using 5V pro mini with device takes 3.3V

[kaos]

I am starting to make a touching screen switch with TFT and pro mini.
I have a few 5V pro mini, but later on I found out my TFT needs to work with 3.3V
I think I can just use a separate 3.3V power source to TFT and connect the other digital/analog pin to work?
or just use a step down regulator(which I have) from 5V pro mini Vcc to down step to 3.3V then connect to the TFT.
But I am not sure the connection between other digital or analog pins has any ill effect or do they even work this way? obviously 3.3V pro mini works at 8Mhz and 5V works at 16 mhz.

[jhsa]

You don't know if the TFT's data pins are 5V tolerant.
In some cases a 5V arduino can work at 3.3V. That is the case of a DIY RF module I have built recently.
So try to power the 5V arduino with 3.3V. I think it will still work at 16Mhz. I wouldn't risk 5V on the TFT's data pins.

Just my 2c

João

[kaos]

the supply voltage is easy to change. My question is does 3.3V TFT(supplied with 3.3V) can even be working with the 5V pro mini (supplied with 5V separately) on those digital/analog pin. or the communication pins' output is independent of the power source they use.

[jhsa]

if the arduino is supplied with 5V then the logical level on its pins will also be 5V. If the TFT is not 5V tolerant.. then... puuuffff

João

[kaos]

In that case I guess I need to get a 3.3V pro mini to work with. Yak!

[jhsa]

Why? As I said before you can try to power the 5V arduino with 3.3V. Get a 3.3V regulator and connect the output to the arduino's VCC pin.

I have several projects working this way.
But of course, you do what you gotta do

João

[kaos]

Does 5v pro nini (16MHz) powered with 3.3V works at 8Mhz? That is where I don't understand. If the TFT is operating at 3.3V will those communication pins work with the 5V pro mini powered at 3.3V reliably?
my understanding the 5V pro mini at 16 Mhz has different crystal than 3.3 V pro mini at 8Mhz. If I change to 3.3V I need to change the crystal as well.

[jhsa]

The pro mini has a 16Mhz crystal/resonator so it will try to oscillate at that frequency. The 8Mhz pro mini obviously has a 8Mhz crystal..

If the pro mini is powered from 3.3V, the logical level on its pins will also be 3.3V
If you have already a 5V pro mini, try it before you buy the other. Unless of course you really need the 8Mhz pro mini..

João

[kaos]

Ok, I will give it a try and ran some test program. If i connect a 3.3v power to the 5V mini, do I NEED to connect to Vcc or I still can connect to RAW?

[jhsa]

VCC..
Watch the polarity of course.. processors don't like reverse polarity very much..

João

[kaos]

Jhsa:
thx for the suggestion.

But no, it won't work. the pro mini light up and loaded a simple demo program, but nothing happens on TFT.
Also I found out the 2.8 TFT is just too big for pro mini to handle, not enough memory to do anything else other than the simplest demo program.
Need to use a UNO or mega2560 for this project.

[jhsa]

The UNO has exactly the same memory as the pro mini.. It has the same chip..

João