> >>>I know that when I've implemented I2C buses
> >
> >> no pullups needed, because you output are push-pull instead of
> >> open-collector.
>
> > A hardware slave I2C PIC (eg F88) has both the usual N-ch / P-ch
> > push-pull i/o FETs.
>
>You probably meant the right thing, but it sounds odd. While the ports do
>have the usual push-pull output FETs, in I2C mode the push-up FET is
>disabled -- as it has to be. So effectively, in I2C mode the SPI data/clock
>lines are open drain outputs -- on both master and slave.

All well and good but perhaps a complex way to explain, that i2c is 
bidirectional and
SPI is not.  SPI is a good old fashioned TTL-style hookup -- everything that
is an output connects only to inputs, and all inputs and outputs stay that 
way.  It's
also why SPI has more wires, because there's a Data Out and an Data In.

Barry

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