JN517X Datasheet, PDF (44/100 Pages) NXP Semiconductors – Supports multiple network stacks

NXP Semiconductors
JN517x
IEEE802.15.4 Wireless Microcontroller
• 41-bit down counter
• Optionally runs during sleep periods
• Clocked by 32 kHz system clock; either 32 kHz RC oscillator or 32 kHz clock input
A wake-up timer consists of a 41-bit down counter clocked from the selected 32 kHz
clock. An interrupt or wake-up event can be generated when the counter reaches zero. On
reaching zero, the counter will continue to count down until stopped, which allows the
latency in responding to the interrupt to be measured. If an interrupt or wake-up event is
required, the timer interrupt should be enabled before loading the count value for the
period. Once the count value is loaded and counter started, the counter begins to count
down; the counter can be stopped at any time through software control. The counter will
remain at the value it contained when the timer was stopped and no interrupt will be
generated. The status of the timers can be read to indicate if the timers are running and/or
have expired; this is useful when the timer interrupts are masked. This operation will reset
any expired status flags.
9.10.3.1 32 kHz RC oscillator calibration
The 32 kHz RC oscillator that can be used to time sleep periods is designed to require
very little power to operate and be self-contained, requiring no external timing
components and hence is lower cost. As a consequence of using on-chip resistors and
capacitors, the inherent absolute accuracy and temperature coefficient is lower than that
of a crystal oscillator, but once calibrated the accuracy approaches that of a crystal
oscillator. Sleep time periods should be as close to the desired time as possible in order to
allow the device to wake up in time for important events, for example beacon
transmissions in the IEEE802.15.4 protocol. If the sleep time is accurate, the device can
be programmed to wake up very close to the calculated time of the event and so keep
current consumption to a minimum. If the sleep time is less accurate, it will be necessary
to wake up earlier in order to be certain the event will be captured. If the device wakes
earlier, it will be awake for longer and so reduce battery life. The oscillator has a default
current consumption of around 0.5 A, which can optionally be reduced to 0.375 A, in
order to improve battery life. However, the calibrated accuracy and temperature
coefficient will be worse as a consequence. For detailed electrical specifications, see
Section 14.3.9.
In order to allow sleep time periods to be as close to the desired length as possible, the
true frequency of the RC oscillator needs to be determined to better than the initial 30 %
accuracy. The calibration factor can then be used to calculate the true number of nominal
32 kHz periods needed to make up a particular sleep time. A calibration reference
counter, clocked from the 16 MHz peripheral system clock, is provided to allow
comparisons to be made between the 32 kHz RC clock and the 16 MHz clock when the
JN517x is awake and running from the 32 MHz crystal.
Wake-up Timer0 counts for a set number of 32 kHz clock periods during which time the
reference counter runs. When the wake-up timer reaches zero the reference counter is
stopped, allowing software to read the number of 16 MHz clock ticks generated during the
time represented by the number of 32 kHz ticks programmed in the wake-up timer. The
true period of the 32 kHz clock can thus be determined and used when programming a
wake-up timer to achieve a better accuracy and hence more accurate sleep periods.
JN517X
Product data sheet
For an RC oscillator running at exactly 32000 Hz, the value returned by the calibration
procedure should be 10000, for a calibration period of twenty 32000 Hz clock periods. If
the oscillator is running faster than 32000 Hz the count will be less than 10000, if running
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.
44 of 100