CY28447
(Ce1,Ce2) should be calculated to provide equal capacitance
loading on both sides.
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)
Figure 1. Crystal Capacitive Clarification
1
CLe
=
1
Ce2 + Cs2 + Ci2
1
Ce1 + Cs1 + Ci1
(
)
+
Calculating Load Capacitors
CL....................................................Crystal load capacitance
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.
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.)
CLK_REQ# Description
The CLKREQ# signals are active LOW inputs used for clean
enabling and disabling selected SRC outputs. The outputs
controlled by CLKREQ# are determined by the settings in
register byte 8. The CLKREQ# signal is a de-bounced signal
in that it’s state must remain unchanged during two consec-
utive rising edges of SRCC to be recognized as a valid
assertion or deassertion. (The assertion and deassertion of
this signal is absolutely asynchronous.)
Clock Chip
Ci2
Ci1
Pin
3 to 6p
X2
X1
CLK_REQ[1:9]# Assertion (CLKREQ# -> LOW)
Cs2
Cs1
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 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 CLKREQ# deassertion to a voltage greater than
200 mV.
Trace
2.8 pF
XTAL
Ce1
Ce2
Trim
33 pF
Figure 2. Crystal Loading Example
CLK_REQ[1:9]# Deassertion (CLKREQ# -> HIGH)
The impact of deasserting the CLKREQ# pins is that all SRC
outputs that are set in the control registers to stoppable via
deassertion of CLKREQ# are to be stopped after their next
transition. The final state of all stopped DIF signals is LOW,
both SRCT clock and SRCC clock outputs will not be driven.
As mentioned previously, the capacitance on each side of the
crystal is in series with the crystal. This means 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 capacitors
CLKREQ#X
SRCT(free running)
SRCC(free running)
SRCT(stoppable)
SRCT(stoppable)
Figure 3. CLK_REQ#[1:9] Deassertion/Assertion Waveform
Rev 1.0,November 20, 2006
Page 10 of 21
SPECTRALINEAR [ SPECTRALINEAR INC ]
