cannot "arc over" from 2-3 feet distance.

Fix:=3D20
A team member recommended to put a capacitor across resistor. Did that =
and
problem went away.

Can anybody explain why it is happening ? Is it noise from Power Supply =
but
how can it defeat a strong pull up ?

Thanks,
Shiraz Bashir
General Electric Healthcare
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