User Manual

Jennic
JennicJennic
Jennic
18 JN-DS-JN5139 v1.5 © Jennic 2008
5 System Clocks
Two separate oscillators are used to provide system clocks: a crystal-controlled 16MHz oscillator, using an external
crystal and an internal, RC-based 32kHz oscillator.
5.1 16MHz Oscillator
The JN5139 contains the necessary on-chip components to build a 16 MHz reference oscillator with the addition of
an external crystal resonator and two tuning capacitors. The schematic of these components are shown in Figure 9.
The two capacitors, C1 and C2, should be 15pF ±5% 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. The electrical specification of the oscillator can be found in section 17.3.6. For detailed application
support and specification of the crystal required see Appendix B.1.
XTALOUT
C2
C1
R1
XTALIN
JN5139
Figure 9: Crystal oscillator connections
The clock generated by this oscillator provides the reference for most of the JN5139 subsystems, including the
transceiver, processor, memory and digital and analogue peripherals.
5.2 32kHz Oscillator
The internal 32kHz RC oscillator requires no external components. It provides a low speed clock for use in sleep
mode. The clock is used for timing the length of a sleep period (see section 16 Power Management and Sleep
Modes) and also to generate the system clock used internally during reset. The internal timing components of the
oscillator have a wide tolerance due to manufacturing process variation and so the oscillator runs nominally at 32kHz
±30%. To make this useful as a timing source for accurate wakeup from sleep, a frequency calibration factor derived
from the more accurate 16MHz oscillator may be applied. The calibration factor is derived through software, details
can be found in section 12.3.1. For detailed electrical specifications, see section 17.3.5.