> The mosfet approach is used mostly where you have a bidirectional data
> line, but SPI lines are all unidirectional. While the mosfet approach will
> definitely work, it has quite a few parts when compared with a single
> level translator chip from Maxim or others. Maxim also has bidirectional
> level translators that use the mosfet trick plus a speed up monostable
> trick.


Ahh, for some reason I read I2C. For unidirectional interfacing in 5 to 3
direction, use an NPN transistor as common-base amplifier - use the
emitter as input, collector as output(with pullup to 3.3 supply) and base
through a resistor to 3.3 supply. In the 3 to 5 direction, on a normal TTL
level pin, just hook it directly. However the SPI interface on the PIC has
Schmitt trigger inputs so that is marginal(at best). You can use the
common base method slightly modified, but at that point your three(or
four) pins have gobbled up a dozen little parts. Cheap parts, but what a
hassle. CD4049 for 5-to-3 + CD40109 for 3-to-5, perhaps?

Cheerful regards,

Bob

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