CC1070-RTR1 datasheet(35/57 Pages) TI2

35 / 57 page

CC1070

SWRS043A

Page 35 of 54

An external clock signal or the internal

crystal oscillator can be used as main

frequency reference. An external clock

signal should be connected to XOSC_Q1,

while XOSC_Q2 should be left open. The

XOSC_BYPASS bit in the INTERFACE

external digital rail-to-rail clock signal is

used. No DC block should be used then. A

sine with smaller amplitude can also be

used. A DC blocking capacitor must then

be used (10 nF) and the XOSC_BYPASS

bit in the INTERFACE register should be

set to ‘0’. For input signal amplitude, see

section 4.2 on page 8.

Using the internal crystal oscillator, the

crystal must be connected between the

XOSC_Q1 and XOSC_Q2 pins. The

oscillator is designed for parallel mode

operation of the crystal. In addition,

loading capacitors (C4 and C5) 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

The parasitic capacitance is constituted by

pin input capacitance and PCB stray

capacitance. Total parasitic capacitance is

typically 8 pF. A trimming capacitor may

be placed across C5 for initial tuning if

necessary.

The crystal oscillator circuit is shown in

Figure 20. Typical component values for

different values of CL are given in Table

21.

The

crystal

oscillator

amplitude

regulated. This means that a high current

is required to initiate the oscillations.

When the amplitude builds up, the current

is reduced to what is necessary to

maintain

approximately

600

mVpp

amplitude. This ensures a fast start-up,

keeps the drive level to a minimum and

makes the oscillator insensitive to ESR

variations. As long as the recommended

load capacitance values are used, the

ESR is not critical.

The initial tolerance, temperature drift,

aging and load pulling should be carefully

specified in order to meet the required

frequency

accuracy

certain

application.

Figure 20. Crystal oscillator circuit

Item

CL= 12 pF

CL= 16 pF

CL= 22 pF

6.8 pF

15 pF

27 pF

6.8 pF

15 pF

27 pF

Table 21. Crystal oscillator component values

XTA

XOSC_Q

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