MC13191 Technical Data, Rev. 1.5

16

Freescale Semiconductor

8

Applications Information

This section provides application specific information regarding crystal oscillator reference frequency, a

basic design example for interfacing the MC13191 to an MCU and recommended crystal usage.

8.1

Crystal Oscillator Reference Frequency

For low long term drift, users may require that several frequency tolerances be kept as low as ± 40 ppm

accuracy. This means that a total offset up to 80 ppm between transmitter and receiver will still result in

acceptable performance. The MC13191 transceiver provides onboard crystal trim capacitors to assist in

meeting this performance.

The primary determining factor in meeting this specification is the tolerance of the crystal oscillator

reference frequency. A number of factors exist that contribute to this tolerance and a crystal specification

will quantify each of them:

1. The initial (or make) tolerance of the crystal resonant frequency itself.

2. The variation of the crystal resonant frequency with temperature.

3. The variation of the crystal resonant frequency with time, also commonly known as aging.

4. The variation of the crystal resonant frequency with load capacitance, also commonly known as

pulling. This is affected by:

a) The external load capacitor values - initial tolerance and variation with temperature.

b) The internal trim capacitor values - initial tolerance and variation with temperature.

c) Stray capacitance on the crystal pin nodes - including stray on-chip capacitance, stray package

capacitance and stray board capacitance; and its initial tolerance and variation with

temperature.

Freescale requires the use of a 16 MHz crystal with a <9 pF load capacitance. The MC13191 does not

contain a reference divider, so 16 MHz is the only frequency that can be used. A crystal requiring higher

load capacitance is prohibited because a higher load on the amplifier circuit may compromise its

performance. The crystal manufacturer defines the load capacitance as that total external capacitance seen

across the two terminals of the crystal. The oscillator amplifier configuration used in the MC13191

requires two balanced load capacitors from each terminal of the crystal to ground. As such, the capacitors

are seen to be in series by the crystal, so each must be <18 pF for proper loading.

In the reference schematic, the external load capacitors are shown as 6.8 pF each, used in conjunction with

a crystal that requires an 8 pF load capacitance. The default internal trim capacitor value (2.4 pF) and stray

capacitance total value (6.8 pF) sum up to 9.2 pF for a total of 16 pF. The value for the stray capacitance

was determined empirically assuming the default internal trim capacitor value and for a specific board

layout. A different board layout may require a different external load capacitor value. The on-chip trim

capability may be used to determine the closest standard value by adjusting the trim value via the SPI and

observing the frequency at CLKO. Each internal trim load capacitor has a trim range of approximately

5 pF in 20 fF steps.

Initial tolerance for the internal trim capacitance is approximately ±15%.