DSI + LTDC frequency ratio.

[af.bulychev9]

I have display with controller ILI9881C, which controlled via mipi-dsi.

I want to use 800x800x24bit mode. How to understand what's the point to use dsi txclkex and dsi line byte frequency? and how it is correlate with ltdc frequency? in display controller dataheet i have the next description for frame rate

how can i calculate pixel timings for Horizontal Back Porch and Horizontal Front Porch? is it must be based on ltdc frequency?

[Tesla DeLorean]

The line and frame totals drive the​ pixel clock. The rate of data out of the LTDC has to be matched to the lanes and bits going over the DSI based on your colour depth and pixel clock.

Write a "fitter" to get parity in the rates based on the limited clock multipler/divider options. Adjust porches to accommodate within specs of display.​

[af.bulychev9]

Just for clarification, must i use LTDC frequency for bitrate ?

in my case - 24 bit on 108 MHz will equals 4.5 Mb/s , right?

[Tesla DeLorean]

The pixel clock for the example there would be 76.2 MHz. with 1.83 Gbps on the DSI

Problem here is going to be you only have 2 lanes, not 4

Is your SDRAM memory interface 16 or 32-bit wide?

Are you doing video play back? What is the hard frame rate for that?

[Tesla DeLorean]

if the display you're talking about is say 1000x1000 @ 60 Hz, that's 60 MHz, and need 1.44 Gbps of DSI bandwidth, perhaps 1.2 Gbps @ 50 Hz

What STM32? F4, F7 or H7 ?

[af.bulychev9]

Stm32f769, 8Mhz hse.

​display is 800x800

Fps is not critical. I just want understand how to calculate values for settings​

[af.bulychev9]

800x800x24@30fps is 460Mbps , correct?

[af.bulychev9]

And a 19.2MHz on ltdc?

[Tesla DeLorean]

But the horizontal and vertical totals will be closer to 1000, check what the display specifies.

The 2 lane DSI gives you a ceiling of 1 Gbps

So perhaps 41.67 Hz @ 24bpp or 62.5 Hz @ 16bpp

60 Hz @ 16bpp -> 0.9 Gbps

[af.bulychev9]

Ok, understand,

But i still confused how to set frequency of ltdc and dsi. i need to set it as maximum from HSE, or i need to make it closer to calculated bitrate?