CRT
CONTROL
interlaced raster
scan
proceeds.
The
basic clock frequency
used by
the
counters
is
CCLk
(Character Clock) supplied
from
the
timing
generator circuit.
The
period of CCLK is
dependent
on
the
mode
selected,
and corresponds
to
the
display
time
of
the
character cell
in
each scan line. CCLK is derived from
the
15
MHz
dot clock.
In
the
text
mode, CCLK is produced by dividing
the
dot
clock by 10 (corresponding
to
10
pixels per character cell). In
the
graphics mode, CCLK is produced by dividing
the
dot
clock by
16
(corresponding
to
16
pixels per graphics cell).
Other
registers
within
the
CRTC
are programmed
with
data for controlling
the
operation
of
the
counters
to
determine
various
parameters
for
the
Display
Unit
(e.g.
active
line
period,
vertical
retrace period etc.). A definition
of each of
the
registers is provided below.
Register Description
The
CRTC
contains
19 registers. 10 of
the
registers are
programmed
with
operational
parameters
to
match
the
Display
Unit
to
the
selected screen
format
(text or graphics).
Four of
the
remaining
registers are involved
with
the
text
cursor.
Two
of
these
registers specify
the
format of
the
cursor;
the
other
two
define
the
cursor
position
on
the
screen.
The
start
address
accounts
for
another
two.
Two
registers are
used
to
store
the
position
of a light pen.
The
remaining
register is
programmed
with
a
pointer
which
specifies
the
address
location
of
the
other
registers. A
summary
of
the
registers
is
provided below.
This
is followed
by a detailed description of
each
individual register.
AR
Address Register
R9
Max scan
line
address
RO
Horizontal
Total
RIO
Cursor format (start)
Rl
Horizontal
Displayed ·
Rl1
Cursor format (end)
R2
HSync
Position
R12
Start Address
(H)
R3
HsyncWidth
R13
S
tart
Address
(L)
R4
Vertical
Total
R14
Cursor
Position
(H)
R5
V.
Total
Adjust
R15
Cursor Position
(L)
R6
Vertical
Displayed
R16
Light
Pen
(H)
R7
VSync
Position
R17
LightPen(L)
R8
InterlaceMode
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