Datasheet

USB 2.0 Hi-Speed Hub Controller
Datasheet
Revision 2.2 (02-17-12) 56 SMSC USB251xB/xBi
DATASHEET
Chapter 7 AC Specifications
7.1 Oscillator/Crystal
Crystal: Parallel resonant, fundamental mode, 24 MHz ±350 ppm
Figure 7.1 Typical Crystal Circuit
Figure 7.2 Formula to Find the Value of C
1
and C
2
Note 7.1 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 7.2.
However, the PCB itself may present a parasitic capacitance between XTALIN and
XTALOUT. For an accurate calculation of C
1
and C
2
, take the parasitic capacitance
between traces XTALIN and XTALOUT into account.
Note 7.2 Each of these capacitance values is typically around 18 pF.
Table 7.1 Crystal Circuit Legend
SYMBOL DESCRIPTION IN ACCORDANCE WITH
C
0
Crystal shunt capacitance
Crystal manufacturer’s specification (Note 7.1)
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 7.2 (Note 7.2)
C
2
C
1
C
2
C
L
1 M
Crystal
XTAL2
(C
S2 =
C
B2
+ C
XTAL2
)
XTAL1
(C
S1 =
C
B1
+ C
XTAL1
)
C
0
C
1
= 2 x (C
L
– C
0
) – C
S1
C
2
= 2 x (C
L
– C
0
) – C
S2