CY28405-2
Calculating Load Capacitors
In addition to the standard external trim capacitors, trace
capacitance and pin capacitance must also be considered to
correctly calculate crystal loading. As mentioned previously,
the capacitance on each side of the crystal is in series with the
crystal. This means the total capacitance on each side of the
crystal must be twice the specified crystal load capacitance
(CL). While the capacitance on each side of the crystal is in
series with the crystal, trim capacitors (Ce1,Ce2) should be
calculated to provide equal capacitive loading on both sides.
Clock Chip
(CY28405-2)
Ci1
Ci2
Pin
3 to 6p
As mentioned previously, the capacitance on each side of the
crystal is in series with the crystal. This mean the total capac-
itance on each side of the crystal must be twice the specified
load capacitance (CL). While the capacitance on each side of
the crystal is in series with the crystal, trim capac-
itors(Ce1,Ce2) should be calculated to provide equal capaci-
tative loading on both sides.
Use the following formulas to calculate the trim capacitor
values fro Ce1 and Ce2.
Cs1
X1
X2
Cs2
Trace
2.8pF
XTAL
Ce1
Ce2
Trim
33pF
Figure 2. Crystal Loading Example
Load Capacitance (each side)
Ce
= 2 * CL – (Cs + Ci)
CLe
Total Capacitance (as seen by the crystal)
=
1
(
Ce1 + Cs1 + Ci1
+
1
1
Ce2 + Cs2 + Ci2
)
CL ................................................... Crystal load capacitance
CLe .........................................Actual loading seen by crystal
......................................using standard value trim capacitors
Ce .....................................................External trim capacitors
Cs .............................................Stray capacitance (trace,etc)
Ci .............Internal capacitance (lead frame, bond wires etc)
PD# (Power-down) Clarification
The PD# (Power Down) pin is used to shut off ALL clocks prior
to shutting off power to the device. PD# is an asynchronous
active LOW input. This signal is synchronized internally to the
device powering down the clock synthesizer. PD# is an
asynchronous function for powering up the system. When PD#
is low, all clocks are driven to a LOW value and held there and
the VCO and PLLs are also powered down. All clocks are shut
down in a synchronous manner so has not to cause glitches
while transitioning to the low ‘stopped’ state.
PD# – Assertion
When PD# is sampled low by two consecutive rising edges of
CPUC clock then all clock outputs (except CPU) clocks must
be held low on their next high to low transition. CPU clocks
must be hold with CPU clock pin driven high with a value of 2x
Iref and CPUC undriven.
Due to the state of internal logic, stopping and holding the REF
clock outputs in the LOW state may require more than one
clock cycle to complete.
Rev 1.0, November 22, 2006
Page 8 of 16
SPECTRALINEAR [ SPECTRALINEAR INC ]
