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

— 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