GitHub - yoprogramo/QRcode_ST7789: Subclasses of QRcodeDisplay to be able to use ST7789 TFT displays.

Open Library Manager (menu Sketch > Include Library > Manage Libraries…) then install the following libraries:

Creating a QRcode is just as simple as (note we are using a Lilygo board that does not expose the SPI interface and we have to use other SPI):

#define TFT_RST       23    
#define TFT_CS        5     
#define TFT_DC        16  
#define TFT_BACKLIGHT 4
#define TFT_MOSI      19
#define TFT_SCLK      18  

#include <Adafruit_GFX.h>      
#include <Adafruit_ST7789.h>   
#include <SPI.h>
#include <qrcode_st7789.h>

Adafruit_ST7789 display = Adafruit_ST7789(&SPI, TFT_CS, TFT_DC, TFT_RST);
QRcode_ST7789 qrcode (&display);


void setup() {

   Serial.begin(115200);
   Serial.println("");
   Serial.println("Starting...");

   // enable debug qrcode
   // qrcode.debug();
   SPI.begin(TFT_SCLK,TFT_RST,TFT_MOSI,TFT_CS);
   display.init(135,240);
   pinMode(TFT_BACKLIGHT, OUTPUT);
   digitalWrite(TFT_BACKLIGHT, HIGH);

   // Initialize QRcode display using library
   qrcode.init();
   // create qrcode
   qrcode.create("Hello world.");

}

void loop() { }