2009 - 2015 Microchip Technology Inc. DS00002005A-page 32
USB2250/50i/51/51i
8.0 AC SPECIFICATIONS
8.1 Oscillator/Crystal
Parallel Resonant, Fundamental Mode, 24 MHz 350 ppm.
Note 8-1 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 8-2, "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 8-2 Each of these capacitance values is typically approximately 18 pF.
FIGURE 8-1: TYPICAL CRYSTAL CIRCUIT
TABLE 8-1: CRYSTAL CIRCUIT LEGEND
Symbol Description In Accordance With
C
0
Crystal shunt capacitance
Crystal manufacturer’s specification (See Note 8-1)
C
L
Crystal load capacitance
C
B
Total board or trace capacitance OEM board design
C
S
Stray capacitance Microchip IC and OEM board design
C
XTAL
XTAL pin input capacitance Microchip IC
C
1
Load capacitors installed on OEM
board
Calculated values based on Figure 8-2, "Capacitance
Formulas" (See Note 8-2)
C
2
FIGURE 8-2: CAPACITANCE FORMULAS
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
