CY28443-3
Table 5. Crystal Recommendations
Frequency
Drive
(max.)
Shunt Cap Motional
(max.)
Tolerance
(max.)
Stability
(max.)
Aging
(max.)
(Fund)
Cut
Loading Load Cap
(max.)
14.31818 MHz
AT
Parallel 20 pF
0.1 mW
5 pF
0.016 pF
35 ppm
30 ppm
5 ppm
The CY28443-3 requires a Parallel Resonance Crystal.
Substituting a series resonance crystal will cause the
CY28443-3 to operate at the wrong frequency and violate the
ppm specification. For most applications there is a 300-ppm
frequency shift between series and parallel crystals due to
incorrect loading.
Clock Chip
Ci2
Ci1
Pin
3 to 6p
Crystal Loading
Crystal loading plays a critical role in achieving low ppm perfor-
mance. To realize low ppm performance, the total capacitance
the crystal will see must be considered to calculate the appro-
priate capacitive loading (CL).
X2
X1
Cs2
Cs1
Trace
2.8pF
The following diagram shows a typical crystal configuration
using the two trim capacitors. An important clarification for the
following discussion is that the trim capacitors are in series
with the crystal not parallel. It’s a common misconception that
load capacitors are in parallel with the crystal and should be
approximately equal to the load capacitance of the crystal.
This is not true.
XTAL
Ce1
Ce2
Trim
33pF
Figure 2. Crystal Loading Example
Use the following formulas to calculate the trim capacitor
values for Ce1 and Ce2.
Load Capacitance (each side)
Ce = 2 * CL – (Cs + Ci)
Total Capacitance (as seen by the crystal)
1
CLe
=
1
Ce2 + Cs2 + Ci2
1
Ce1 + Cs1 + Ci1
(
)
+
Figure 1. Crystal Capacitive Clarification
Calculating Load Capacitors
CL....................................................Crystal load capacitance
CLe......................................... Actual loading seen by crystal
using standard value trim 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.
Ce..................................................... External trim capacitors
Cs..............................................Stray capacitance (terraced)
Ci ...........................................................Internal capacitance
(lead frame, bond wires etc.)
CLK_REQ[0:1]# Description
The CLKREQ#[A:B] signals are active LOW inputs used for
clean enabling and disabling selected SRC outputs. The
outputs controlled by CLKREQ#[A:B] are determined by the
settings in register byte 8. The CLKREQ# signal is a
de-bounced signal 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.)
CLK_REQ[A:B]# Assertion (CLKREQ# -> LOW)
All differential outputs that were stopped are to resume normal
operation in a glitch-free manner. The maximum latency from
the assertion to active outputs is between 2–6 SRC clock
periods (2 clocks are shown) with all SRC outputs resuming
Rev 1.0,November 20, 2006
Page 11 of 23
SPECTRALINEAR [ SPECTRALINEAR INC ]
