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'[PIC] PIC32 Vddcore'
2009\04\15@010859 by Tony Vandiver

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Hi All,

   I seem to get a lot of noise (measuring 1.9Vp-p with an antenna, uh,
I mean a scope probe) on the Vddcore pin when switching a lot of I/O at
once.  The PIC32MX440F512 is running at 80MHz, and I'm switching 12 I/O
lines on portd all at once at around 2MHz for testing purposes.  There
is a clear correlation of the noise to the I/O switch.  I've got a 0.1uF
ceramic in parallel with a 10uF LESR tantalum (0.9ohms) on the Vddcore
line, and the total distance from the cpu to the caps is around 400
mils.  Any suggestions on either rounding the outputs off a little or
should I try switching a single pin at a time (incredibly slow), or
changing the filter or it's location wrt the cpu on Vddcore?  I believe
that the noise on this line is causing some unintended POR resets, but
I've got the same looking Vddcore noise on two boards and only one
resets (showing POR as the source) so I'm not sure.

Thanks,

Tony



2009\04\15@043225 by Mike Harrison

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On Wed, 15 Apr 2009 00:09:02 -0500, you wrote:

>Hi All,
>
>    I seem to get a lot of noise (measuring 1.9Vp-p with an antenna, uh,
>I mean a scope probe) on the Vddcore pin when switching a lot of I/O at
>once.  The PIC32MX440F512 is running at 80MHz, and I'm switching 12 I/O
>lines on portd all at once at around 2MHz for testing purposes.  There
>is a clear correlation of the noise to the I/O switch.  I've got a 0.1uF
>ceramic in parallel with a 10uF LESR tantalum (0.9ohms) on the Vddcore
>line, and the total distance from the cpu to the caps is around 400
>mils.  Any suggestions on either rounding the outputs off a little or
>should I try switching a single pin at a time (incredibly slow), or
>changing the filter or it's location wrt the cpu on Vddcore?  I believe
>that the noise on this line is causing some unintended POR resets, but
>I've got the same looking Vddcore noise on two boards and only one
>resets (showing POR as the source) so I'm not sure.

I've just read a a Xilinx appnote about decoupling which said that at higher frequencies, lower
value caps are more effective than 100nfs due to lower inductance
http://www.xilinx.com/support/documentation/application_notes/xapp623.pdf

Simultaneous switching always increases noise and possibly ground bounce - the latter may be the
cause of your resets - even switching half at a time may help a lot. Adding series Rs to the lines
close to the pins will also help reduce the effect of switching the capacitive load of the tracks.

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