Datasheet
Revision 1.5 Page 28 of 41
nRF905 Product Specification
11 Peripheral RF Information
11.1 Crystal Specification
Tolerance includes initial accuracy and tolerance over temperature and aging.
Table 22. Crystal specification of nRF905
To achieve a crystal oscillator solution with low power consumption and fast start up time, it is recom-
mended to specify the crystal with a low value of crystal load capacitance. Specifying a lower value of crys-
tal parallel equivalent capacitance, Co=1.5pF is also good, but this can increase the price of the crystal
itself. Typically Co=1.5pF at a crystal specified for Co_max=7.0pF.
The crystal load capacitance, C
L
, is given by:
C
1
and C
2
are 0603 SMD capacitors as shown in the application schematics. C
PCB1
and C
PCB2
are the lay-
out parasitic on the circuit board. C
I1
and C
I2
are the capacitance seen into the XC1 and XC2 pin respec-
tively; the value is typical 1pF.
11.2 External Clock Reference
An external reference clock, such as an MCU clock, may be used instead of a crystal. The clock signal
should be applied directly to the XC1 pin, the XC2 pin can be left high impedance. When operating with an
external clock instead of a crystal the clock must be applied in standby mode to achieve low current con-
sumption. If the device is set into standby mode with no external clock or crystal then the current consump-
tion increases up to a maximum of 1mA.
11.3 Microprocessor Output Clock
By default a microprocessor clock output is provided. Providing an output clock increases the current con-
sumption in standby mode. The current consumption in standby depends on frequency and load of exter-
nal crystal, frequency of output clock and capacitive load of the provided output clock. Typical current
consumption values are found in
Table 9. on page 13.
Frequency
C
L
ESR
C
0max
Tolerance @
868/915 MHz
Tolerance @
433 MHz
4MHz 8pF – 16pF 150
Ω 7.0pF ±30ppm ±60ppm
8MHz 8pF – 16pF 100
Ω 7.0pF ±30ppm ±60ppm
12MHz 8pF – 16pF 100
Ω 7.0pF ±30ppm ±60ppm
16MHz 8pF – 16pF 100
Ω 7.0pF ±30ppm ±60ppm
20MHz 8pF – 16pF 100
Ω 7.0pF ±30ppm ±60ppm
22221111
21
21
'',
''
''
IPCBIPCBL
CCCCandCCCCwhere
CC
CC
C ++=++=
+
⋅
=