UM0851 datasheet(196/245 Pages) STMICROELECTRONICS

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Audio/video drivers

UM0851

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Doc ID 16604 Rev 2

hold a pixel, pixclock, HSYNC duration, VSYNC duration, and margin lengths. These

values are user programmable:

struct fb_var_screeninfo {

__u32 xres; /* Visible resolution in the X axis */

__u32 yres; /* Visible resolution in the Y axis */

/* ... */

__u32 bits_per_pixel; /* Number of bits required to hold a pixel */

/* ... */

__u32 pixclock;

/* Pixel clock in picoseconds */

__u32 left_margin;

/* Time from sync to picture */

__u32 right_margin; /* Time from picture to sync */

/* ... */

__u32 hsync_len; /* Length of horizontal sync */

__u32 vsync_len; /* Length of vertical sync */

/* ... */

};

Fixed information about the video hardware, such as the start address and size of

frame buffer memory, is held in struct fb_fix_screeninfo. These values cannot be

altered by the user:

struct fb_fix_screeninfo {

char id[16]; /* Identification string */

unsigned long smem_start; /* Start of frame buffer memory */

__u32 smem_len; /* Length of frame buffer memory */

/* ... */

};

The struct fb_cmap specifies the color map, which is used to convey the user's

definition of colors to the underlying video hardware. You can use this structure to

define the RGB (Red, Green, and Blue) ratio that you desire for different colors: Device

independent color-map information. You can get and set the color-map using the

FBIOGETCMAP and FBIOPUTCMAP ioctls:

struct fb_cmap {

__u32 start; /* First entry */

__u32 len;/* Number of entries */

__u16 *red; /* Red values */

__u16 *green; /* Green values */

__u16 *blue; /* Blue values */

__u16 *transp; /* Transparency */

};

Sample User application

Using the steps given below, you can prepare a simple user application, which works over

the framebuffer API. The program shows three color bands on the screen by operating on

/dev/fb0, the framebuffer device node corresponding to the display.

After opening the framebuffer device node (/dev/fb0) the program gets the fixed screen and

variable screen information. It first deciphers the visible resolutions and the bits per pixel in a

hardware-independent manner using the framebuffer ioctl FBIOGET_VSCREENINFO. This

interface command obtains the display's variable parameters by operating on the struct

fb_var_screeninfo. The command FB_ACTIVATE_FORCE enables and activates the CLCD

. The program then goes on to mmap () the framebuffer memory and writes color data in

each constituent pixel bit.

UM0851 Datasheet(PDF) 196 Page - STMicroelectronics

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