RaspberryPi-NHD-2.4-TFT

C executable for the RaspberryPi to output an area of the framebuffer to the NHD-2.4-240320CF TFT (ST7789, 16 bit parallel)

This is a driver for a Newhaven 2.4" TFT display (http://www.newhavendisplay.com/tfts-24-tfts-c-1_590.html). It maps the framebuffer of the main display (/dev/fb0) to memory, extracts a 320x240 pixel sized area and sends this to the TFT display via the 16 bit parallel interface.

A 320x240 area of the Raspberry Pi's desktop rendered on the TFT

The driver init code sets up the 16-bit parallel interface to the color mode 65K RGB with 5/6/5 bits for the red/green/blue pixel values.

The pixel data in the framebuffer is 32 bits per pixel, 8/8/8/8 bits for the alpha/red/green/blue pixel values. This 32 bit data is converted to the 5/6/5 16 bit format and send to the TFT driver IC via the GPIOs with a frame rate of around 20 fps.

Physical setup

Part list:

• Raspberry Pi 2 or 3
• NHD-2.4-240320CF-CSXN-F TFT display with ST7789S driver IC
• NHD-FFC40 Breakout-Board (40 pin, 0.5mm pitch)

Connecting the NHD TFT to the Raspberry Pi

Connect the TFT Pins from the FFC40 breakout to the Raspberry's GPIO (I used simple jumper wires) like this:

          TFT   Raspberry  
    ---------   ---------   
       GND  1   GND  
        NC  2   -  
        NC  3   -    
        NC  4   -  
        NC  5   -   
        NC  6   -  
       VDD  7   3.3v   
     IOVDD  8   3.3v (typical 1.8V, max 3.3V)   
        NC  9   -   
       /CS 10   GND (= CS is always enabled)   
       D/C 11   PIN 07 / GPIO04   
       /WR 12   PIN 12 / GPIO18   
       /RD 13   3.3v (the read function is not used)   
       DB0 14   PIN 13 / GPIO27   
       DB1 15   PIN 15 / GPIO22    
       DB2 16   PIN 16 / GPIO23    
       DB3 17   PIN 18 / GPIO24    
       DB4 18   PIN 22 / GPIO25  
       DB5 19   PIN 24 / GPIO08    
       DB6 20   PIN 26 / GPIO07  
       DB7 21   PIN 29 / GPIO05    
       DB8 22   PIN 31 / GPIO06    
       DB9 23   PIN 32 / GPIO12    
      DB10 24   PIN 33 / GPIO13    
      DB11 25   PIN 35 / GPIO19    
      DB12 26   PIN 36 / GPIO16    
      DB13 27   PIN 37 / GPIO26    
      DB14 28   PIN 38 / GPIO20    
      DB15 29   PIN 40 / GPIO21    
      /RES 30   PIN 11 / GPIO17   
       IM0 31   GND (= selects the 16 bit interface)   
        NC 32   -   
       GND 33   GND   
    LED-K1 34   GND   
    LED-K2 35   GND   
    LED-K3 36   GND   
    LED-K4 37   GND   
     LED-A 38   3.3v    
       GND 39   GND   
        NC 40   -    

Compiling and running the executable

• Download the file NHD_24_240320_Framebuffer2GPIO.c from this repository to your Raspberry Pi's home folder
• Open a terminal on the Raspberry Pi and enter this line:
  gcc -o NHD_24 NHD_24_240320_Framebuffer2GPIO.c -lrt -O3
  This will generate the executable NHD_24 in the same folder as the .c file
• To run the executable, enter
  sudo ./NHD_24
  into the terminal
• CTRL-C stops the executable

Offsetting the TFT display area

The 320x240 pixel area that is output to the TFT has its origin by default at the upper left corner (0/0) of the display. If you want to move the area to some other location within the display's resolution, you can change the values of the #defines X_START and Y_START in the .c file, lines 24 and 25.