Pin Assignment for Display on RP2040

[nadavmatalon]

A description of the problem and the conditions that cause it to occur:

My hardware setup is very basic:

(1) WAVESHARE RP2040 ZERO

and,

(2) WAVESHARE 1.9" 170x320 LCD SPI Display (driver: ST7789)
(see also the Wiki if needed)

I have a custom "Setup500_WaveShare_RP2040_Zero_SPI_Display.h.zip" file (attached here) with the relevant parameters for this particular display.

The problem:
The display only works as expected with the following pins assignments:

#define TFT_MOSI 3       // DIN pin
#define TFT_SCLK 2       // Clock pin

However, if I assign either of these pins to different pins on the RP2040 Zero in the "user_settings" file, I get a blank screen, for example:

#define TFT_MOSI 0      // DIN pin
#define TFT_SCLK 4      // Clock pin

(these are the pins I actually need for my project to work)

To clarify, the code itself compiles fine with any pin assignment, however, if the pin assignment for MOSI and SCLK isn't 2 & 3, then the display only shows a blank screen and none of the graphics.

After much testing, I can also confirm that the other display pins (DC, CS, RST) can be assigned to any of the RP2040 Zero without an issue (as long as MOSI & SCLK are assigned as above).

To rule-out hardware issues, I looked into the RP2040 Zero's schematics and it appears that all GPIO pins are identical in that they are broken-out directly from the RP2040 ic to the Zero's pin header (that is, no pull-up or pull-down resistors or anything else on them that might cause an issue).

So, bottom line, with everything else exactly the same (both hardware and software), when pins 2 & 3 are used for MOSI & SCLK respectively, the display works great. However, assigning these functions to any other pin on the RP2040 Zero causes the display to show a blank screen.

Given all this, I'm almost certain the issue is somewhere on the library's software side, though I couldn't figure out what it might be.

Environment details:

Arduino IDE: Ver. 1.8.19

TFT_eSPI Library: Ver. 2.5.0

Help please?

[Bodmer]

Only certain pins can be used with the built in SPI peripheral. This is a hardware level constraint, see section 2.4.3 of the RP2040 datasheet.
So for example MOSI (SPI TX) cannot be allocated to GPIO 0.

An alternative approach based on a PIO SPI implementation is possible, see setup file 61.

[nadavmatalon]

Hi again,
Thank you very much for taking the time to look into this and the detailed info - much appreciated!