Digi NS9750 - Manual (Page 39)

w w w . d i g i e m b e d d e d . c o m 1 5
A b o u t N S 9 7 5 0
USB clock
USB is clocked by a separate PLL driven by an external 48 MHz crystal, or it can be
driven directly by an external 48 MHz oscillator.
Figure 5: USB clock
Y1_PWR
**
**
Y1_OUT
**
TANK_LC
3R3V
C17
10pF
**
3
4 1
2
X1
48.0000MHz
C16
100pF_5%
NS9750
L4
1uH_5%
TANK_RC
X2_USB_OSC
Y1
EC2600_TTS_48M
4
2
1
3
VCC
GND
TEST
OUT
R7
68R1
NOTE: ** = OPTIONAL
Crystal circuit
C15
10pF
X1_IN
X1_USB_OSC
R8 1.5M
Tank Circuit
X1_USB
**
**
C9
100nF
R9
100 OHM
**
X2_USB
R6
100
TB1
BEAD_0805_601
**
X1 is a 48-MHz 3rd harmonic crystal. It has
the same physical characteristics as a 16
MHz crystal. The circuit may have a tendency
to oscillate at 16 MHz unless precautions are
taken. A LC-tank circuit is added to provide a
“low impedance” for the 16 MHz oscillation to
ground.