C9830
133 MHz Clock Generator for Intel 820 Chipset
Preliminary
Suggested Oscillator Crystal Parameters (Cont.)
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.
CL = (CXINPCB + CXINFTG + CXINDISC) X (CXOUTPCB + CXOUTFTG + CXOUTDISC
)
(CXINPCB + CXINFTG + CXINDISC) + (CXOUTPCB + CXOUTFTG + COUTDISC
)
Where:
CXTAL
= the load rating of the crystal
CXOUTFTG = the clock generators XIN pin effective device internal capacitance to ground
CXOUTFTG = the clock generators XOUT pin effective device internal capacitance to ground
CXINPCB = the effective capacitance to ground of the crystal to device PCB trace
CXOUTPCB = the effective capacitance to ground of the crystal to device PCB trace
CXINDISC = any discrete capacitance that is placed between the XIN pin and ground
CXOUTDISC = any discrete capacitance that is placed between the XOUT pin and ground
XIN
CXINPCB
CXINDISC
CXINFTG
CXOUTPCB
CXOUTDISC
CXOUTFTG
XOUT
Clock Generator
As an example, and using this formula for this datasheet’s device, a design that has no discrete loading capacitors
(CDISC) and each of the crystal to device PCB traces has a capacitance (CPCB) to ground of 4pF (typical value) would
calculate as:
CL = (4pF + 36pF + 0pF) X (4pF + 36pF + 0pF) = 40 X 40
(4pF + 36pF + 0pF) + (4pF + 36pF + 0pF) 40 + 40
= 1600
80
= 20pF
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.
INTERNATIONAL MICROCIRCUITS, INC. 525 LOS COCHES ST
MILPITAS, CA 95035, USA TEL: 408-263-6300. FAX 408-263-6571
http://www.imicorp.com/
Rev 1.1
4/25/2000
Page 13 of 14
CYPRESS [ CYPRESS ]
