C9726
Clock Generator for VIA VT8371/Athlon (K7) Chipset Systems
Approved Product
Cypress Semiconductor Corporation
525 Los Coches St.
Milpitas, CA 95035. Tel: 408-263-6300, Fax: 408-263-6571
http://www.cypress.com
Document#: 38-07047 Rev. **
05/03/2001
Page 13 of 16
Suggested Oscillator Crystal Parameters
Characteristic
Symbol
Frequency
F
o
Tolerance
T
C
T
S
T
A
Operating Mode
-
Load Capacitance
C
XTAL
Effective Series
Resistance (ESR)
Note1:
For best performance and accurate frequencies from this device, It is recommended but not mandatory that the chosen
crystal meets or exceeds these specifications
Note 2: Larger values may cause this device to exhibit oscillator startup problems
Min
12.00
-
-
-
-
-
-
Typ
Max
16.00
+/-100
+/- 100
5
-
-
-
Units
MHz
PPM
PPM
PPM
Conditions
14.31818
-
-
-
-
20
40
Note 1
Stability (T
A
-10 to +60C) Note 1
Ageing (first year @ 25C) Note 1
Parallel Resonant, Note 1
The crystal’s rated load. Note 1
Note 2
pF
R
ESR
Ohms
To obtain the maximum accuracy, the total circuit loading capacitance should be equal to C
XTAL
. This loading capacitance is the
effective capacitance across the crystal pins and includes the clock generating device pin capacitance (C
FTG
), any circuit traces
(C
PCB
), and any onboard discrete load capacitors (C
DISC
).
The following formula and schematic may be used to understand and calculate either the loading specification of a crystal for a
design or the additional discrete load capacitance that must be used to provide the correct load to a known load rated crystal.
C
L
= (C
XINPCB
+ C
XINFTG
+ C
XINDISC
) X (C
XOUTPCB
+ C
XOUTFTG
+ C
XOUTDISC
)
(C
XINPCB
+ C
XINFTG
+ C
XINDISC
) + (C
XOUTPCB
+ C
XOUTFTG
+ C
OUTDISC
)
Where:
C
XTAL
C
XOUTFTG
= the clock generators XIN pin effective device internal capacitance to ground
C
XOUTFTG
= the clock generators XOUT pin effective device internal capacitance to ground
C
XINPCB
= the effective capacitance to ground of the crystal to device PCB trace
C
XOUTPCB
= the effective capacitance to ground of the crystal to device PCB trace
C
XINDISC
= any discrete capacitance that is placed between the XIN pin and ground
C
XOUTDISC
= any discrete capacitance that is placed between the XOUT pin and ground
= the load rating of the crystal
C
XINPCB
C
XOUTPCB
C
XOUTDISC
C
XINDISC
C
XINFTG
C
XOUTFTG
XIN
XOUT
Clock Generator
As an example, and using this formula for this datasheet’s device, a design that has no discrete loading capacitors (C
DISC
) and each
of the crystal to device PCB traces has a capacitance (C
PCB
) to ground of 4pF (typical value) would calculate as:
C
L
= (4pF + 36pF
+ 0pF) X (4pF + 36pF
+ 0pF) = 40 X 40 = 1600 = 20pF
(4pF + 36pF
+ 0pF) + (4pF + 36pF
+ 0pF) 40 + 40 80
Therefore to obtain output frequencies that are as close to this data sheets specified values as possible, in this design example, you
should specify a parallel cut crystal that is designed to work into a load of 20pF.