JN516X Datasheet, PDF (18/94 Pages) NXP Semiconductors – IEEE802.15.4 Wireless Microcontroller

5 System Clocks
Two system clocks are used to drive the on-chip subsystems of the JN516x. The wake-up timers are driven from a
low frequency clock (notionally 32kHz). All other subsystems (transceiver, processor, memory and digital and
analogue peripherals) are driven by a high-speed clock (notionally 32MHz), or a divided-down version of it.
The high-speed clock is either generated by the accurate crystal-controlled oscillator (32MHz) or the less accurate
high-speed RC oscillator ( 27-32MHz calibrated). The low-speed clock is either generated by the accurate crystal-
controlled oscillator (32.768kHz), the less accurate RC oscillator (centered on 32kHz) or can be supplied externally
5.1 High-Speed (32MHz) System Clock
The selected high-speed system clock is used directly by the radio subsystem, whereas a divided-by-two version is
used by the remainder of the transceiver and the digital and analogue peripherals. The direct or divided down version
of the clock is used to drive the processor and memories (32, 16, 8, 4, 2 or 1MHz).
32MHz Crystal
Oscillator
Div by 2
PERIPHERAL SYSTEM CLOCK
High Speed
RC Oscillator
Div by 1,2,4,8,16 or 32
CPU CLOCK
Figure 7 System and CPU Clocks
Crystal oscillators are generally slow to start. Hence to provide a fast start-up following a sleep cycle or reset, the fast
RC oscillator is always used as the initial source for the high-speed system clock. The oscillator starts very quickly
and will run at 25-32MHz (uncalibrated) or 32MHz +/-5% (calibrated). Although this means that the system clock will
be running at an undefined frequency (slightly slower or faster than nominal), this does not prevent the CPU and
Memory subsystems operating normally, so the program code can execute. However, it is not possible to use the
radio or UARTs, as even after calibration (initiated by the user software calling an API function) there is still a +/-5%
tolerance in the clock rate over voltage and temperature. Other digital peripherals can be used (eg SPI Master/Slave),
but care must be taken if using Timers due to the clock frequency inaccuracy.
Further details of the High-Speed RC Oscillator can be found in section 19.3.11.
On wake-up from sleep, the JN516x 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.
The use of the fast RC Oscillator at wake-up means there is no need to wait for the 32MHz crystal oscillator to
stabilise Consequently, the application code will start executing quickly using the clock from the high-speed RC
oscillator.
5.1.1 32MHz Crystal Oscillator
The JN516x 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 8.
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
18
JN-DS-JN516x v1.1 Production
© NXP Laboratories UK 2013