CC2400-RTR1 datasheet(49/84 Pages) TI2

49 / 84 page

CC2400

SWRS042A

Page 49 of 83

43 Crystal Oscillator

An external clock signal or the internal

crystal oscillator can be used as main

frequency

reference.

The

reference

frequency must be 16 MHz. Because the

crystal frequency is used as reference for

the data rate as well as other internal

signal

processing

functions,

other

frequencies cannot be used.

If an external clock signal is used this

should be connected to XOSC16_Q1,

while XOSC16_Q2 should be left open. If

rail-to-rail (1.8V) square-wave signal is

used, the MAIN.XOSC16M_BYPASS bit

must be set. It is also possible to use a

sine-wave input. A voltage swing of 200

mV peak-to-peak is recommended in this

case.

Using the internal crystal oscillator, the

crystal must be connected between the

XOSC16_Q1 and XOSC16_Q2 pins. The

oscillator is designed for parallel mode

operation of the crystal. In addition,

loading capacitors (C5 and C6) for the

crystal are required. The loading capacitor

values

depend

the

total

load

capacitance, CL, specified for the crystal.

The total load capacitance seen between

the crystal terminals should equal CL for

the crystal to oscillate at the specified

frequency.

parasitic

431

421

The parasitic capacitance is constituted by

pin input capacitance and PCB stray

capacitance.

The

total

parasitic

capacitance is typically 5 pF.

The crystal oscillator circuit is shown in

Figure 23. Typical component values for

different values of CL are given in Table

20. Note that these values will depend on

the PCB layout and the crystal used.

Determination of the values should be

done by measuring RF frequency on

several boards and adjusting the values of

the loading capacitors accordingly.

The

crystal

oscillator

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

a stable oscillation. 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 (see the

Electrical Specifications section).

A small SMD crystal is used in the

reference design; note that the crystal

package strongly influences the price. In a

low-cost design, it may be preferable to

use a larger crystal package.

The required accuracy of the crystal is

determined by the receive filtering. Figure

19 shows how sensitivity varies with the

frequency offset between the transmitter

and the receiver. It is important to take the

total

tolerance

the

crystal

into

consideration; this consists of the initial

tolerance, drift due to temperature and

aging.

C431

C421

XTA L

XOSC16_Q1

XOSC16_Q2

C431

C421

XTA L

XOSC16_Q1

XOSC16_Q2

Figure 23. Crystal oscillator circuit

Item

CL= 16 pF

C421

22 pF

C431

22 pF

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