Datasheet

USB2422
DS00001726B-page 38  2013 - 2015 Microchip Technology Inc.
6.0 AC SPECIFICATIONS
6.1 Oscillator/Crystal
Parallel Resonant, Fundamental Mode, 24 MHz ±350 ppm.
Note 6-1 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 6-2, "Formula to Find the
Value of C1 and C2". 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 6-2 Each of these capacitance values is typically approximately 18 pF.
FIGURE 6-1: TYPICAL CRYSTAL CIRCUIT
TABLE 6-1: CRYSTAL CIRCUIT LEGEND
Symbol Description In Accordance with
C
0
Crystal shunt capacitance
Crystal manufacturer’s specification (See Note 6-1)
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 6-2, "Formula to Find the
Value of C1 and C2" (See Note 6-2)
C
2
FIGURE 6-2: FORMULA TO FIND THE VALUE OF C
1
AND C
2
C
1
C
2
C
L
Crystal
C
0
XTALIN
(C
S1
= C
B1
+ C
XTALIN
)
XTALOUT
(C
S2
= C
B2
+ C
XTALOUT
)
C
1
= 2 x (C
L
– C
0
) – C
S1
C
2
= 2 x (C
L
– C
0
) – C
S2