AVR-Chat

--- In AVR-Chat@yahoogroups.com, "Bob Gardner" <bob.linda.gardner@...> 
wrote:

> ... I don't want to cut corners.

Then there's one more trick that might be worth bringing up.  All the 
level converter ICs mentioned will work great, but you probably don't 
have them in your spares drawer.  For two connections at fairly low 
speed (I'm still assuming you're using the UARTS) you will also be able 
to use a couple of transistors in common-base configuration as level 
converters.  Any old NPN small signal transistors will do, such as the 
2N3904.  Pull the base up to VCC on the driving side with a fairly high 
value resistor.  Connect the driving port bit to the emitter.  Pull the 
collector up to VCC on the driven side with a medium value resistor and 
connect it to the driven port bit.  Without doing the math, I would try 
something like 4.7 kohms on the collector and 47 kohms on the base, but 
anything in this area should work.  When the driving output is higher 
than the base resistor, the transistor is off and the collector 
resistor pulls the driven input up.  When the driving output goes low, 
the transistor turns on and pulls the driven input down to about 0.2 V 
above the driving output voltage.  You lose a small amount of noise 
margin, but in many projects this is of little consequence and the big 
advantage is you can often pull this trick with parts on hand.

Graham.