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Forum Index : Microcontroller and PC projects : tft lcd display chip select???

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viscomjim
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Joined: 08/01/2014
Location: United States
Posts: 925

Posted: 07:51am 30 May 2015

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While waiting for my 2.4" touch displays to arrive, I wired up a 1.8" display that had no touch. I left the cs pin on the display wired to ground as it states in the manual that without touch, it is not needed.

My 2.4" displays arrived and I plugged it into the breadboard in the same position where the 1.8" was and added the extra wiring for the touch option. I accidentally left the display cs wired to ground instead of wiring it to a pin on the uMite.

I ran the gui and touch tests and also the meter code from Zonker which has touch buttons. Everything worked correctly as far as I can tell. What does this pin do that it needs to be controlled by the uMite.

Hopefully not a stupid question????

Thanks!

matherp
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Joined: 11/12/2012
Location: United Kingdom
Posts: 10250

Posted: 07:59am 30 May 2015

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Quote What does this pin do that it needs to be controlled by the uMite

The CS pin tells the relevant chip that it should listen to the SPI bus. What you are doing by not controlling it is sending the same commands to the display controller and to the touch controller. It is pure luck whether or not the display controller will respond to the touch commands. If it does then you could have one controller trying to set the MISO data pin high and the other setting it low - bad!!! or touch commands could just corrupt the display.

It sounds like by luck you are getting away with it but it is bad practice and if you want to use another device also on the SPI bus then you may well have problems.

viscomjim
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Joined: 08/01/2014
Location: United States
Posts: 925

Posted: 08:22am 30 May 2015

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I guess it was just a fluke that things worked. I set it up properly now. I'm glad I didn't accidentally kill one of the displays doing that. Thanks for the info!

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