ACT apricot - Crtcdetail; General

CRT CONTROL
CRTCDETAIL
General
The basis of the CRT control circuit is the Motorola
MC6845 CRT Controller (CRTC). Once initialised with
operational parameters for the Display Unit (scan line rate,
frame rate, mode, etc.)'
the CRTC:
(a) Automatically and repetitively generates addresses
which access data from the System Ram to refresh the
screen of the display.
(b) Generates horizontal (line) and vertical (field) sync
pulses for controlling the scanning of the electron
beam across the screen.
(c) Generates an output signal which defines the active
display period of the screen.
(d) Allows the software to control the movement of a
cursor (in
the text mode only) and also scroll the screen.
LIGHT PEN
INPUT
PERIPHERALS
DATA BUS
SYSTEM CONTROL BUS (R/W. D
CRTC SELECT
At
,------,
PARALLEL
I INTERFACE I
L -1
T----1
1 I
RES
DISPLAY
ENABLE
LPSTB
VSYNC
HSYNC
DO
TO
MAO
D7
CRTC
TO
MC6845
MAIO
R/W. E
RAO
TO
RA3
cs
RS
CURSOR
CLK
CCLK
- -
- -
, ,
A/G SELECT I TIMING I
----~~: GENERATOR :
L _____ -1
TO MEMORY
VIDEO INTE
J
CONTROL AND
RfACE
TO DISPLAY UNIT
MEMORY REfRESH ADDRESS )
TO SCREEN RAM
RASTER ADDRESS
TO SYSTEM RAM
TO VIDEO INTERfACE
Figure 4. CR TC block diagram