NRF51802-QFAA-R datasheet(23/85 Pages) NORDIC

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Page 23

nRF51802 Product Specification v1.2

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 Table 9.

Table 9 Clock properties

Figure 5 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 MHz2

2. The HFCLK will be 16 MHz for both the 16 and 32 MHz crystal option.

External clock reference3

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 reference3

32.768 kHz

Synthesized from HFCLK

32.768 kHz

Internal RC Oscillator (RCOSC)

32.768 kHz

oscillator

32.768 kHz

crystal

oscillator

16 MHz

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

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