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NXP Semiconductors
JN-RM-2080
K32W module development reference manual
JN-RM-2080
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© NXP Semiconductors N.V. 2020. All rights reserved.
Reference manual
Rev. 1.0
— 27 Mar 2020
14 of 30
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4.9 Reference Oscillator
The NXP K32W061/041 device contains the necessary on-chip components to build a
32-MHz reference oscillator with the addition of an external crystal resonator. There is no
need to use external capacitors because the K32W061/041 includes a bank of
switchable capacitors that can be tuned to adjust the load capacitance (C
L
) that needs to
be seen by the XTAL.
The reference crystal serves many purposes, including the provision of a reference for
the 32-bit ARM processor, PHY controller, radio synthesizer and analogue peripherals. In
addition, the crystal provides timing references for external I/O (e.g. on-chip UARTs) and
timer counters. Thus, it is important that the crystal reference is specified and built
correctly to ensure that the system functions properly.
The choice of crystal resonator is important for the following reasons:
Resonator tolerance:
A number of parameters, ranging from on-chip timings to
radio centre-frequency, are derived directly from the tolerance of the crystal. As
indicated in the component list, we recommend that a total tolerance of less than
±25 ppm is used, as the maximum permissible offset specified in BLE 5
specification is ±50 ppm and ±40ppm in the IEE802.15.4 spec. Also, note that
this tolerance should include both temperature and ageing effects imparted on
the resonator.
Resonator load capacitance:
The active oscillator components on the
K32W061/041 devices are designed for a crystal resonator with load
capacitance (C
L
) of 6 pF. This is a standard load and resonators of this type are
widely available.
Lay-out recommendations:
Route the connections from the 32 MHz XTAL to the chip oscillator pins with traces as
short as possible. The layout of the oscillator circuit is such that tracks between
components are as short as possible. This improves the performance of the oscillator by
reducing stray capacitance which can introduce frequency errors.
The XTAL should be placed away from high frequency devices and traces in order to
reduce the capacitive coupling between XTAL pins and PCB traces.
Keep other digital signal lines, especially clock lines and frequently switching signal lines,
as far away from the crystal connections as possible.
Caution
: Adherence to NXP’s recommendations will ensure that the module performs
correctly. The substitution of components is not recommended, as this may lead to both
oscillator start-up and frequency tolerance issues.
4.10 Decoupling
4.10.1 General considerations
Decouple the power supplies or regulated voltages as close as possible to the supply pin
of the IC. The decoupling capacitors must be connected to a localized ground pad on the
top layer that is connected to the main ground plane layer through multiple vias.
Ensure that each decoupling capacitor has its own via connection to ground. When
