How to determine frame rate from pixel clock

[sid_27]

Hi I have designed VGA controller for XGA 1024x768 and its pixel freq is 65 MHz. How do I determine the frame rate (fps) for this.

 

[TrickyDicky]

you know you get 1024x768 (786,432) pixels per frame. 65,000,000/768432 = 82.65. But theres probably some blanking overhead, so its likely to be 60Hz. You need to read the timing specs.

 

[sid_27]

Is Screen refresh rate 60 Hz is same as 60 fps. What should be the pixel clock if I want 1024x768, 30fps

 

[mrflibble]

Strictly speaking fps doesn't matter for pixel clock, it's the refresh rate that matters for pixel clock. And no, refresh != fps. For example it's perfectly fine to have a 120 Hz refresh with only 30 fps. And in that example the pixel clock will be dictated by the 120 Hz refresh.

 

[sid_27]

What is the formula to calculate the fps, If my VGA Controller is 1024x768 and pixel clock is 65 MHz. What adjustment i have to do if I want 30fps output.

 

[TrickyDicky]

There isnt one. The FPS is just how often you update the output. You could have a 60Hz refresh rate (ie, you output 60 frames per second) but if the frame buffer is only updated every other refresh, then the frame rate is only 30 fps.
So there is no formula to work out from the resolution and pixel clock. The FPS is determined by you.

 

[mrflibble]

Take a display where you change the content once per day, but obviously still have a refresh rate of 60 Hz. Then the formula for your number of fps = 1/(24*3600). There we go, formula, problem solved.

Also, 1 microsecond of googling: https://en.wikipedia.org/wiki/Frame_rate

 

[FvM]

If the question is about CRT interface standards, there's a fixed relation between pixel and vertical frequency. The popular XGA standard uses 65 MHz pixel frequency with 60 Hz vertical scanning frequency. It's available both in analog VGA and digital DVI interface.