Framebuffer test for STM32H735G-DK does not work

[Southbranch]

Hi,

My aim is to bring up custom display with the STM32H735G-DK board. As a first simple step I am using the existing display with an internal (small) framebuffer following the instructions given here: 3. Display with framebuffer in internal RAM | TouchGFX Documentation

See code snippets below, I am just trying to set some RGB888 colors in the framebuffer and loop through repeatedly by calling a simple method from the main(). But any other color than black or white just fails, typically shows as grey strips on the display...

Maybe I have missed something around how the RGB888 colors are stored..

Code to declare frame buffer:

// The display on the STM32H735G-DK is a RGB888 with 480 x 272 px.
// Lets use a small framebuffer for a portion of the display that
// could fit into internal RAM.
uint32_t *framebuffer[480*100];
uint32_t bufferSize = sizeof(framebuffer)/sizeof(framebuffer[0]);

Method called from main()

static void FillDisplay(void)
{
	// BLACK - works OK
	uint32_t RGB888_color = 0x00000000;
	for (uint32_t i = 0; i < bufferSize; i++)
	{
		*(uint32_t*)(framebuffer + i)= RGB888_color;
	}
 
	// WHITE works OK
	HAL_Delay(1500);
	RGB888_color = 0xFFFFFFFF;
	for (uint32_t i = 0; i < bufferSize; i++)
	{
		*(uint32_t*)(framebuffer + i)= RGB888_color;
	}
 
	// PINK does not work..
	HAL_Delay(1500);
	RGB888_color = 255 << 16 | 0 << 8 | 255 << 0;
	for (uint32_t i = 0; i < bufferSize; i++)
	{
		*(uint32_t*)(framebuffer + i)= RGB888_color;
	}
}

Code that executes in the main():

 /* USER CODE BEGIN 2 */
 // Enable the display with setting high pins
 // for LCD_DISP and LCD_BL_CTRL for STM32H735G-DK
 HAL_GPIO_WritePin(GPIOD, LCD_DISP_Pin,1);
 HAL_GPIO_WritePin(GPIOG, LCD_BL_CTRL_Pin,1);
 
 // Set frame buffer to LTDC
 HAL_LTDC_SetAddress(&hltdc, (uint32_t)framebuffer, LTDC_LAYER_1);
 /* USER CODE END 2 */
 
 /* Infinite loop */
 /* USER CODE BEGIN WHILE */
 while (1)
 {
	 /* USER CODE END WHILE */
 
	 /* USER CODE BEGIN 3 */
	 FillDisplay();
	 HAL_Delay(1500);
 }
 /* USER CODE END 3 */
 

What am I missing here...?

Many thanks in advance for any hints

[MM..1]

Try explain this FYI RGB888 is 3 byte format not UINT32 pointer format

uint32_t *framebuffer[480*100];

and show LTDC layer config.

[Southbranch]

Many thanks for your reply.

Please see attached screenshots for LTDC config.

I am probably mixing something up in regards to store RGB888 3-bytes into the 4-byte format. According to Color Formats | TouchGFX Documentation a purple color RGB(255,0,255) should be represented using: uint32_t brightPurpleRGB888 = 255 << 16 | 0 << 8 | 255 << 0;

But I just can't get it to work..

[Southbranch]

And here is the remaining LTDC config.

[MM..1]

Your code is complete bad. You set RGB888 for layer then data need place in memory every 3 bytes.

#define BUFFERSIZE 480*100*3
uint8_t framebuffer[BUFFERSIZE];

static void FillDisplay(void)
{
	// any collor
	
	for (uint32_t i = 0; i < BUFFERSIZE; i+=3)
	{
		framebuffer[i]= 0x00; //R or B collor
		framebuffer[i+1]= 0x00; //G collor
		framebuffer[i+2]= 0x00; //R or B collor
	}

[Southbranch]

Yes, you are right, my code was bad and yours works just fine, thank you so much! :beaming_face_with_smiling_eyes:

I also got it to work with external RAM and posting it here:

#define BUFFERSIZE 480*100*3
uint8_t* framebuffer = (uint8_t*)0x70000000;
 
static void FillDisplay2(uint8_t red, uint8_t green, uint8_t blue)
{
	for (uint32_t i = 0; i < BUFFERSIZE; i+=3)
	{
 // Works
		*(__IO uint8_t*) (framebuffer + i)= blue; //R or B collor
		*(__IO uint8_t*) (framebuffer + i+1)= green; //G collor
		*(__IO uint8_t*) (framebuffer + i+2)= red; //R or B collor
 
		// Does not work...
		//*(__IO uint32_t*) (framebuffer + i) = red << 16 | green << 8 | blue << 0;
	}
}

[MM..1]

Fine , but this all is better doing with D2DMA and for external RAM over linker section script and declare array same way as in internal.

TouchGFX handle this all for you.

[Southbranch]

Thanks, yes TouchGFX will be next step for sure:thumbs_up:

I just wanted to run this simple tests to get a better understanding from ground level.