Thanks!  This is exactly the answer I was looking for.

- Ted


-----Original Message-----
From: Spehro Pefhany [mailto:speff@INTERLOG.COM]
Sent: Friday, July 16, 2004 10:56 AM
To: PICLIST@MITVMA.MIT.EDU
Subject: Re: [EE:] SPI bus w/multiple devices...

At 09:50 AM 7/16/2004 -0700, you wrote:
>I am just starting to fool with SPI.  I am more familiar with I2C.
>
>What I can't seem to figure out is, if I have multiple SPI devices on
>the same bus, is there anything I need to do to the bus lines, like in
>I2C, you need some 2k pullups?

No.

>One reason I ask is, how do I make sure that the SO lines don't cause
>shorting problem.  If a slave on the SPI bus is not selected, how can I
>be sure that the SO line is tri-stated, and not low, when someone else
>drives it high for transmitting data?

You control the individual /CE line on each slave devices, so just bring it
high on the currently selected device for long enough (usually << 1usec)
before enabling another device. In some special sitations you might want to
use a pull-up or pull-down, but in most it is not required.

>Can someone point me at some document that clarifies the spec?

Some of the old Motorola data perhaps, but really you need to look at the
data sheets for each device. There are a few parts that are "receive
only/daisy chainable" that don't tristate the SDO line when /CE is inactive,
so watch out for those.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the
reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

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