How do I use the extra GPIO pins in Adafruit's TFT 3.5"

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Poi
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How do I use the extra GPIO pins in Adafruit's TFT 3.5"

Postby Poi » Wed Mar 13, 2019 6:15 pm

I got the Adafruit PiTFT 3.5" display, this one here https://www.adafruit.com/product/2441.

It has an additional 2x13 GPIO connector header in the back, but I am not exactly sure how you are supposed to use it.

I know the display doesn't use the 40 pins of the Pi, but it takes up all the space when you connect it.

Are the signals from the unused pins in the Pi routed to the pins in the back of the display?
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Ultibo
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Re: How do I use the extra GPIO pins in Adafruit's TFT 3.5"

Postby Ultibo » Wed Mar 13, 2019 11:39 pm

Poi wrote:Are the signals from the unused pins in the Pi routed to the pins in the back of the display?

Yes, if you look at the Adafruit page for the PiTFT 3.5 it says this:

There's a 2x13 'classic Pi' connection GPIO header on the bottom, you can connect a 26-pin Pi GPIO cable to it to use any of the other pins as you like. The other GPIO are broken out into solder pads at the bottom, in case you want to use more of the GPIO.

Which means that the 26 pin connector on the back of the board is wired exactly the same as the 26 pin connector on the earlier Pi A and B models and is exactly the same as the first 26 pins of the current 40 pin connector. The few extras that were not available on the 26 pin header have also been brought out to solder pads on the back of the board.

So you can use the GPIO pins on the Pi TFT connector exactly the same way you would use the GPIO on the Pi itself and everything should work unchanged. Just remember that the Pi TFT uses the 5 SPI pins as well as GPIO 24 and 25 so those won't be available for other things.
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Poi
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Re: How do I use the extra GPIO pins in Adafruit's TFT 3.5"

Postby Poi » Thu Mar 14, 2019 12:57 am

Great, thanks for that.

Just remember that the Pi TFT uses the 5 SPI pins as well as GPIO 24 and 25 so those won't be available for other things.


Does that mean in reality you can only use 19 pins out of the original 26? Or are the ones used by the display still useful somehow? Just asking out of curiosity, I think I will be just fine with 19 pins :P
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Re: How do I use the extra GPIO pins in Adafruit's TFT 3.5"

Postby Ultibo » Thu Mar 14, 2019 9:16 am

Poi wrote:Does that mean in reality you can only use 19 pins out of the original 26? Or are the ones used by the display still useful somehow? Just asking out of curiosity, I think I will be just fine with 19 pins :P

You could probably add another SPI device on the SCK, MOSI, MISO lines but there is no opportunity to use those pins for any other function and GPIO 24 and 25 will not be available, otherwise you're free to use whatever you need.
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