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Author Topic: 3.3v vs 5v, can't make up my mind  (Read 1869 times)
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Ok, I am working on a wireless display.
The LCD is 3.3 volt and the IO's are 5v tolerant. I will be using an XBee which is also 3.3v. These items will talk to an Arduino Pro mini (I'll call it Pro1). Pro1 will get it's data from a second Arduino Pro Mini (Pro2) which is 5v running at 16Mhz.

Because Pro2 is 5v should I make Pro1 5 volts as well? Should to pro1 operate at the same 16Mhz or does it not really matter. What type of performance hit will Pro1 take if I make it 3.3v? Obviously if Pro1 is 3.3 it will interface with the LCD and XBee units without the need for logic level shifting.

I can't make up my mind  smiley-twist
« Last Edit: March 23, 2012, 07:01:19 am by Jassper » Logged

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3.3V is just a little low for running an ATmega at 16 MHz.  Many people do it without problems but to keep within specs you need, I think, 3.6V for 16 MHz operation or run at 8 MHz.

Is Pro1 running on batteries?  If that is the case you would probably want 3.3V and 8 MHz because both will extend battery life.

I would tend toward 3.3V just to simplify the peripheral connections.
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Thanks for the reply,

Neither Pro1 and Pro2 are powered by batteries.

As long as Pro1 @ 3.3v and 8Mhz can communicate just fine with Pro2 @ 5v 16Mhz then I have to agree, using 3.3v on Pro 1 will save a bit on added components to communicate with the LCD and XBee.

btw, Pro 1 and Pro2 will be communicating over RS-232 via a MAX232 RS-232 Level converter. This is because some times Pro2 will be connected to a computer or other display instead of the Pro1 display requiring a standard RS-232 signal.

So, with that said, it shouldn't matter either way. The only time I see that I may wish I used a 5v on Pro1 is if I need to use the Analog in as all my sensors are 0-5v outputs.
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Quote
btw, Pro 1 and Pro2 will be communicating over RS-232 via a MAX232 RS-232 Level converter.
Minor detail smiley

Quote
So, with that said, it shouldn't matter either way.
Correct.

It sounds like the LCD would be happy with 5v, that just leaves the Xbee. You could deal with that using two resistors or a TXB0102 chip.

That way everything is back to 5v so you sensors are good.
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You could deal with that using two resistors or a TXB0102 chip.

Would you have an example use for this chip?
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It has two VCC pins, VCCA and VCCB, connect VCCB to 5v and VCCA to 3v3 then run your signals through the data pins.

The data sheet is quite clear I think.

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It has two VCC pins, VCCA and VCCB, connect VCCB to 5v and VCCA to 3v3 then run your signals through the data pins.

The data sheet is quite clear I think.

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The one section I wasn't clear on was the pull ups,
Quote
Pullup or Pulldown Resistors on I/O Lines
The TXB0102 is designed to drive capacitive loads of up to 70 pF. The output drivers of the TXB0102 have low
dc drive strength. If pullup or pulldown resistors are connected externally to the data I/Os, their values must be
kept higher than 50 kΩ to ensure that they do not contend with the output drivers of the TXB0102.
For the same reason, the TXB0102 should not be used in applications such as I2C or 1-Wire where an
open-drain driver is connected on the bidirectional data I/O. For these applications, use a device from the TI
TXS01xx series of level translators.

But I'm thinking I don't need them anyways.

Thanks
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This is for the Xbee right?

AFAIK Xbees are normal push/pull IO so yes you don't need pullup resistors.

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And the OE pin should be tied to Vcca, correct?

Thanks for your help
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But you still need a 3.3V power source for the Xbee. Where is that coming from with all else running at 5V?
I would stay with pro1 at 3.3V and pro2 at 5V.  Let the RS232 chips deal with the +/- signals, read this:

http://www.maxim-ic.com/app-notes/index.mvp/id/836

MAX3232 for the 3.3V supply, and standard MAX232 for the 5V supply.
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There will be a SOT 3.3v reg on board powered from the 7805 reg. I'm laying out both systems then will weigh pros and cons of each. One big thing for me is not needing to have 2 different Arduino pro's on hand and not have to worry about mixing them up.

I'm sure it will boil down to personal preference, but any input is welcome. smiley

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I see. Then I'd go with 5V/16MHz on each, 3.3V regulator for Xbee, and level translation chip such as TXB0102 for the Xbee Rx/Tx.
http://www.ti.com/product/txb0102
Then can use same RS232 chip on both ends also.
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And the OE pin should be tied to Vcca, correct?
Correct.

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Rob
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