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

[Scott McMorrow <scott@xxxxxxxxxxxxx>]

Shiraz

Sounds like something that Doug would be able to help with.

Scott

Scott McMorrow
Teraspeed Consulting Group LLC
121 North River Drive
Narragansett, RI 02882
(401) 284-1827 Business
(401) 284-1840 Fax

http://www.teraspeed.com

Teraspeed® is the registered service mark of
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Bashir, Shiraz (GE Healthcare) wrote:
> 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:=20
> 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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