Freescale USB Development Board USB-KW40Z User’s Guide, Rev. 0, 10/2015
Freescale Semiconductor, Inc
11
USB-KW40Z development board
Figure 6. USB-KW40Z RF circuitry
4.2.2
Clocks
The USB-KW40Z provides two clock sources to the MKW40Z160:
•
32 MHz Reference Oscillator -
Figure 7
shows the external 32 MHz external crystal Y1. This
mounted crystal must meet the specifications outlined in the AN3251 application note. The IEEE
Std. 802.15.4 requires that the frequency be accurate to less than
±
40 ppm.
— Capacitors C4 and C5 provide the bulk of the crystal load capacitance. At 25 °C, it is desired
to have the frequency accurate to ±10 ppm or less to allow for temperature variation.
•
Optional 32.768 kHz Crystal Oscillator: Provision is also made for a secondary 32.768 kHz crystal
Y2 (see
Figure 8
). This oscillator can be used for a low power accurate time base.
— The module comes provided with this Y2 crystal and its load capacitors C6 and C7.
— Load capacitors C6 and C7 provide the entire crystal load capacitance; there is no onboard trim
capacitance.
Figure 7. USB-KW40Z board’s 32 MHz reference oscillator circuit
