Datasheet

USB251xB/xBi
DS00001692C-page 44  2010 - 2015 Microchip Technology Inc.
7.0 AC SPECIFICATIONS
7.1 Oscillator/Crystal
Crystal: Parallel resonant, fundamental mode, 24 MHz 350 ppm.
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.
Note: The USB251xB/xBi contains an internal 1 M resistor between the XTALIN and XTALOUT pins.
FIGURE 7-1: TYPICAL CRYSTAL CIRCUIT
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 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 7-2 (Note 7-2)
C
2
FIGURE 7-2: FORMULA TO FIND THE VALUE OF C
1
AND C
2
C
1
C
2
C
L
Crystal
XTALOUT
(C
S2 =
C
B2
+ C
XTAL2
)
XTALIN
(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