7
AT43USB324
1941A–02/01
Multimedia
Keyboards
The hardware of the AT43USB324 supports a wide variety of USB keyboards with an
embedded hub. The exact configuration and function of the keyboard/hub is defined by
the firmware programmed into its masked ROM.
Oscillator and PLL
All the clock signals required to run the AT43USB324 are derived from on-chip oscilla-
tor. To reduce EMI and power dissipation, the oscillator is designed to operate with a
6 MHz crystal. An on-chip PLL generates the high frequency for the clock/data separa-
tor of the serial interface engine. In the suspended state, the oscillator circuitry is turned
off.
The oscillator of the AT43USB324 is of a special low drive type, designed to work with
most crystals without any external components. The crystal must be of the parallel reso-
nance type requiring a load capacitance of about 10 pF. If the crystal requires a higher
value capacitance, external capacitors can be added to the two terminals of the crystal
and ground to meet the required value. 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
100 ppm. The use of a ceramic resonator in place of the crystal is not recommended
because a resonator would not have the necessary frequency accuracy and stability.
The clock can also be externally sourced. In this case, connect the clock source to the
XTAL1 pin, while leaving XTAL2 pin floating. The switching level at the XTAL1 pin can
be as low as 0.47V (see Electrical Specifications section on page 10) and a CMOS
device is required to drive this pin to maintain good noise margins at the low switching
level.
For proper operation of the PLL, an external RC filter consisting of a series RC network
of 100
Ω
and 0.22 µF in parallel with a 0.01 µF capacitor must be connected from the
LFT pin to VSS
(see Figure 4). Use only high quality ceramic capacitors.
Figure 4.
External RC Filter
I/O Pins
The keyboard matrix strobe output pins, PA[0:7], PB[0:7] and PE[0,1] have controlled
slope drives. With a load of 100 pF, the output fall time ranges between 75 ns and
300 ns. The keyboard matrix strobe input pins, PC[0:7] have built-in pull-up resistors, 20
k
Ω
nominal value, to the internal 3.3V power supply.
Y1
6.000 MHz
R1
100
C1
0.22 µF
C2
0.01 µF
XTAL1
XTAL2
LFT
U1
AT43USB324
