STM32F7 Discovery LTDC unit - changing system clock

Question

I have strange experiences with display on my F7 Discovery board. When working with 216MHz system clock, the display (LTDC) works properly. However after changing to 120MHz (using CubeMX, so the app ensures other clock buses are correctly configured, LTDC works with 25MHz) I can see only tearing as in a photo below. Should I change something else in addition to changing clock frequency?

Answer 1

Probably the problem is related to insufficient SDRAM memory bandwidth. Changing system clock to 120 MHz causes SDRAM controller to work at 60MHz. That may not be enough for ltdc to fetch data from frame buffer especially if double layering is enabled. I had similar issue after changing TFT display to higher resolution one. At 25MHz pixel clock, memory bandwidth requirement is roughly 100MB/s per layer. With double layers enabled it is twice that. SDRAM bandwidth at 60MHz and 16bit bus is less than 120MB/s. Try disabling second layer in ltdc although even then bandwidth is barely enough.