Peritek VCD-V, Руководство пользователя

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Peritek
Programming On-board Devices and Memories 5-47
5.7 BT468 - Color Map Controller for the VCU-V
Note
This section describes the features of the BT468. Some of these features
are not directly used or supported in the Peritek
PX Windows including
overlays (but this will change with X11R6).
On the VCU-V, the composite video output is generated by the Brooktree
BT468 high performance monolithic color map/cursor controller. The
BT468 latches eight pixels of 8-bit primary screen data plus four bits for
overlay. The data is synchronized internally through another register, and
then fed pixel-by-pixel through the chip.
The BT468 provides composite sync generation, the color map lookup
tables, cursor bit map and position control, primary and overlay plane
blinking and blanking, and video output DACs. In addition to the
customary 256 word translation map for the 8-bit primary graphics plane,
it contains 15 additional table entries for the 4-bit graphics overlay planes
and 3 for the 2-bit cursor. Each table entry is a 24 bit value (8 bits each for
R, G, and B). It occupies four register locations in the VCU Device Buffer.
The registers use only the low 8 bits of each word.
The BT468 is controlled through an 8 bit I/O port. It functions exactly the
same way as the BT463, described in the previous section. The same goes
for the ICS1562 programmable pixel clock generator.
The BT468 has a two bit graphics cursor. It contains a 64 x 64 x 2 bit map,
position match registers, and counters triggered by the dot clock and
referenced to horizontal and vertical sync. The match registers compare a
programmed value (corresponding to an X-Y position on the screen) to the
counters. When coincidence occurs, the cursor output become active and
select one of the three cursor colors. By setting the correct internal control
bits, the BT468 can supply a bit-map cursor and/or a cross-hair cursor. The
display window for the cross-hair is programmable.
The details of the internal control registers are documented in the BT468
data sheet.