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PiTFT 5V divided to 3.3V for PiTFT 3.3V input power?

Posted: Sat Oct 28, 2017 4:28 pm
by welshgeekboy
Hello,

I have a 10 pin connector which I'd like to use as the connector between a Pi Zero and a PiTFT 3.5". I've already desoldered the PiTFT headers, and will use a 10 pin header soldered to the PiTFT with a 10 pin connector bridging the gap between Pi and screen.

PiTFT pinout here: https://pinout.xyz/pinout/pitft_plus_35#

However, the PiTFT needs 8 GPIO pins and 3 power pins (5V, 3V3, 0V) which is a total of 11 pins. So my thought was to take the 5V at the PiTFT end (power comes from the Pi) and use a voltage divider soldered directly to the back of the PiTFT which will turn the 5V to 3V3. Is this likely to cause issues? Presumably the 3V3 is used by the PiTFT to communicate with the Pi. If the two seperate 3V3 sources (one on the Pi, one on the PiTFT) are not exactly the same voltage, or not connected together and therefore equalised, are there likely to be any problems? Can I leave them as two seperate 3V3 sources that are not connected?

That's one question - the main one. But also, if the above idea does not work, does the PiTFT really need board pins 18, 22, 24, 26? Is there a way to leave out one of those pins? I'm presuming not.

Of course, I could just add an extra connection between the PiTFT and the main board, but it's neater not to.

Thanks!

Re: PiTFT 5V divided to 3.3V for PiTFT 3.3V input power?

Posted: Sat Oct 28, 2017 5:25 pm
by pcmanbob
Hi.
Looking at the information you provided.

It says gpio18 can be used to pwm dim the back light. So if you don't need that function you could leave out gpio18 connection. Then you will only need 10 connections including using 3.3v supply from pi.

Re: PiTFT 5V divided to 3.3V for PiTFT 3.3V input power?

Posted: Sat Oct 28, 2017 5:45 pm
by welshgeekboy
Sorry, I didn't properly RTFM. It looks like the backlight is set to be on automatically- I presumed I had to have a pin controlling it at all times. So in theory I could have a spare pin on the 10 pin connector, but though I don't need to be able to set the brightness of the screen, but I would like to be able to control whether the backlight is on or off.

I've been looking at what the Adafruit site tells us about the backlight: are they saying that the touchscreen driver effectively creates more pins (STMPE GPIO) for use on the PiTFT, that don't use physical pins on the Pi, but which can still be set ON/OFF from the Pi using software? So I could still have digital backlight control, without using a Pi pin?
https://learn.adafruit.com/adafruit-pi ... stmpe-gpio

Re: PiTFT 5V divided to 3.3V for PiTFT 3.3V input power?

Posted: Wed Nov 01, 2017 12:28 pm
by welshgeekboy
To answer my own question- the PiTFT creates extra GPIO that can be accessed from the Pi using simple code.

https://learn.adafruit.com/adafruit-pit ... ht-control

Thank you for your help.