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Epson Research and Development Page 15
Vancouver Design Center
Programming Notes and Examples S1D13705
Issue Date: 02/01/22 X27A-G-002-03
4 Look-Up Table (LUT)
This section is supplemental to the description of the Look-Up Table architecture found in
the S1D13705 Hardware Functional Specification. Covered here is a review of the LUT
registers, recommendations for the color and gray shade LUT values, and additional
programming considerations for the LUT. Refer to the S1D13705 Hardware Functional
Specification, document number X27A-A-001-xx for more detail.
The S1D13705 Look-Up Table consists of 256 indexed red/green/blue entries. Each entry
is 4 bits wide. Two registers, REG[15h] and REG[17h], control access to the LUT.
Each Look-Up Table entry consists of a red, green, and blue component. Each component
consisting of four bits, or sixteen intensity levels. Any Look-Up Table element can be
selected from a palette of 4096 (16x16x16) colors.
In color display modes, pixel values are used as an index to an RGB value stored in the
Look-Up Table. In monochrome modes, pixel values still index into the LUT, but only the
green component is used to determine display intensity.
The selected color depth determines how many index positions are used for image display.
For example at one bit-per-pixel (bpp) only index positions 0 and 1 of the Look-Up Table
are used. At 4-bpp the first 16 index positions of the Look-Up Table are used and at 8-bpp
all 256 Look-Up Table index positions are used.
The Look-Up Table mechanism itself consists of an index register and a data register. The
index, or address, register determines which element of the Look-Up Table will be
accessed. After setting the index the LUT may be read or written through the data register.
The first data element read or written is the red component of the entry. Subsequent
read/write operations access the green and then the blue elements of the Look-Up Table.
The S1D13705 LUT architecture is designed to provide a high degree of similarity in
operation to a standard VGA RAMDAC. However, there are two considerations which
must be kept in mind.
• The S1D13705 Look-Up Table has four bits (16 levels) of intensity per primary color.
The standard VGA RAMDAC has six bits (64 levels). This four to one difference must
be taken into consideration when converting from a VGA palette to a LUT palette. One
suggestion is to divide the VGA intensity level by four to arrive at the LUT intensity.
However, most applications specify the red, green and blue components as eight bit
intensities. To determine the appropriate S1D13705 Look-Up Table value we recom-
mend using the four most significant bits.