Bob Blick wrote:
>>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. 


Isn't it simpler to use a diode? You have a diode drop from the
transistor emitter anyway.

      p
      u
      l
      l
      u
      p
3.3V-+--|>|--5V


> 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

So use a series resistor from the 3V with a pullup on the 5V side
to move the 3.3V range up over the s/t threshold.

> 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?

Robert
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