JN516X Datasheet, PDF (87/94 Pages) NXP Semiconductors – IEEE802.15.4 Wireless Microcontroller

B.3 32kHz Oscillator
In order to obtain more accurate sleep periods, the JN516x contains the necessary on-chip components to build an
optional 32kHz oscillator with the addition of an external 32.768kHz crystal and two tuning capacitors. The crystal
should be connected between XTAL32K_IN and XTAL32K_OUT (DIO9 and DIO10), with two equal capacitors to
ground, one on each pin. The schematic of these components are shown in Figure 52. The two capacitors, C1 and
C2, will typically be in the range 10 to 22pF ±5% and use a COG dielectric. As with all crystal oscillators the PCB
layout is especially important, both to keep parasitic capacitors to a minimum and to reduce the possibility of PCB
noise being coupled into the oscillator.
32KXTALIN
JN516x
32KXTALOUT
Figure 52: 32kHz Crystal Oscillator Connections
The electrical specification of the oscillator can be found in 19.3.9. The oscillator cell is flexible and can operate with a
range of commonly available 32kHz crystals with load capacitances from 6 to 12.5p, and ESR up to 80KΩ. It
achieves this by using automatic gain control (AGC), which senses the signal swing. As explained in Appendix B.1.3
there is a trade-off that exists between the load capacitance and crystal ESR to achieve reliable performance. The
use of an AGC function allows a wider range of crystal load capacitors and ESR’s to be accommodated than would
otherwise be possible. However, this benefit does mean the supply current varies with the supply voltage (VDD),
value of the total capacitance at each pin, and the crystal ESR. This is described in the table and graphs below.
32kHz Crystal Requirements
Parameter
Min
Typ
Max
Notes
Crystal Frequency
32kHz
Supply Current
1.4µA
Vdd=3v, temp=25 C, load
cap =9pF, Rm=25K
Supply Current Temp. Coeff.
0.1%/C
Vdd=3v
Crystal ESR Range (Rm)
10KΩ
25KΩ
80KΩ
See below for more details
Crystal Load Capacitance
Range (CL)
6pF
9pF
12.5pF
See below for more details
Not all Combinations of Crystal Load Capacitance and ESR are Valid
© NXP Laboratories UK 2013
JN-DS-JN516x v1.1 Production
87