Product Specs

19 CLOCK — Clock control
Page
102
When the system requests one or more clocks from the HFCLK controller, the HFCLK controller will
automatically provide them. If the system does not request any clocks provided by the HFCLK controller, the
controller will enter a power saving mode.
These clocks are only available when the system is in ON mode. When the system enters ON mode, the
internal oscillator (HFINT) clock source will automatically start to be able to provide the required HFCLK
clock(s) for the system.
The HFINT will be used when HFCLK is requested and HFXO has not been started. The HFXO is started by
triggering the HFCLKSTART task and stopped using the HFCLKSTOP task. A HFCLKSTARTED event will
be generated when the HFXO has started and its frequency is stable.
The HFXO must be running to use the RADIO, NFC module or the calibration mechanism associated with
the 32.768 kHz RC oscillator.
19.1.1 64 MHz crystal oscillator (HFXO)
The 64 MHz crystal oscillator (HFXO) is controlled by a 32 MHz external crystal
The crystal oscillator is designed for use with an AT-cut quartz crystal in parallel resonant mode. To achieve
correct oscillation frequency, the load capacitance must match the specification in the crystal data sheet.
Figure 18: Circuit diagram of the 64 MHz crystal oscillator on page 102 shows how the 32 MHz crystal is
connected to the 64 MHz crystal oscillator.
XC1 XC2
C1 C2
32 MHz
crystal
Figure 18: Circuit diagram of the 64 MHz crystal oscillator
The load capacitance (CL) is the total capacitance seen by the crystal across its terminals and is given by:
C1 and C2 are ceramic SMD capacitors connected between each crystal terminal and ground. For more
information, see Reference circuitry on page 546. C
pcb1
and C
pcb2
are stray capacitances on the PCB. C
pin
is the pin input capacitance on the XC1 and XC2 pins. See table 64 MHz crystal oscillator (HFXO) on page
109. The load capacitors C1 and C2 should have the same value.
For reliable operation, the crystal load capacitance, shunt capacitance, equivalent series resistance, and
drive level must comply with the specifications in table 64 MHz crystal oscillator (HFXO) on page 109. It is
recommended to use a crystal with lower than maximum load capacitance and/or shunt capacitance. A low
load capacitance will reduce both start up time and current consumption.