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

["Hill, John" <jhill@xxxxxxxxxx>]

Hello Doug,

I think you misunderstand. We do not use the 100 ohm resistor to detect
the ESD. We use the 100 ohm resistor to detect which chip is driving the
Reset line low. Most current microprocessors can drive the Reset line
low. There is also the power on reset circuit driving this line.=20

The 100 ohm resistor can also be used to form a RC low pass filter, just
add a capacitor. It also lowers the Q on the series tuned tank of the
line and the input capacitance.

Hence, many times just adding a 100 ohm resistor can resolve the issue
because of the lower Q. The question is how much energy will be absorbed
by the loop antenna of the Reset line. By lowering the Q, the antenna
will ring less due to the impulse of the ESD event.

The antenna is the output capacitance of the output driver, the
inductance of the trace, the inductance of the ground and the input
capacitance of the input. The bigger the loop, the better the antenna.
The 100 ohm resistor helps to make this loop a bad antenna.

Note that just adding a capacitor makes a high Q loop antenna. We need
the resistor.

Best regards,

John


=20

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-----Original Message-----

From: Doug Smith [mailto:doug@xxxxxxxxxx]=20
Sent: Wednesday, June 07, 2006 12:54 AM
To: Hill, John
Cc: a.ingraham@xxxxxxxx; SI-List
Subject: Re: [SI-LIST] Re: Strange resets happening in one of our
circuit boards

Hi John and the group,

Scope probes of the kind you are thinking of cannot be used if=20
impulses from ESD in the same room are suspected. The probe will show=20
a pulse that is not really there. They make great ESD event detectors.

Generally, a custom measurement setup must be designed for the=20
product. Often this is a shielded magnetic loop for measuring voltages=20
in the circuit (yes voltages! see some of the papers at=20
http://emcesd.com ,  my site) or a home built voltage probe of very=20
small dimensions. In any case, the measurement must be proven to be=20
correct before using the result. I call this a "null experiment." For=20
a voltage probe it is shorting the probe to the circuit ground and=20
making sure what you see is much smaller than the measurement of the=20
node you want.

Doug

Hill, John wrote:

> Hello,
>=20
> I like to add a series 100 ohm resistor at the reset pin on the
> microprocessor. With a 10K pull-up the voltage divider is not an
issue,
> but with a scope you can see what is going on.=3D20
>=20
> This does two things. First, there is a little RC to the input.
Second,
> if there are multiple open collectors on the reset pin, you can tell
> which pin is driving the reset signal low.=3D20
>=20
> When you show a programmer that current is going into their chip, they
> believe that it is really his chip pulling the reset low.
>=20
> Best regards,
>=20
> John
>=20
>=20
> =3D20
>=20
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> The information in this email and attachments hereto may contain
legally =3D
> privileged, proprietary or confidential information that is intended
for =3D
> a particular recipient. If you are not the intended recipient(s), or
the =3D
> employee or agent responsible for delivery of this message to the =3D
> intended recipient(s), you are hereby notified that any disclosure, =
=3D
> copying, distribution, retention or use of the contents of this e-mail
=3D
> information is prohibited and may be unlawful. When addressed to
Takata =3D
> customers or vendors, any information contained in this e-mail is =3D
> subject to the terms and conditions in the governing contract, if =3D
> applicable. If you have received this communication in error, please =
=3D
> immediately notify us by return e-mail, permanently delete any =3D
> electronic copies of this communication and destroy any paper copies.
> ---------------------------------------
> -----Original Message-----
>=20
> From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]
> On Behalf Of Andrew Ingraham
> Sent: Friday, June 02, 2006 11:43 PM
> To: SI-List
> Subject: [SI-LIST] Re: Strange resets happening in one of our circuit
> boards
>=20
>=20
>>Even though reset durations may be long, you still need to bring
>>everybody out of reset in a timely fashion.
>=20
> ...
>=20
>>Thus the asserting edge of reset might be slow, but the negating
>>edge may need to be faster.
>=20
>=20
> And in other cases the asserting edge needs to be reasonably fast too.
> Sometimes it's necessary to "get everyone off the bus" (disable all
> output
> buffers) quickly.  "Quickly" here may mean on the order of 10s of
> nanoseconds; not exactly lightning fast, but not so slow that you
don't
> care
> either.
>=20
> The designer needs to understand the system's requirements,
> specifications,
> and Reset timing.  Each case may be different.
>=20
> Regards,
> Andy
>=20
>=20
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