to ttl or not to ttl..!

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sirmorris
Posts: 2811
Joined: Thu May 08, 2008 5:45 pm

to ttl or not to ttl..!

Post by sirmorris »

I've been playing with my userport add-on. The documentation says that the outputs are ttl. Now I always understood this to mean 0 and 5v.

BUT NO!

If you already knew this and are now :lol: at me, then please continue until you are satisfied :D

How can I guarantee a particular voltage level?

Transistor and 2 resistors like this?

Code: Select all


                  collector to VCC
                /
port>--[====]--K
                \
                 +--- tap VCC here
                 Z
                 Z
                 |
                 emitter to ground through suitable current-limiting resistor

The thing is, I'd like to guarantee 0 to VCC levels. I could use a zener to clamp to a maximum voltage, but it would allow lower ones through. I think the 4050 works this way too?

:?
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siggi
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Re: to ttl or not to ttl..!

Post by siggi »

I would use TTL chips with open-collector outputs. Your external resistor pulls the voltage to your "high" level (which could be much more than +5V, if you use them as level converters) and the internal transistor pulls the voltage to 0V. Works fine, if you do not draw too much current.

Siggi
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sirmorris
Posts: 2811
Joined: Thu May 08, 2008 5:45 pm

Re: to ttl or not to ttl..!

Post by sirmorris »

Aah yes, I didn't think about that :)
ksng36

Re: to ttl or not to ttl..!

Post by ksng36 »

There are what it is level translator chips that can safety change your logic levels around. The one's I have seen will translate from 5V to 3.3V or back the other way. Can't think of the part numbers, though.
sirmorris
Posts: 2811
Joined: Thu May 08, 2008 5:45 pm

Re: to ttl or not to ttl..!

Post by sirmorris »

Would that be a 4050?
ksng36

Re: to ttl or not to ttl..!

Post by ksng36 »

I don't believe the 4050 is a level translator chip. No, what I'm referring to is a type of buffer chip which will take 5V logic levels and convert them to 3.3V logic levels. And I believe they are bidirectional. I would go and check out the Maxim website. Just type in "Maxim" or "Dallas Semiconductor" into your favorite web search engine and navigate to it.
sirmorris
Posts: 2811
Joined: Thu May 08, 2008 5:45 pm

Re: to ttl or not to ttl..!

Post by sirmorris »

Ah yes I know these devices - If they were available in anything other than uMax and uDFN packaging they'd be usable..! I'm not ready to start hand-soldering packages that tiny ;)
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