at43312 ATMEL Corporation, at43312 Datasheet - Page 8

• Manufacturer Part Number: at43312
• Description: Self- And Bus- Powered Usb Hub Controller
• Manufacturer: ATMEL Corporation

Oscillator and Phase-Locked-Loop
All the clock signals required to run the AT43311 are
derived from an on-chip oscillator. To reduce EMI and
power dissipation in the system, the oscillator is designed
to operate with a 6 MHz crystal. An on-chip PLL generates
the high-frequency for the clock/data separator of the Serial
Interface Engine. In the suspended state, the oscillator cir-
cuitry is turned off. To assure quick startup, a crystal with a
high Q, or low ESR, should be used. To meet the USB hub
frequency accuracy and stability requirements for hubs, the
crystal should have an accuracy and stability of better than
500 PPM. Even though the oscillator circuit would work
with a ceramic resonator, its use is not recommended
because a resonator would not have the frequency accu-
racy and stability.
A 6 MHz series resonance quartz crystal with a load capac-
itance of approximately 10 pF is recommended. If the crys-
tal recommended load capacitor is larger, external capaci-
tors added to pins OSC1 and OSC2 are recommend. The
values for these capacitors depends on the crystal and the
layout of the board, but a starting value of 22 pF at OSC1
and OSC2 should work fine in most cases. If the crystal
used cannot tolerate the drive levels of the oscillator, a
series resistor between OSC2 and the crystal pin is recom-
mended.
The clock can also be externally sourced. In this case, con-
nect the clock source to the OSC1 pin, while leaving OSC2
pin floating. The switching level at the OSC1 pin can be as
low as 0.47V (see “Oscillator Signals: OSC1, OSC2” table
on page 18) and a CMOS device is required to drive this
8
AT43312
pin to maintain good noise margins at the low switching
level.
Figure 4. Oscillator and PLL Connections
For proper operation of the PLL, an external RC filter con-
sisting of a series RC network of 100
lel with a 2 nF capacitor must be connected from the LFT
pin to V
To provide the best operating condition for the AT43312,
careful consideration of the power supply connections are
recommended. Use short, low impedance connections to
all power supply lines: V
decoupling capacitors to reduce noise: 0.1 F decoupling
high-quality capacitors, soldered as close as possible to
the package pins are recommended.
C3
10nF
22pF
22pF
C1
C2
SS
100
R2
.
C4
2nF
Y1
6.000 MHz
100
R1
CC
, V
CCA
U1
OSC1
OSC2
LFT
and V
and 10 nF in paral-
AT43312
SS
. Use sufficient