nRF24LE1 Nordic VLSI, nRF24LE1 Datasheet - Page 117

• Manufacturer Part Number: nRF24LE1
• Manufacturer: Nordic VLSI

nRF24LE1 Preliminary Product Specification
The start-up time is typically less than 1ms for a crystal with 12pF load capacitance, 3pF shunt capacitance
and an ESR of 50 Ω .
The crystal oscillator is normally running only when the system is in active or standby mode. It is possible
to keep it on in register retention mode as well, by writing a ‘1’ to bit 7 in the CLKCTRL register (see
57. on page
reason is that the additional current drawn during start-up makes it more power-efficient to let the oscillator
run for a few extra milliseconds than to restart it.
13.3.2
The 16MHz RC oscillator (RCOSC16M) is used primarily to provide a high speed clock while the crystal
oscillator is starting up. It starts in just a few microseconds, and has a frequency accuracy of ± 5%.
By default, the 16MHz RC and crystal oscillators are started simultaneously. The RC oscillator supplies the
clock until the crystal oscillator has stabilized. The system then makes an automatic switch to the crystal
oscillator clock, and turns off the RC oscillator to save power. Bit 3 in the CLKCTRL register can be polled
to check which oscillator is currently supplying the high speed clock.
The system can be configured to start only one of the two 16MHz oscillators. Write bit 4 and 5 in the CLKC-
TRL register to choose the desired behavior. Note that the RF Transceiver cannot be used while the high
frequency clock is sourced by the RC oscillator. The ADC may also have reduced performance.
13.3.3
The nRF24LE1 may be used with an external 16MHz clock applied to the XC1 pin. Write a ‘1’ to bit 6 in the
CLKCTRL register if the external clock is a rail-to-rail digital signal. The input signal may also be analog,
coming from e.g. the crystal oscillator of a microcontroller. In this case the crystal oscillator on the
nRF24LE1 must also be enabled, since it is used to convert the analog input into a digital clock signal.
CLKCTRL 6 must be ‘0’, and CLKCTRL 5:4 must be ‘10’ to enable the oscillator. An input amplitude of
0.6V peak-to-peak or higher is recommended to achieve low current consumption and a good signal-to-
noise ratio. The DC level is not important as long as the applied signal never rises above VDD or drops
below VSS. The XC1 pin will load the microcontrollers crystal with approximately 1pF in addition to PCB
routing. XC2 shall not be connected.
13.3.4
The 32kHz crystal oscillator (XOSC32K) is operational in all system modes except deep sleep and mem-
ory retention, timer off. It is enabled by writing ‘000’ to CLKLFCTRL 2:0.
A crystal must be connected between port pins P0.0 and P0.1, which are automatically configured as crys-
tal pins when the oscillator is enabled. To achieve correct oscillation frequency it is important that the load
capacitance matches the specification in the crystal datasheet. The load capacitance is the total capaci-
tance seen by the crystal across its terminals:
Revision 1.1
Note: A frequency accuracy of ± 60 ppm or better is required to get a functional radio link.
107). This is recommended if the system is expected to wake up again in less than 5ms. The
16MHz RC oscillator
External 16MHz clock
32kHz crystal oscillator
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Table