Document Number: 38-07012 Rev. *K Page 4 of 14
resolution of 0.375 pF for a total crystal load range of 6 pF to
30 pF.
For driven clock inputs the input load capacitors may be
completely bypassed. This enables the clock chip to accept
driven frequency inputs up to 166 MHz. If the application requires
a driven input, then XTALOUT must be left floating.
Crystal Drive Level and Power
Crystals are specified to accept a maximum drive level.
Generally, larger crystals can accept more power. The drive level
specification in the table below is a general upper bound for the
power driven by the oscillator circuit in the CY22381.
For a given voltage swing, power dissipation in the crystal is
proportional to ESR and proportional to the square of the crystal
frequency. (Note that actual ESR is sometimes much less than
the value specified by the crystal manufacturer.) Power is also
almost proportional to the square of C
L
.
Power can be reduced to less than the DL specification in the
table below by selecting a reduced frequency crystal with low C
L
and low R
1
(ESR).
Output Configuration
Under normal operation there are four internal frequency
sources that may be routed through a programmable crosspoint
switch to any of the three outputs through programmable
seven-bit output dividers. The four sources are: reference, PLL1,
PLL2, and PLL3. The following is a description of each output.
CLKA’s output originates from the crosspoint switch and goes
through a programmable seven-bit post divider. The seven-bit
post divider derives its value from one of two programmable
registers controlled by FS.
CLKB’s output originates from the crosspoint switch and goes
through a programmable seven-bit post divider. The seven-bit
post divider derives its value from one of two programmable
registers controlled by FS.
CLKC’s output originates from the crosspoint switch and goes
through a programmable seven-bit post divider. The seven-bit
post divider derives its value from one programmable register.
The Clock outputs have been designed to drive a single point
load with a total lumped load capacitance of 15 pF. While driving
multiple loads is possible with the proper termination, it is
generally not recommended.
Power-Saving Features
When configured as OE, the general-purpose input three-states
all outputs when pulled LOW. When configured as Shutdown, a
LOW on this pin three-states all outputs and shuts off the PLLs,
counters, the reference oscillator, and all other active
components. The resulting current on the V
DD
pins is less than
5 A (typical). After leaving shutdown mode, the PLLs has to
relock.
When configured as SUSPEND
, the general-purpose input can
be configured to shut down a customizable set of outputs and/or
PLLs, when LOW. All PLLs and any of the outputs can be shut
off in nearly any combination. The only limitation is that if a PLL
is shut off, all outputs derived from it must also be shut off.
Suspending a PLL shuts off all associated logic, while
suspending an output forces a three-state condition.
Improving Jitter
Jitter optimization control is useful in mitigating problems related
to similar clocks switching at the same moment and causing
excess jitter. If one PLL is driving more than one output, the
negative phase of the PLL can be selected for one of the outputs.
This prevents the output edges from aligning, allowing superior
jitter performance.
CyClocks RT Software
CyClocks RT is our second-generation application that allows
users to configure this device. The easy-to-use interface offers
complete control of the many features of this family including
input frequency, PLL and output frequencies, and different
functional options. Data sheet frequency range limitations are
checked and performance tuning is automatically applied. You
can download a free copy of CyClocks RT on Cypress’s web site
at http://www.cypress.com.
