The Palette
7.0 The Palette
VIDC20 has a 28-bit wide 256-entry palette which is constructed out of 3 8-bit wide look-up-tables (LUTs), each with 256 entries, named Red, Green, and Blue, and one 4 bit wide LUT with 16 entries, named Ext. The Red, Green and Blue LUTs each drive their respective DACs, and the Ext LUT is normally configured to drive the ED[3:0] output port, except when Hires mode or LCD mode is selected. These bits may be used outside the chip for a variety of purposes such as supremacy, fading, Hi-Res and LCD driving. The ED[7:41 output port is normally driven from the Ext register, ereg[7:4], which may be written at any time, so these bits can be used as DC control bits.
The mapping of the logical colours through the LUTs is dependent on the mode in use, as follows:
•	In 1,2,4 bits/pixel modes, the logical data is fed simultaneously to all 4 LUTs. This gives a fully flexible palette with any logical colour being mapped to any physical colour, and any ED[3:0] value. The palette will give 16 colours from a selection of 224.
In 8 bits/pixel modes, the logical data is fed simultaneously to a114 LUTs. This gives a fully flexible palette with any logical colour being mapped to any physical colour. Logical colours 0-15 access the Ext LUT, and logical colours 16-255 access location 0 of the Ext LUT. The Ext LUT again drives ED[3:0]. The palette will give 256 colours from a selection of 224.
In the 16 bits/pixel mode, a patented technique has been developed. This approach is highly flexible and allows many different addressing modes e.g. 5-5-5, 5-b-5 etc. In this mode 216 colours are available from a selection of 22a. Please see the Applications Note "76 bit Colour".
In the 32 bits/pixel mode, 24 bits from the logical field will drive the 256 entries in each of the colour LUTs (8 bits to each LUT) and 4 bits will drive the Ext LUT. The upper 4 bits are discarded.The palette will give the full range of 224 colours.
Note that where a logical field does not drive all the palette entries (such as in 4 bits/pixel mode) only the lower part of the palette is used. Unused sections need not be programmed.
When HiRes mode or LCD mode is selected, then the palette must be set up in a predetermined configuration. This is explained in Chapter 9.0 Hi-Res Support and Chapter 10.0 Liquid Crystal Displays respectively.
7.1 Palette Updating
A signal FLYBK is provided which is identical to that on VIDC10. i.e. FLYSK goes HIGH at the start of the first raster which is not display, and goes LOW at the start of the first raster which is display. The rising edge of this signal may cause an interrupt in the memory controller, and the palette should be updated at this time for flicker-free updating.