TWR-KW2xDxxx
Tower KW2xDxxx Hardware Reference Manual, Rev. 0.2
3-6
Freescale Semiconductor
3.2.2
Clocks
The TWR-KW2xDxxx provides two clocks:
•
32 MHz Reference Oscillator -
Figure 3-4
shows the external 32 MHz external crystal Y1. This
mounted crystal must meet the MKW2XDXXX specifications. The IEEE 802.15.4 Standard
requires that the frequency be accurate to less that +/-40 ppm.
— Capacitors C21 and C22 provide the bulk of the crystal load capacitance. Onboard trim
capacitors can be programmed to center the frequency. At 25°C, it is desired to have the
frequency accurate to +/-10 ppm or less to allow for temperature variation.
— To measure the 32 MHz oscillator frequency, signal CLK_OUT (PTA18) can optionally be
programmed to provide a buffered output clock signal
— The TWR-KW2xDxxx has provision for injecting an external 32 MHz clock source as an
alternative to use of the onboard crystal:
– The crystal Y1 should be removed
– C20 must be mounted
– The external 32 MHz source is connected to 2-pin header J5; the frequency accuracy of the
external source must meet the +/-40 ppm of the IEEE 802.15.4
Figure 3-4. 32 MHz Reference Oscillator Circuit
•
Optional 32.768 kHz Crystal Oscillator - Provision is also made for a secondary 32.768 kHz crystal
Y2 as shown in
Figure 3-5
. This oscillator can be used for a low power accurate time base.
— The module comes provided with the 32.768 kHz crystal.
— Load capacitors C23 and C24 provide the entire crystal load capacitance; there is no onboard
trim capacitance.
