
CY28446
Document #: 001-00168 Rev *D Page 10 of 21
Use the following formulas to calculate the trim capacitor
values for Ce1 and Ce2.
CL ...................................................Crystal load capacitance
CLe .........................................Actual loading seen by crystal
using standard value trim capacitors
Ce .....................................................External trim capacitors
Cs..............................................Stray capacitance (terraced)
Ci ...........................................................Internal capacitance
(lead frame, bond wires etc.)
OE# Description
The OE# signals are active LOW inputs used for clean
enabling and disabling selected SRC outputs. The outputs
controlled by OE[A,B]# are determined by the settings in
register byte 3 and byte 8. OE[0,1,3,6]# controls SRC[0,1,3,6],
respectively. The OE# signal i s a debounced signa l in that its
state must remain unchanged during two consecutive rising
edges of SRCC to be recognized as a valid assertion or
deassertion. (The assertion and deassertion of this signal is
absolutely asynchronous.)
OE# Assertion (OE# -> LOW)
All differential outputs that were stopped are to resume normal
operation in a glitch-fre e manner. The maximum latency from
the assertion to active outputs is between 2 and 6 SRC clock
periods (2 clocks are shown) with all SRC outputs resuming
simultaneously . All stopped SRC outputs must be driven HIGH
within 10 ns of OE# deassertion to a voltage greater than
200 mV.
OE# Deassertion (OE# -> HIGH)
The impact of deasserting the OE# pins is that all SRC outputs
that are set in the control registers to stoppable via deassertion
of OE# are to be stopped a fter their next transition. The final
state of all stopped SRC clocks is Low/Low.
PD (Power-down) Clarification
The VTT_PWRGD# /PD pin is a dual-function pin. During
initial power-up, the pin functions as VTT_PWRGD#. Once
VTT_PWRGD# has been sampled LOW by the clock chip, the
pin assumes PD functionality. The PD pin is an asynchronous
active HIGH input used to shut off all clocks cleanly prior to
shutting off power to the device. This signal is synchronized
internal to the device prior to powering down the clock synthe-
sizer. PD is also an asynchronous input for powering up the
system. When PD is asserted HIGH, all clocks need to be
driven to a low value and held prior to turning off the VCOs and
the crystal oscillator.
PD (Power-down) Asser tio n
When PD is sampled HIGH by two consecutive rising edges
of CPUC, all single-ended outputs will be held LOW on their
next HIGH-to-LOW transition and dif ferential clocks must held
HIGH or tri-stated (depending on the state of the control
register drive mode bit) on the n ext diff clock# HIGH-to-LOW
transition within 4 clock periods. When the SMBus PD drive
mode bit corresponding to the differential (CPU, SRC, and
DOT) clock output of interest is progra mmed to ‘0’, the clock
output are held with “Diff clock” pin driven HIGH and “Diff
clock#” driven LOW. If th e control register PD drive mode bit
corresponding to the output of interest is progra mmed to “1”,
then both the “Diff clock” and the “Diff clock#” are LOW. Note
Figure 4 shows CPUT = 133 MHz and PD drive mode = ‘1’ for
all differential outputs. This diagram and description is appli-
cable to valid CPU frequenci es 100, 1 33, 166, and 200 MHz.
In the event that PD mode is desired as the initial power-on
state, PD must be asserted HIGH in less than 10 µs after
asserting Vtt_PwrGd#. It should be noted that 96_100_SSC
will follow the DOT wavefo rm is selected for 96 MHz and the
SRC waveform when in 100-MHz mode.
Load Capacitan ce (each side)
Total Capacitance (as seen by the crystal)
Ce = 2 * CL – (Cs + Ci)
Ce1 + Cs1 + Ci1
1+Ce2 + Cs2 + Ci2
1
()
1
=
CLe
Figure 3. OE# Deassertion/Assertion Waveform
SRCT(stoppable)
SRCT(stoppable)
SRCC(free running)
SRCT(free running)
OE#