Peritek VCD-V, Руководство пользователя

Страница: 158 / 183

Peritek
5-62 Programming On-board Devices and Memories
The Case 1 cursor function is described in the BT459, BT468, and BT482
data sheets.
The Case 2 cursor is a little bit unusual. Two of the three analog outputs
from a BT482 color map/cursor controller are converted to digital. These
outputs are then used as a 2-bit cursor for the DLUT (see Section 5.10).
Needless to say the programming must be done carefully and the cursor is
not well suited to a variety of video clock rates because of the analog
signal output conversion process requires some "tuning" of the A-D
circuit. The BT482 cursor is mapped to the cursor 0 address.
The Case 3 cursor, which is used on the VCT-V, implements a two-bit
hardware cursor using two BT431 cursor controllers chips (see Section
5.11 for a functional description). They can be accessed through the
register area with decoding which allows addressing both chips together or
as individual chips. The dual addressing allows both chips to be loaded
with position data at the same time.
The cursor colors on the BT463 are not used, because to do so requires
special gating to force the high four bits in the BT463 to be $E or $F (this
method was used, however, on VCT-V revisons 0-2). Instead, the cursors
use the high two bits of the 4-bit window type table inputs (see BT463
data sheet). This requires some unusual programming of the window type
table inputs and means you effectively only get four unique tables.
Each BT431 has four internal registers, and occupies four register
locations in the graphics board (VCD-V or VCT-V) register block. The
registers use only the low 8 bits of each word. In order to provide the
greatest flexibility, the BT431s can be individually addressed. Multiple
addressing is provided so that if all you need to do is to change position
registers, both chips can be changed at one time. See the chart below
which shows how this works.
By setting the correct internal control bits, the BT431 can supply a bit-map
cursor and/or a cross-hair cursor. The display window for the cross-hair is
programmable. The BT431 cursor does not have an internal blink function.
The BT431 is accessed in much the same way as the BT459. For
functional details, please refer to the BT431 data sheet.