A couple of days after this thread on SPI series resistors came up I
coincidentally had a problem with ringing on an SPI bus. I followed
Michael's suggestion of installing 220 ohm resistors in series and
viola! the problem was solved. It was certainly handy that this
solution had just been mentioned on the piclist.

We also experimented with filtering but the series resistors worked
best. So if anyone else has this problem I can definitely recommend
series resistors as a fix for SPI ringing.

Cheers,
Zik

On 7/20/07, Michael Rigby-Jones <Michael.Rigby-Jones@bookham.com> wrote:
>
>
> >-----Original Message-----
> >From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu]
> >On Behalf Of David VanHorn
> >Sent: 19 July 2007 21:04
> >To: Microcontroller discussion list - Public.
> >Subject: Re: [EE] Resistors on SPI bus pins?
> >
> >
> >On the AVR, and probably on the pics as well, the SPI is how
> >you program the chip, so you have to design the hardware such
> >that any peripheral devices can't interfere with the
> >programmer.  The series resistors insure that if the
> >peripherals try to drive the bus, they will not be able to
> >affect the logic levels.
> >--
>
> Another reason for series resistors on the outgoing signals is to reduce ringing.  The rise/fall time of the SPI clock and data can be fast enough that the signals can be reflected due to imepdance mismatch, causing ringing.  The resistors slow down the edges helping to prevent ringing and also crosstalk.  Typicaly we always put a few hundred ohms close to the SDO, SCK and CS outputs of the micro, and on the data output of each peripheral.
>
> Regards
>
> Mike
>
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