User Manual

Jennic
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Jennic
© Jennic 2008 JN-DS-JN5139 v1.5 77
B.3.3 Supply Decoupling
C12 is the decoupling capacitor for the analogue areas of IC1. It is placed as close as possible to the IC1 pin VDD1.
C13 is the decoupling capacitor for the digital areas of IC1. It is also used to decouple the supply on the Flash
memory due to:
• placement of the Flash memory power pin (IC2 Pin 8) next to the IC1 Pin VDD2
• the fact that the Flash memory is only used during booting (unless reprogramming), so the RF areas of the
device are not active.
B.3.4 Reference Oscillator Requirements
The device contains the necessary on-chip components to build a 16-MHz reference oscillator with the addition of an
external crystal resonator. The schematic in Figure 50: Application Schematic shows the crystal circuit in the form
of capacitors C10 and C11, together with a crystal resonator Y1.
The reference crystal serves many purposes, including the provision of a reference for the 32-bit RISC processor,
radio and analogue peripherals. In addition, the crystal also 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 external crystal resonator, Y1, is connected to IC1 via two coupling capacitors, C10 and C11, that should be
15pF ±5% and use a C0G dielectric – the 15pF will need to vary for alternate crystals. This is important, in order to
ensure that the oscillator Q-factor is maximised and the temperature co-efficient is minimised.
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 ±35ppm is used, as the maximum permissible offset specified in IEEE 802.15.4 is ±40
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 JN5121 and JN5139 series devices
are designed for a crystal resonator with load capacitance of 9 pF. This is a standard loading, and resonators
of this type are widely available.
B.3.5 Reference Oscillator Layout Considerations
The layout of the oscillator circuit is such that the components are close together. This improves the performance of
the oscillator by reducing parasitic impedance and the likelihood of cross-talk.
We also recommend that the symmetry of layout be maximised in order to avoid uneven loading of the crystal
resonator.
B.3.6 VCO Tune Circuit Component Specifications
Jennic wireless microcontroller devices employ an RF Phase Locked Loop (PLL). With respect to the schematic in
Figure 50: Application Schematic , the only external components required on the printed circuit board are two
capacitors, C8 and C9, together with the resistor R4.
! Caution: It is essential that the component values advised here
are followed, since their substitution could lead to failure in the
PLL.