JN517X Datasheet, PDF (19/100 Pages) NXP Semiconductors – Supports multiple network stacks

NXP Semiconductors
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JN517x
IEEE802.15.4 Wireless Microcontroller
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Fig 9. System and CPU clocks
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Crystal oscillators are generally slow to start. Hence to provide a fast start-up following a
sleep cycle or reset, the fast RC oscillator is always used as the initial source for the
high-speed system clock. The oscillator starts very quickly and will run at 25 MHz to
32 MHz (uncalibrated) or 32 MHz 5 % (calibrated). Although this means that the system
clock will be running at an undefined frequency (slightly slower or faster than nominal),
this does not prevent the CPU and memory subsystems operating normally, so the
program code can execute. However, it is not possible to use the radio or UARTs, as even
after calibration (initiated by the user software calling an API function) there is still a 5 %
tolerance in the clock rate over voltage and temperature. Other digital peripherals can be
used (eg SPI-bus master/slave), but care must be taken if using Timers due to the clock
frequency inaccuracy.
Further details of the high-speed RC oscillator can be found in Section 14.3.9
On wake-up from sleep, the JN517x uses the fast RC oscillator. It can then either:
• Automatically switch over to use the 32 MHz clock source when it has started up
• Continue to use the fast RC oscillator until software triggers the switch-over to the
32 MHz clock source, for example when the radio is required
• Continue to use the RC oscillator until the device goes back into one of the sleep
modes
The use of the fast RC Oscillator at wake-up means that there is no need to wait for the
32 MHz crystal oscillator to stabilize.
9.4.1.1 32 MHz crystal oscillator
The JN517x contains the necessary on chip components to build a 32 MHz reference
oscillator with the addition of an external crystal resonator and 2 tuning capacitors. The
schematics of these components are shown in Figure 10. The 2 capacitors, C1 and C2,
should typically be 12 pF and use a COG dielectric. Due to the small size of these
capacitors, it is important to keep the traces to the external components as short as
possible. The on-chip transconductance amplifier is compensated for temperature
variation, and is self-biasing by means of the internal resistor R1. This oscillator provides
the frequency reference for the radio and therefore the reference PCB layout and BOM
must be carefully followed. Refer to Section 14.3.11 for development support with the
crystal oscillator circuit. The oscillator includes a function which flags when the amplitude
of oscillation has reached a satisfactory level for full operation, and this is checked before
the source of the high-speed system clock is changed to the 32 MHz crystal oscillator.
JN517X
Product data sheet
All information provided in this document is subject to legal disclaimers.
Rev. 2.0 — 8 November 2016
© NXP Semiconductors N.V. 2016. All rights reserved.
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