Data Sheet

5 System Clocks

Two system clocks are used to provide timing references into the on-chip subsystems of the JN5142. A 16MHz clock,

generated by a crystal-controlled 32MHz oscillator, is used by the transceiver, processor, memory and digital and

analogue peripherals. A 32kHz clock is used by the sleep timer and is generated by one of two on-chip oscillators or

can be supplied externally.

5.1 16MHz System Clock

The 16MHz system clock is used by the digital and analogue peripherals and the transceiver. A scaled version

(1,2,4,8,16 or 32MHz) of this clock is also used by the processor and memories. For most operations it is necessary

to source this clock from the 32MHz oscillator.

Crystal oscillators are generally slow to start. Hence to provide a faster start-up following a sleep cycle a fast RC

oscillator is provided that can be used as the source for the 16MHz system clock. The oscillator starts very quickly

and can run at 27MHz or 32MHz (calibrated), giving system clock speeds of either 13.5MHz or 16MHz. Using the

oscillator at 27MHz scales down the speed of the processor and any peripherals in use. For the SPI interface this

causes no functional issues as the generated SPI clock is slightly slower and is used to clock the external SPI slave.

Use of the radio or UART is not possible when using the high-speed RC oscillator, as even after calibration there is a

+/- 7.5% tolerance. Additionally, timers should be used with care as the exact frequency will not be known.

On wake-up from sleep, the JN5142 uses the Fast RC oscillator. It can then either:

 Automatically switch over to use the 32MHz clock source when it has started up.

 Continue to use the fast RC oscillator until software triggers the switch-over to the 32MHz 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.

Compared to the JN5148, the use of the fast RC Oscillator at wake-up means, there is no need to wait for the 32MHz

crystal oscillator to start, if it is necessary to download code from the external memory. Consequently, in this

situation, application code will start executing earlier, with a typical improvement of 550µsec.

5.1.1 32MHz Oscillator

The JN5142 contains the necessary on chip components to build a 32MHz 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 typically be 15pF 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 it is essential that the

reference PCB layout and BOM are carefully followed. The electrical specification of the oscillator can be found in

Section 19.3.10. Please refer to Appendix B for development support with the crystal oscillator circuit.

XTALOUT

XTALIN

JN5142

Figure 9: 32MHz Crystal Oscillator Connections