Apple Lisa - Contrast Control

Confidential
Lisa Hardware Reference Manual 7-Jul-81
The video circuit fetches a 16 bit word from main memory once every four
CPU clock cycles, and shifts it out to the CRT to provide a bit mapped
display. The data in the bit map is displayed as an image of the bits
in memory. A one corresponds to a dark (off) dot, and a zero corresponds
to a bright (on) dot. Wait states are inserted into each memory fetch
so that it takes a multiple of four cycles.
A periodic interrupt, the vertical retrace interrupt, is generated at the
start of each vertical retrace. The display is refreshed at 60 Hz.
A
state machine in the video circuit counts the number of words
displayed and generates the horizontal and vertical sync pulses.
The ROM which controls this state machine also contains the system
serial number.
The vertical retrace interrupt can be disabled. A write to the address
$00E01A in 1/0 space enables the interrupt and a write to $00E018 disables
the interrupt. Since the Reset line resets this bit, the vertical retrace
interrupt is masked off at power on.
The video circuit controls the low order .15_~its of the video address
to refresh the display. The high order six bits remain fixed. The video
address latch at $00E800 in the 1/0 address space provides the high order
six bits. By writing 'to this register the programmer can position the
video memory anywhere in main memory. At power on the register is in an
indeterminate state.
DIS - D6 D5
D4
D3
D2
D1
DO
----+----
I
.
------+
unused VAlO
VA19 VAlS
I
VAl7 VAl 6
VAlS
I
I
------+
+-------~..-.--~-------+------~----~--
CONTRAST CONTROL
The contrast control circuitry is located on the 1/0 board. A
digital to analog converter controls the CRT display contrast. This
converter is controlled by a latch connected to the Slow Data bus
(the hard disk interface 6522 Peripheral A Port). To write a value
to the contrast latch:
1. Set the DEN bit (PB2) to 1.
2. Put the desired contrast value ($00 to $FF) on the Peripheral
A Port of the hard disk 6522.
3. Execute MOVE.L DO,$OODOIC. This instruction causes a pulse
on the contrast latch clock input.
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