Datasheet

USB 2.0 Hi-Speed Hub Controller
Datasheet
SMSC USB251x 69 Revision 1.1 (04-26-10)
DATASHEET
Chapter 10 AC Specifications
10.1 Oscillator/Crystal
Crystal: Parallel Resonant, Fundamental Mode, 24/48
1
MHz ±350 ppm.
External Clock: 50% Duty cycle ± 10%, 24/48 MHz ± 350 ppm, Jitter < 100 ps rms.
Figure 10.1 Typical Crystal Circuit
Table 10.1 Crystal Circuit Legend
Figure 10.2 Formula to Find the Value of C
1
and C
2
Note 10.1 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 10.2, "Formula to Find the Value of C1 and C2". However, the OEM 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 10.2 Each of these capacitance values is typically approximately 18 pF.
1.Only when SEL48 is available and supported.
SYMBOL DESCRIPTION IN ACCORDANCE WITH
C
0
Crystal shunt capacitance
Crystal manufacturer’s specification (See Note 10.1)
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 10.2, "Formula
to Find the Value of C1 and C2" (See Note 10.2)
C
2
XTAL1
(C
S1 =
C
B1
+ C
XTAL1
)
XTAL2
(C
S2 =
C
B2
+ C
XTAL2
)
C
1
C
2
C
L
1 M
Crystal
C
0
C
1
= 2 x (C
L
– C
0
) – C
S1
C
2
= 2 x (C
L
– C
0
) – C
S2