>>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.

No, common base does not work like that. Collector-emitter voltage is the
saturation value of the transistor, you can get it down to 50 millivolts
pretty easily.

>> 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.

Then you miss the bottom threshold, and your total voltage swing is
reduced. You need more voltage swing, not less.

Cheerful regards,

Bob


-- 
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist