ICS1562B
Pipeline Delay Reset Function
The
ICS1562B
implements the clocking sequence required to
reset the pipeline delay on Brooktree RAMDACs when the
LOAD output is programmed for a modulus of either 3, 4, 5,
6, 8 or 10. This sequence can be generated by setting the
appropriate register bit (DACRST) to a logic 1 and then reset-
ting to logic 0.
When changing frequencies, it is advisable to allow 500 mi-
croseconds after the new frequency is selected to activate the
reset function. The output frequency of the synthesizer should
be stable enough at that point for the video DAC to correctly
execute its reset sequence. See Figure 4 for a diagram of the
pipeline delay reset sequence.
Pipeline Delay Reset Timing
If an external reference frequency source is to be used with the
ICS1562B.
it is important that it be jitter-free. The rising and
falling edges of that signal should be fast and free of noise for
best results.
The loop phase is locked to the falling edges of the XTAL1
input signals if the REFPOL bit is set to logic 0.
Internal Feedback
The
ICS1562B
supports LOAD (N1) and N2 divider
chains to act as the feedback divider for the PLL.
The N1 and N2 divider chains allow a much larger modulus to
be achieved than the PLL’s own feedback divider. Additionally,
the output of the N2 counter is accessible off-chip for perform-
ing horizontal reset of the graphics system, where necessary.
This mode is set under register control (ALTLOOP bit). The
reference divider (R counter) will ordinarily be set to divide by
1 in this mode, and the reference input will be supplied to
the XTAL1 input. The output frequency of the synthesizer
will then be:
F
(CLK)
: = F (XTAL1) . N1 . N2.
STROBE
or
DATCLK
CLK+
10
9
11
T
CLK
LOAD
12
Figure 4
Reference Oscillator and Crystal
Selection
The
ICS1562B
has circuitry on-board to implement a Pierce
oscillator with the addition of only one external component, a
quartz crystal. Pierce oscillators operate the crystal in anti-
(also called parallel-) resonant mode. See the AC Charac-
teristics for the effective capacitive loading to specify when
ordering crystals.
Series-resonant crystals may also be used with the
ICS1562B.
Be aware that the oscillation frequency will be slightly higher
than the frequency that is stamped on the can (typically 0.025-
0.05%).
As the entire operation of the phase-locked loop depends on
having a stable reference frequency, we recommend that the
crystal be mounted as closely as possible to the package. Avoid
routing digital signals or the
ICS1562B
outputs underneath or
near these traces. It is also desirable to ground the crystal can
to the ground plane, if possible.
By using the phase-detector hardware disable mode, the PLL
can be made to free-run at the beginning of the vertical interval
of the external video, and can be reactivated at its completion.
ICS1562B-001
The
ICS1562B-001
supports phase detector
disable via a special control mode. When the
PDRSTEN (phase detector reset enable) bit is
set and the last address latched is 15 (0Fh), a
high level on AD3 will disable PLL locking.
ICS1562B-201
The
ICS1562B-201
supports phase detector
disable via the BLANK pin. When the
PDRSTEN bit is set, a high level on the
BLANK input will disable PLL locking.
4
ICS [ INTEGRATED CIRCUIT SYSTEMS ]
