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question about 3.3V TTL UART with Arduino
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question about 3.3V TTL UART with Arduino

by redsparrow on Sat Apr 25, 2009 10:03 pm

Hi, I'm new to microcontrollers and I have a few questions. (My background is computer science and software design.)

I was looking at getting a Duemilanove, a GPS shield and an EM-406A, but I'm pretty excited about the Venus634LPx (with flash) they have a SparkFun. I'm thinking I might try it with the GPS shield. The Venus uses 3.3V for its serial communication where, it's my understanding that, the Arduino uses 5V.

For the power itself, I see the GPS shield has a couple of unlabeled holes near R1 and R2 that are 3.3V, one from the Arduino (limited to 50mA, I believe) and the other looks to come from IC1 on the shield, which should be good. (I could try the Lassen iQ surface mount, but those holes should work, right?)

My real question though is about the communication. What would I have to do to accommodate the 3.3V TTL UART, if anything? The Lassen uses the same Tx and Rx as the EM-406A, so maybe I wouldn't have to do anything different for IO?

As a beginner, I may end up going with the EM-406A anyway, but I thought I should scope out my options. Having the EM-406A surface mount on the shield certainly helps... Although, I could buy an EM-406A cable and use it for connecting the Venus's Tx, Rx, PPS and GND and get the 3.3V from the previously mentioned spot... Hm...

So many decisions.

Thanks for taking the time to read this, and for any help you might be able to offer...
redsparrow
 
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Re: question about 3.3V TTL UART with Arduino

by redsparrow on Sat Apr 25, 2009 10:26 pm

I see in the specs it actually says 3.3V LVTTL. From the Googling I've done, I get the impression that means it's happy with low voltage, but should also be tolerant of high (5V) voltage. Is this a sound assumption?
redsparrow
 
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Joined: Thu Apr 23, 2009 10:07 pm

Re: question about 3.3V TTL UART with Arduino

by trialex on Sun Apr 26, 2009 5:19 am

No i think LVTTL means "Low Voltage Transistor-Transistor Logic" - I don't think that necessarily implies it is 5V compatible - in fact I'd guess that it wasn't.

Translating between logic levels is a pain. You could buy an arduino that runs at 3.3V - like the "skinny" from sparkfun. The skinny doesn't have communications on board - you'd need to buy a USB->TTL cable like the FTDI one for programming. And it'd need to be the 3.3V one.

Obviously up to you though, but especially for a first project, sticking with 5V versions of everything is easier.
trialex
 
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Re: question about 3.3V TTL UART with Arduino

by redsparrow on Sat May 02, 2009 3:03 pm

Thanks, trialex. I'm going to go with the 5V for now. Once I'm comfortable with everything else, I can learn more about translating between the voltages (or go for a 3.3V Arduino...)
redsparrow
 
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Joined: Thu Apr 23, 2009 10:07 pm

Please be positive and constructive with your questions and comments.