Basic knowledge about framebuffer

Horizontal and vertical are concepts introduced by CRT. the electron beam draws a horizontal line from left to right and returns to the beginning of the next line to continue the next line of scanning until the bottom line of the screen, then return to the top of the screen to continue the next frame scan.

Horizontal sweep: the time required for the electron beam to move from the end of the previous row to the start point of the next row.

Frame return: the time required for the electron beam to move from the bottom of the screen to the top of the screen

 

How to calculate the refresh screen rate of CRT:

Assume that the dotclock of the video card is 28.37516 MHz.

1. The scanning time for each point is 1/(28.37516e6Hz) = 35.242e-9 s

2. If the screen resolution is 640x480, it takes 640*35.242e-9 S = 22.555e-6 s to refresh a row. Considering that the horizontal sweep takes some time, we suppose 272 'pixels' is used'

(640 + 272) * 35.242e-9 S = 32.141e-6 s

3. the horizontal scan frequency is

1/(32.141e-6 S) = 31.113e3hz, that is, 31 kHz

4. For a frame of 480 rows, add frame return, assuming that the frame return is 49 'Lines'

(480 + 49) * 32.141e-6 S = 17.002e-3 s

It can be calculated that the frame Scan Frequency is:

1/(17.002e-3 S) = 58.815Hz

That is to say, the update rate is 59Hz.

 

 

Row synchronization hsync: each time a pulse is sent, it means that a new row of data is sent.

Frame Synchronization vsync: each time a pulse is sent, it means that the new image data is sent.

 

In both frame synchronization and row synchronization, the head and tail must have a return time. This arrangement originated from the time required for the electronic gun deflection of the CRT Display and became the standard. Therefore, the tft LCD screen also contains the return time.

 

 

Xres: horizontal screen resolution

Yres: the vertical resolution of the screen.

The LCD resolution of common embedded devices is mainly 320*240,640*480.

 

 

 

 

Refer to kernel/documentation/FB/framebuffer.txt.