20X2 and XBee Voltage Question

dougmd

New Member
Hi Ladies and Gentlemen,

I recently bought two 20X2 and two Xbee modules. I created a 3.125v voltage supply and ran the X2's just fine with it. I also powered up the Xbee modules which showed an LED attached to the power on pin as working. The 20X2 has an internal LDO correct? Is the 20X2 all internally 5 volt logic or is it truly a 3.3v? If its truly 3.3v then I can connect the pins between the Xbee and the 20X2 without voltage dividers correct? I searched around a while and read some manuals, but still not sure and didn't want to damage a device.

Thank you,

Doug
 

srnet

Senior Member
Why 3.125v ?

Easiest to run the 20X2 and the Xbee from the same 3.0V or 3.3v supply and connect the pins direct.
 

dougmd

New Member
Why 3.125v ?

Easiest to run the 20X2 and the Xbee from the same 3.0V or 3.3v supply and connect the pins direct.
I used a 19v power supply and used a 5v voltage regulator and variable voltage regulator. Using the resistors I had available I was able to get the variable voltage regulator within spec for both the Xbee and the 20X2.

I read in the manual

"The 20X2 takes a slightly different approach. It has an internal 3.3V silicon die, but also contains an internal Low
Drop Out Regulator, which is automatically enabled when required. This means the normal 20X2 can be used
across the entire 1.8 to 5V voltage range"
So 3.3v silicon die means approximately 3.3v threshold for a logic '1' correct? I read the voltage off a pin I set high and it read around 3.1v. Is that because the voltage feeding the 20X2 is 3.125? That seems incorrect to me since there's an LDO feeding the 3.3V silicon die inside. The Xbee is 3.3v silicon die and the 20X2 is 3.3v silicon die...so logic thresholds for 1 and 0 should correspond correct?
 

dougmd

New Member
I guess to be clearer I'm wondering what the line logic voltage levels are for a 20x2. Given the LDO, is it steady at some line logic voltage or variable depending on the voltage of the power line?

Thank you!
 

SAborn

Senior Member
From my understanding is you supply the 20x2 with 5v you will get 5v logic level outputs, if you supply the 20x2 with 3.3v you will get 3.3v logic level outputs, so the logic out will be at supply voltage level regardless of what supply voltage is used.

If you are unsure it is always a good practice to place a limiting resistor in the logic lines between picaxe and device, something like a 330R or 1K resistor should work, this way the device is somewhat protected with current limiting should you get something wrong.
 

srnet

Senior Member
PIC (and thus PICAXE) logic levels relate to the supply volts of the chip.

The detail of the logic levels will be found in the data sheet for the 20X2s native PIC, to be found here;

http://www.picaxe.com/What-is-PICAXE/PICAXE-Chip-Labels/

Like I said earlier, just run the 20X2 and Xbee from the same supply and it should work just fine.

The only time you really need to consider the actual voltages of logic devices is when you are trying to connect two devices that have different supply voltages.
 
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Goeytex

Senior Member
The 20X2 does not have an internal LDO. With a 5V supply the logic level out will be about 4.8V - 4.9V. With a 3.3v supply the logic level out will be about 3.1 to 3.2 Don't over complicate this. Simply use the nominal supply voltage specified by the Xbee module. Unless ..... The Xbee requires a 5v supply and has 3.3v I/O, in which case the inputs "should" be 5v "tolerant". A situation then could arise with the Xbee and Picaxe both using the same 5V supply, where Xbee outputs being 3.3v and the Picaxe expecting 5.0v. Even in this condition the Picaxe seems to work ok, even though the logic levels do not precisely match. In this condition it is probably a good idea to use a current limit resistor on the Picaxe outputs.

I have run into several RF modules and one serial /TTL adapter that use or require a 5v supply but then only use 3.3v I/O. I also have a graphic LCD that specifies a 5v nominal supply but then uses 3.3v I/0.

If you will post a link to the datasheet for the Xbee module, better and more precise advise can be given.
 
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Technical

Technical Support
Staff member
The 20X2 does not have an internal LDO
Actually it does. However the end user does not need to worry about it, as it is only for the internal core and is fully automatic.
So the i/o pin voltage will be quite simply at whatever the supply voltage is - so use 2V up to 5V, whatever you choose will be the i/o voltage.
 
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