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%.
MC13191 Technical Data, Rev. 1.6
16
Freescale Semiconductor
FREESCALE [ FREESCALE SEMICONDUCTOR, INC ]
