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Chipkit Uno and SD card on TFT board

Created Sat, 10 Oct 2015 16:40:53 +0000 by YOBE.


Sat, 10 Oct 2015 16:40:53 +0000


I have 2 types of TFT boards: 1.8 and 2.2 version. Steering the TFT works but when I tried to include the SD card, I could not setup communication. First doubting my program, I then found out that TFT 2.2 was working fine, but TFT 1.8 was not.

After measuring, I discovered that the 3 serial resistors on the SPI lines are different between 2.2 and 1.8 version. 1.8 has 1K version and 2.2 has the 100ohm value.

So seems that the 100ohms works but on the TFT 1.8, it doesnt work.

2 questions: a. Do you know why we have these installed (linked to max speed?) because they are not voltage deviders (I use 3.3V board)? b. Am I allowed to just bridge the 3 resistors on the 1.8 version to make it work?



Sat, 10 Oct 2015 20:27:23 +0000

Does the 1.8@ TFT use SPI for communication? If so then that's most likely your problem - you are using hardware SPI to communicate with the TFT but the SD card uses software bit-banging (yes, I know it's horrible) SPI through the same pins. So there's a big conflict there.

Better SD support will come, one day...


Sun, 11 Oct 2015 02:51:45 +0000

Hi Mat,

Not quite sure if your right. In the library it uses SD with DSPI, so to my understanding this is the hardware SPI port for the SD Card? Wrong? Yes, both 1.8 and 2.2 are using the SPI port. But 2.2 works, seems the resistors are the issue. Because for the TFT screen I dont use the library, too much code for what I need. I wrote my own protocol with hardware SPI ;)

So, my conclusion, the resistors on the 1.8 have a too high resistance to work with 3.3V environment. I think....


Sun, 11 Oct 2015 09:29:14 +0000

Hmm, ok - might you be using the stable version of UECIDE on there then? I re-wrote part of the SD library to use hardware SPI on there.

Ok, so yes it may be the resistors. There may be a resistive divider there to drop 5V signals down to 3.3V, or there may be a buffer chip on there to reduce the 5V to 3.3V - and of course we don't send 5V signals.

What display is it that you're having trouble with?


Sun, 11 Oct 2015 11:55:42 +0000


I connected my SPI signals directly to the SD card (so eleminating the resistors that where is serial) and this solved the problem. Only had to do this for the 1.8 inch version as the 2.2 inch version had lower resistors and this did not cause a problem.

Solved for me