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ST7789 TFT Readme

Image TFT

Table of contents

Overview

  • Name: ST7789_TFT
  • Description:
  1. C++ Library for a TFT SPI LCD, ST7789 Driver
  2. Dynamic install-able Raspberry Pi C++ library.
  3. Inverse colour, rotate, sleep modes supported.
  4. Graphics + print class included.
  5. 24 bit colour , 16 bit color & bi-color Bitmaps supported.
  6. Hardware and Software SPI
  7. Dependency: lgpio Library
  8. NB 'spidev.bufsiz' setting must be 65536 or greater see Notes section.
  • Author: Gavin Lyons

Software

User options

In the example files. There are 3 sections in "Setup()" function where user can make adjustments to select for SPI type used, and screen size.

  1. USER OPTION 1 GPIO/SPI TYPE
  2. USER OPTION 2 SCREEN SECTION
  3. USER OPTION 3 SPI SETTINGS

USER OPTION 1 SPI TYPE / GPIO

This library supports both Hardware SPI and software SPI. The SetupGPIO function is overloaded(2 off one for HW SPI the other for SW SPI). The parameters set for SetupGPIO define which is used.

USER OPTION 2 Screen size + Offsets

User can adjust screen pixel height, screen pixel width and x & y screen offsets. These offsets can be used in the event of screen damage or manufacturing errors around edge such as cropped data or defective pixels. The function InitScreenSize sets them.

USER OPTION 3 SPI Settings

InitSPI function is overloaded(2 off, one for HW SPI the other for SW SPI).

parameter default value note SPi type
HWSPI_DEVICE 0 A SPI device, >= 0. which SPI interface to use , ls /dev/spi* Hardware
HWSPI_CHANNEL 0 A SPI channel, >= 0. Which Chip enable pin to use usually 0 or 1 Hardware
HWSPI_SPEED 1000000 The speed of serial communication in bits per second. Hardware
HWSPI_FLAGS 0 mode 0 for this device Hardware
GPIO_CHIP_DEVICE 0 gpio chip device >= 0, check ls/dev/gpiochip* both
SWSPI_CommDelay 0 uS delay for GPIO software

The user can adjust if having reliability issues with SW SPI in some setups. This is a microsecond delay in SW SPI GPIO loop. It is set to 0 by default, Increasing it will slow down SW SPI further.

File system

In example folder: The Main.cpp file contains tests showing library functions. A bitmap data file contains data for bi-color bitmaps and icons tests. The color bitmaps used in testing are in bitmap folder, 3 16-bit and 5 24-bit images.

# example file name Desc
1 Hello_world Basic use case
2 Text_Graphic_Functions Tests text,graphics & function testing
3 Bitmap_Tests bitmap
4 Frame_rate_test_bmp Frame rate per second (FPS) bitmaps
5 Frame_rate_test_two rame rate per second (FPS) text
6 Hello_world_SWSPI Basic use case software SPI

Hardware

Tested and developed on:

  • Size 1.69 inch IPS color TFT LCD
  • Resolution: 240 (H) RGB x280 (V)
  • Control chip: st7789v2
  • Display area 27.972 (H) x 32.634 (V)
  • Panel size 30.07 (H) x37.43 (V) x1.56 (d)
  • Logic voltage 3.3V

Connections as setup in main.cpp test file.

TFT PinNum Pindesc RPI HW SPI RPI SW SPI
1 LED VCC VCC
2 SS/CS SPI_CE0 GPIO21
3 DC GPIO24 GPIO24
4 RESET GPI025 GPIO25
5 SDA SPI_MOSI GPIO5
6 SCLK SPI_CLK GPIO6
7 VCC VCC VCC
8 GND GND GND
  1. Connect LED backlight pin 1 thru a resistor to VCC.
  2. This is a 3.3V logic device do NOT connect the I/O logic lines to 5V logic device.
  3. Pick any GPIO you want for SW SPI, for HW SPI: reset and DC lines are flexible.
  4. User can select SPI_CE0 or SPI_CE1 for HW SPI
  5. Backlight control is left to user.

Output

output pic

Notes

spidev buf size

The maximum transaction size that 'lgpio' can handle by default is 65536 bytes, So this is also the maximum transaction size of Display_lib_RPI, if more than that needs to be sent it is sent in blocks of 65536 bytes. Certain 'Display_lib_RPI' functions use buffered writes, the ones that draw bitmaps, write fonts, fill rectangles, clear screen, etc. In order for this to work the spidev.buf size setting must be at least 65536. To check the setting on your device run command:

cat /sys/module/spidev/parameters/bufsiz

If it is lower than 65536 you can change it by adding this to the start of line in file /boot/firmware/cmdline.txt. (raspberry pi ) Make sure everything is on one line and there is space ' ' between this parameter and next one. Then reboot machine. Verify again by running last cat command above

spidev.bufsiz=65536

bufsiz defines the number of bytes that the SPI driver will use as a buffer for data transfers.

Display offsets

The display initialisation requires an offset calculation which differs for different size and resolution displays. This is in the code(Function AdjustWidthHeight()) but the many different size displays are not available for testing or dealt with. If using a display other than 240x320(the default and size of ST7789 VRAM) and if user finds they cannot address all screen or their data is offset. Try Setting the pixel width and height of your screen to 240X320 and do not write as much as possible to the part of the Video RAM you cannot see. For example if you have a 240X280 display in 0 degree rotation

  1. Set pixel Width = 240 and pixel height = 320
  2. Do not write to the missing 40 pixels in the Y-axis, you still can but it is inefficient.