C2510Fx / CC2511Fx
10.3 Crystal
The crystal oscillator for the
CC2510Fx
uses an
external crystal X1, with two loading capacitors
(C201 and C211).
The
CC2511Fx
should use a 48 MHz
fundamental (X3) or a 48 MHz 3
rd
overtone low
cost external crystal (X4). Depending on the
option selected, different loading capacitors
(C203 and C214 or C202, C212, and C213)
must be used. When X4 is used, an inductor
(L281) must also be connected in series with
C212.
Note: The high speed crystal oscillator
must be stable (SLEEP.XOSC_STB=1)
before using the radio.
The recommended application circuits also
show the connections for an optional 32.768
kHz crystal oscillator with external crystal X2
and loading capacitors C181 and C171. This
crystal can be used by the Sleep Timer if more
accurate wake-up intervals are needed than
what the internal RC oscillator can provide.
When not using X2, P2_3 and P2_4 may be
used as general IO pins.
10.4 USB (
CC2511Fx
)
For the
CC2511Fx
, the DP and DM pins need
series resistors R262 and R263 for impedance
matching and the D+ line must have a pull-up
resistor, R264. The series resistors should
match the 90
Ω
±15% characteristic
impedance of the USB bus.
Notice that the pull-up resistor must be tied to
a voltage source between 3.0 and 3.6 V
(typically 3.3 V). The voltage source must be
derived from or controlled by the V
BUS
power
supply provided by the USB cable. In this way,
10.5 Power Supply Decoupling
The power supply must be properly decoupled
close to the supply pins. Note that decoupling
capacitors are not shown in the application
circuit. The placement and the size of the
decoupling capacitors are very important to
achieve the optimum performance. TI provides
reference designs that should be followed
closely ([1] and [2]).
the pull-up resistor does not provide current to
the D+ line when V
BUS
is removed. The pull-up
resistor may be connected directly between
V
BUS
and the D+ line. As an alternative, if the
CC2511Fx
firmware needs the ability to
disconnect from the USB bus, an I/O pin on
the
CC2511Fx
can be used to control the pull-up
resistor.
The loading capacitor values depend on the
total load capacitance, C
L
, specified for the
crystal. The total load capacitance seen
between the crystal terminals should equal C
L
for the crystal to oscillate at the specified
frequency. For the
CC2510Fx
using the crystal
X1, the load capacitance C
L
is given as:
C
L
=
1
1
1
+
C
211
C
201
+
C
Parasitic
The parasitic capacitance is constituted by pin
input capacitance and PCB stray capacitance.
Total parasitic capacitance is typically 2.5 pF.
The crystal oscillator is amplitude regulated.
This means that a high current is used to start
up the oscillations. When the amplitude builds
up, the current is reduced to what is necessary
to maintain approximately 0.4 Vpp signal
swing. This ensures a fast start-up, and keeps
the drive level to a minimum. The ESR of the
crystal should be within the specification in
order to ensure a reliable start-up
SWRS055D
Page 34 of 243
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