ICS5342 Datasheet, PDF (11/36 Pages) Integrated Circuit Systems – 16-Bit Integrated Clock-LUT-DAC

ICS5342
GENDAC
Double Termination (Figure 1)
For this termination scheme, a load resistor is placed at both
the DAC output and the monitor input. The resistor values
should be equal to the characteristic impedance of the line.
Double termination of the DAC output allows both ends of the
transmission line between the DAC outputs and the monitor
inputs to be correctly matched.The result should be an ideal
reflection-free system.
This arrangement is relatively tolerant of variations in trans-
mission line impedance (e.g. a mismatched connector) since
no reflections occur from either end of the line. A doubly ter-
minated DAC output will rise faster than any singly terminat-
ed output because the rise time of the DAC outputs is
dependent on the RC time constant of the load.
ICS5342
MONITOR
R_LOAD
R_LOAD
Ground
Double Termination
Ground
5342_05
If the GENDAC drives large capacitive loads (for instance
long cable runs), it may be necessary to buffer the DAC out-
puts. The buffer will have a relatively high input impedance.
The connection between the DAC outputs and the buffer in-
puts should also be considered as a transmission line. The
buffer output will have a relatively low impedance. It should
be matched to the transmission line between it and the monitor
with a series terminating resistor. The transmission line
should be terminated at the monitor.
R_S
ICS5342
R_LOAD
MONITOR
R_T
Ground
Buffered Signal
Ground
5342_06
SENSE Output
The GENDAC contains three comparators, one for each of the
DAC output R, G and B lines. The reference voltage to the
comparators is proportional to the V_REF (internal or external)
and is typically 0.330 for V_REF=1.235 Volts. The SENSE*
pin will be driven low when any analog video output is above
0.385 mV. SENSE* output will be high when all analog out-
puts are below 275 mV. This signal is used to detect the type
of (or lack of) monitor connected to the system.
PLL Clock
The ICS5342 has dual PLL frequency generators for generat-
ing the video clock (CLK0) and memory clock (CLK1) need-
ed for graphics subsystems. Both of these clocks are
generated from a single 14.318 MHz crystal or they can be
driven from an external clock source. The chip includes the
capacitors for the crystal and all of the components needed for
the PLL loop filters, minimizing board component count.
There are eight possible video clock, CLK0, frequencies (f0-
f7) which can be selected by the external pins CS1-CS0. All
clocks are software selectable by setting a bit in the PLL con-
trol register. Frequencies f0-f7 can be programmed for any
frequency by writing appropriate parameter values to the PLL
parameter registers. The default frequencies on power up are
commonly used video frequencies (see table “Video Clock
Default Frequency Registers”). At power up, the frequencies
can be selected by pins CS2-CS0. There are two programma-
ble memory clock frequencies (fA, fB). On power up this fre-
quency defaults to the frequency given in the table:
“MemoryClock Default Frequency Registers.” The memory
clock transition between frequencies is smooth and glitch free
if the N2 PLL parameter is not changed from its previous set-
ting.
Video Clock (CLK0) Default Frequency Register *
fn VCLK ^{M & N Comments}
(MHz) ^Code
f0 25.175 7D 50 VGA0 (VGA Graphics)
f1 28.322 55 49 VGA1 (VGA Text)
f2 31.500 2A 43 VESA 640 x 480 @72 Hz
f3 36.00 77 4A VESA 800 x 600 @56 Hz
f4 40.00 79 49 VESA 800 x 600 @60 Hz
f5 44.889 6F 47 1024 x 768 @43 Hz Inter-
laced
f6 65.00 74 2B 1024 x 768 @ 60 Hz,
640 x 480 Hi-Color @ 72
Hz
f7 75.00 71 29 VESA 1024 x 768 @ 70
Hz,
True Color 640 x 480
* With 14.318 MHz input.
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