Data Sheet
Page 27
nRF51822 Product Specification v3.1
3.6 Clock management (CLOCK)
The advanced clock management system can source the system clocks from a range of internal or external
high and low frequency oscillators and distribute them to modules based upon a module’s individual
requirements. This prevents large clock trees from being active and drawing power when system modules
needing this clock reference are not active.
If an application enables a module that needs a clock reference without the corresponding oscillator
running, the clock management system will automatically enable the RC oscillator option and provide the
clock. When the module goes back to idle, the clock management will automatically set the oscillator to idle.
To avoid delays involved in starting a given oscillator, or if a specific oscillator is required, the application can
override the automatic oscillator management so it keeps oscillators active when no system modules
require the clock reference.
Clocks are only available in System ON mode and can be generated by the sources listed in Tab le 1 1.
Table 11 Clock properties
Figure 6 Clock management
Clock Source Frequency options
High Frequency Clock (HFCLK)
1
1. External Crystal must be used for Radio operation.
External Crystal (XOSC) 16/32 MHz
2
2. The HFCLK will be 16 MHz for both the 16 and 32 MHz crystal option.
External clock reference
3
3. See the nRF51 Series Reference Manual for more details on external clock reference.
16 MHz
Internal RC Oscillator (RCOSC) 16 MHz
Low Frequency Clock (LFCLK)
External Crystal (XOSC) 32.768 kHz
External clock reference
3
32.768 kHz
Synthesized from HFCLK 32.768 kHz
Internal RC Oscillator (RCOSC) 32.768 kHz
32.768 kHz
RC
oscillator
32.768 kHz
crystal
oscillator
16 MHz
RC
oscillator
16/32 MHz
crystal
oscillator
LFCLK
clock control
HFCLK
clock control
HFCLK
LFCLK
LFCLKSRC
LFCLKSTART
LFCLKSTOP
LFCLKSTARTED
HFCLKSTART
HFCLKSTOP
HFCLKSTARTED
Request from the
rest of the system
XL1 XL2 XC1 XC2
32.768 kHz
synthesizer
HFCLKSRC