SMSC USB4640i ユーザーズマニュアル
High Speed Inter-Chip USB 2.0 Hub and Flash Media Controller
Datasheet
Revision 1.0 (06-01-09)
54
SMSC USB4640/USB4640i
DATASHEET
Chapter 9 AC Specifications
9.1
Oscillator/Crystal
Parallel Resonant, Fundamental Mode, 24 MHz
± 350 ppm.
Figure 9.1 Typical Crystal Circuit
Table 9.1 Crystal Circuit Legend
Figure 9.2 Capacitance Formulas
Note 9.1
C
0
is usually included (subtracted by the crystal manufacturer) in the specification for C
L
and should be set to ‘0’ for use in the calculation of the capacitance formulas in Figure 9.2,
"Capacitance Formulas". However, the OEM PCB itself may present a parasitic
capacitance between XTAL1 and XTAL2. For an accurate calculation of C
"Capacitance Formulas". However, the OEM PCB itself may present a parasitic
capacitance between XTAL1 and XTAL2. For an accurate calculation of C
1
and C
2,
take
the parasitic capacitance between traces XTAL1 and XTAL2 into account.
Note 9.2
Each of these capacitance values is typically approximately 18 pF.
SYMBOL
DESCRIPTION
IN ACCORDANCE WITH
C
0
Crystal shunt capacitance
C
L
Crystal load capacitance
C
B
Total board or trace
capacitance
OEM board design
C
S
Stray capacitance
SMSC IC and OEM board design
C
XTAL
XTAL pin input capacitance
SMSC IC
C
1
Load capacitors installed on
OEM board
Calculated values based on Figure 9.2,
C
2
C
1
= 2 x (C
L
–
C
0
) – C
S1
C
2
= 2 x (C
L
–
C
0
) – C
S2