DISPLAY BASICS [1]
In general, the display subsystem of an embedded system
is designed to transfer the data from a block of internal processor memory
called a Frame Buffer (FB), which software running on the main Processor
updates to change the image being displayed. The display data consists of some
number of bits for each pixel of the displayed area. In most cases there are
specific bits which describe the intensity of the red, green and blue (RGB)
components of each pixel, although other formats such as YUV/YCrCb (luminance,
red chrominance, blue chrominance) are occasionally used.
A logic block generally referred to as a Display
Controller fetches data from the Frame Buffer, formats it according to the
desired display interface, and transmits it to the Display. Figure 1 shows a
basic system, where the Display Controller is internal to the Embedded
Processor and communicates directly to the Display
In many systems the Embedded Processor includes a Display
Interface Controller, which creates an intermediate communication structure
which is separate from the Display Interface. In this case the Display
Controller is external to the Embedded processor, and is often included within
the Display itself. This external Display Controller often includes its own
Frame Buffer. Systems of this type are shown in Figure 2. Note that it is also
possible to connect to an external Display Controller via a standard bus such
as PCI.
The intensity of each pixel on a display, whether it is a
CRT, LCD or other type, generally must be periodically refreshed. The
architecture of this function was developed in the days of CRTs, but has
remained quite consistent as shown in Figure 3. The refresh is usually done by
"raster scanning", which starts at the first pixel (typically the
upper left hand corner), generates an intensity for that pixel, and then moves
horizontally through all pixels of the first scan line. At that point a "horizontal
synchronization" or HSYNC signal occurs, which causes the refresh to move
to the beginning of the second line, and so on. This process continues until
all lines have been refreshed, at which point a "vertical
synchronization" or VSYNC signal occurs. This causes the refresh to return
to the first pixel, and the process repeats.
The physical nature of the display generally dictates
that the response to HSYNC or VSYNC is not instantaneous, and thus nothing is
refreshed for a time after each signal is received. These times are referred to
as "blanking periods". They are shown as the dashed lines In Figure
3.
Command
Mode Panels (Generally used in high end smartphones)
•
DSI (Display Serial Interface) in command
mode expects the panel to have an internal frame buffer. The job of DSS
(Display Sub System) is just to push out a frame to the panel. The panel has a
controller inside it which refreshes the screen using the content of the
internal buffer; the panel's controller refreshes the screen using its own
timings.
•
In DSI command mode, you control the rate at
which data is pushed out to the panel. So if application generates 50 fps of
data. It pushes out data at that rate. However, since the panel's controller is
refreshing at 60 Hz by itself, you will see tearing since the internal buffer
is filled slower than 60 Hz.
Video
Mode Panels (Generally used in Tablets)
•
DSI in video mode requires us to push out
data continuously to the panel. The DSS completely controls the panels timings
and hence the rate of refresh of the panel. This is needed as the panel has no
internal buffer and needs the host to pump out data to it continuously.
•
In the video mode, the DSS driver provides
some API which tells an upper layer the right time to present a new frame to
DSS, this is the time when a vsync occurs. If you use this API to wait for a
vsync, because of its slow rate of pushing out data, the application is never
ready with a frame for every alternate vsync from the panel, hence leading to
half the frame rate.
Conclusion
In general command mode
panels have better frame rates than video mode panels for browser use cases
because of additional internal frame buffer.
References
(1) http://www.kozio.com/view_files/Embedded_Display_Interfaces_WP_final.pdf
