When sharing SPI, display can show glitches/artefacts if touch used called immediately after

[AgainPsychoX]

Problem

When sharing SPI with the touch sensor, display can show glitches/artefacts if touch used called immediately after. I noticed it the most while drawing strings with font 2.

My setup

Hardware

Microcontroller: Raspberry Pi Pico
Display: 4'' TFT SPI ST7796 480x320
Display driver: ST7796(S)
Touch chipset: XPT2046

Pin	GPIO	Notes
1	0	Reserved for UART0 TX
2	1	Reserved for UART0 RX
3	GND	Ground
4	2	SPI0 SCK, used for display & touch (shared)
5	3	SPI0 TX (MOSI), used for display & touch (shared)
6	4	SPI0 RX (MISO), used for display & touch (shared)
7	5	Chip select for the display
8	GND	Ground
9	6	Reset for the display & touch (shared?)
10	7	Register select (Data/Command) for the display
11	8	Chip select for the touch

Software

TFT_eSPI library version: 2.5.43

I use PlatformIO with earlephilhower Arduino Core:

platform = https://github.com/maxgerhardt/platform-raspberrypi.git
board = pico
framework = arduino
board_build.core = earlephilhower

User setup done also via PlatformIO build flags:

	; TFT driver configuration
	-DUSER_SETUP_LOADED=1
	-DST7796_DRIVER=1
	-DTFT_WIDTH=320
	-DTFT_HEIGHT=480
	-DTFT_MISO=4
	-DTFT_MOSI=3
	-DTFT_SCLK=2
	-DTFT_CS=5
	-DTFT_DC=7
	-DTFT_RST=6
	;-DTFT_BL=-1 (hardware)
	-DTOUCH_CS=8
	-DLOAD_GLCD=1
	-DLOAD_FONT2=1
	-DLOAD_FONT4=1
	-DLOAD_FONT6=1
	-DLOAD_FONT7=1
	-DLOAD_FONT8=1
	-DLOAD_GFXFF=1
	-DSMOOTH_FONT=1
	-DSPI_FREQUENCY=80000000
	; -DSPI_READ_FREQUENCY=20000000 ; same as default
	; -DSPI_TOUCH_FREQUENCY=2500000 ; same as default

Minimalistic code would be (approximately):

// ... #include's and tft object

void setup() {
	tft.init();
	tft.setRotation(1);
	// ... calibration (optional)
	tft.fillScreen(TFT_BLACK);
	tft.setTextFont(2); // most noticeable on this font
}

void loop() {
	tft.drawString("Hello! 123", 0, 0);
	
	uint16_t x, y;
	tft.getTouchRaw(&x, &y); // not even used, but it matters
}

On the photo below I used different code, but it all comes to the drawing. If necessary I might confirm the problem with the minimalistic code itself.

Result

String, the characters are drawn, but last character has weird, random blue/red/black dots artefacts.

Photo

Expected result

Normal operation without any glitches/artefacts.

Workarounds

1. Use tft.setTextColor(TFT_WHITE); - it makes rendering (font 2) characters using slower method, which cuts the glitch visibility.
2. OR, Use different font which uses flower drawing, again, cuts the glitch visibility.
3. OR, add some delay(2) before using touch related function (2 ms just from my testing on my setup).
4. OR, add spaces to the end of the text to make last character readable, glitch persists..
5. OR, maybe lowering display SPI speed could help (but it makes it slower to draw/update, so it's sad).

Other notes

I guess it's due to (shared) SPI speed changes: graphics operations are put to SPI buffer to be pushed out, but before it finishes the speed changes, which makes mess. I tried putting SPI_BUSY_CHECK in the beginning of TFT_eSPI::begin_touch_read_write but it didn't help.

Bug is most noticeable with font 2 due to Faster drawing of characters and background using block write in TFT_eSPI::drawChar. Making it slower by setting textcolor == textbgcolor by setTextColor(...) (which makes it use a lot of drawPixel calls), or using different font (which doesn't have fast drawing) makes it far less noticeable.

[Bodmer]

See this post and check if your display needs a modification.
#898

[AgainPsychoX]

Yes, it seems the modification helps. It's working without glitches now. Thank you.