NXP Semiconductors
JN-RM-2079
QN9090 module development reference manual
JN-RM-2079
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© NXP Semiconductors N.V. 2020. All rights reserved.
Reference manual
Rev. 1.0
— 17 Jan 2020
13 of 31
Fig 7. GND path between C10/C12 and C19
4.9 Reference Oscillator
The NXP QN9090 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 QN9090 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. 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 QN9090
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.
C
1
0
C
1
2
C
19
