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

["Peterson, James F \(FL51\)" <james.f.peterson@xxxxxxxxxxxxx>]

An observation :
It looks like since the reset can be driven from two places (JTAG port
and Power monitor), these drivers must be of the open-drain or
open-collector sort. This means that when at a logic 1 level they are
not driving anything - the only thing keeping it to a logic 1 level is
your 1K ohm resistor. This will not take near as much energy to toggle
than if it was driven high by some low-Z logic device.

regards,
Jim



-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On Behalf Of Carlton Ross-ra8188
Sent: Friday, June 02, 2006 10:23 AM
To: SI-List
Subject: [SI-LIST] Re: Strange resets happening in one of our circuit
boards

Doug Smith tells a classic chair ESD story in his training. I think this
is your problem.=3D20

The likely case is that ESD from the chair is coupling into your JTAG
cable. Since your JTAG cable contains a conductor connected to the micro
RESET pin, a susceptibility problem should be no surprise. You may want
to try shielding the cable or placing ferrite(s) at the board connector.
Alternately, you could provide protection for your RESET pin by
implementing an RC low-pass filter at the micro. You can test this by
soldering a leaded 100nF capacitor from the RESET pin to the nearest VSS
pin.=3D20

Ross Carlton
Freescale Semi.


-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On Behalf Of Zhangkun
Sent: Thursday, June 01, 2006 8:20 PM
To: Shiraz.Bashir@xxxxxx; SI-List
Subject: [SI-LIST] Re: Strange resets happening in one of our circuit
boards

Shiraz Bashir

Normally, the power ground noise could not be so strong to reset the
system.

Best Regards

Zhangkun
2006.6.2

----- Original Message -----
From: "Bashir, Shiraz (GE Healthcare)" <Shiraz.Bashir@xxxxxx>
To: "SI-List" <si-list@xxxxxxxxxxxxx>
Sent: Friday, June 02, 2006 7:42 AM
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.
>=3D20
> 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
>=3D20
> OR
>=3D20
> 2. If we keep solder wires for scoping purposes.
>=3D20
> It doesn't happen when Unit is enclosed in it's metal housing.
>=3D20
> 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
>=3D20
> 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.
>=3D20
> Fix:=3D3D20
> A team member recommended to put a capacitor across resistor. Did that

>and problem went away.
>=3D20
> Can anybody explain why it is happening ? Is it noise from Power
Supply
> but how can it defeat a strong pull up ?
>=3D20
> Thanks,
> Shiraz Bashir
> General Electric Healthcare
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