ICS2694 Просмотр технического описания (PDF) - Integrated Circuit Systems
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ICS2694 Datasheet PDF : 8 Pages
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ICS2694
Frequency Reference
The internal reference oscillator contains all of the passive
components required. An appropriate series-resonant crystal
should be connected between XTAL1 (1) and XTAL2 (2). In
IBM-compatible applications, this will typically be a
14.31818 MHz crystal, but fundamental mode crystals be-
tween 10 MHz and 25 MHz have been tested. Maintain short
lead lengths between the crystal and the ICS2694. In order to
optimize the quality of the quartz crystal oscillator, the input
switching threshold of XTAL1 is VDD/2 rather than the con-
ventional 1.4V of TTL. Therefore, XTAL1 may not respond
properly to a legal TTL signal since TTL is not required to
exceed VDD/2. Therefore, another clock input CLKIN (pin 13)
has been added to the chip which is sized to have an input
switching point of 1.4V. Inside the chip, these two inputs are
ANDED. Therefore, when using the XTAL1 and XTAL2,
CLKIN should be held high. (It has a pull-up.) When using
CLKIN, XTAL1 should be held high. (It does not have a
pull-up because a pull-up would interfere with the oscillator
bias.)
It is anticipated that some applications will use both clock
inputs, properly gated, for either board test or unique system
functions. By generating all the system clocks from one refer-
ence input, the phase and delay relationships between the
various outputs will remain relatively fixed, thereby eliminat-
ing problems arising from totally unsynchronized clocks inter-
acting in a system.
Power Supply Conditioning
The ICS2694 is a member of the second generation of dot clock
products. By incorporating the loop filter on chip and upgrad-
ing the VCO, the ease of application has been substantially
improved over earlier products. If a stable and noise-free power
supply is available, no external components are required. How-
ever, in some applications it may be judicious to decouple the
power supply as shown in Figures 1 or 2. Figure 1 is the normal
configuration for 5 volt only applications. Which of the two
provides superior performance depends on the noise content of
the power supplies. In general, the configuration of Figure 1 is
satisfactory. Figure 2 is the more conventional if a 12 volt
analog supply is available, although the improved performance
comes at a cost of an extra component; however, the cost of the
discretes used in Figure 1’s are less than the cost of Figure 1’s
discrete components.
Since the ICS2694 outputs a large number of high-frequency
clocks, conservative design practices are recommended. Care
should be exercised in the board layout of supply and ground
traces, and adequate power supply decoupling capacitors con-
sistent with the application should be used.
+5
C1
+50
C1
DVDD
DVDD
.µ1F
.µ1F
33
+5
R1 C2
C3 AVDD
+120
470
R1 D1
C2 AVDD
22µV .µ1F
VSS, AVSS
4.7V .µ1F
VSS, AVSS
Figure 1
Figure 2
3
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