NEC uPD72257, Предварительное руководство пользователя

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6.7 Dithering
The dither units adds random noise to a number of low order bits of each colour
value of each RGB(888) pixel colour.
The random noise is generated by a pseudo random number generator RNG. The
noise is evenly distributed.
The extent to which noise is added can be configured by
VInCONTROL.DITHEN[3:0]. VInCONTROL.DITHEN[3:0] determines the number
of low order bits of each colour value, which are replaced by bits, generated by
the pseudo random number generator.
In the end the RGB(888) colour data is truncated to RGB(565) format for storage
in the framebuffer finally.
The dithering performs and addition of k bits, specified by
VInCONTROL.DITHEN[3:0], to the k (for R and B) respectively k-1 (for G) lower
bits of of the colour values. Finally the 24-bit R(888) values are cropped to 16-bit
RGB(565) values to be stored in the framebuffer.
The number of manipulated bits of the green colour value is always one less than
for red and blue.
The data manipulation of the dithering unit can be represented as follows:
• R
5
[4:0] = [ (R
8
[7:0] + RNG[k:0]) and 0xF8 ] >> 3
• G
6
[5:0] = [ (G
8
[7:0] + RNG[k-1:0]) and 0xFC ] >> 2
• B
5
[4:0] = [ (B
8
[7:0] + RNG[k:0]) and 0xF8 ] >> 3
6.8 Video Data FIFO and Framebuffer addressing
This section describes the function of the video data FIFO and the way how the
FIFO data is transferred to the memory.
6.8.1 Video data FIFO
The 32 x 32-bit FIFO gathers the RGB(565) data of 64 pixels. If the data of 64
pixels is collected, the entire FIFO content is written to the framebuffer via the
AHB bus by using a burst transfer.
The data format of each pixel after dithering is 16 bit wide. Hence two consecutive
RGB(565) values are combined to a single 32-bit data word, stored in the 32-bit
wide FIFO - and later on in the framebuffer.
At the beginning of each new scanline the read and write addresses of the FIFO
are cleared - which actually means a FIFO reset. The first 16-bit RGB(565) value
is stored in the lower 16-bit of the first 32-bit FIFO entry, the next RGB(565) goes
to the higher 16 bit.
Video Input (Ravin-M only) Chapter 6
Preliminary User's Manual S19203EE1V3UM00
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