8.3.5 External Crystal
If a crystal or ceramic resonator is used as the external oscillator, the crystal/resonator and a 10 MΩ resistor must be wired across the
XTAL1 and XTAL2 pins. Appropriate loading capacitors should be added to XTAL1 and XTAL2, and both pins should be configured for
analog I/O with the digital output drivers disabled.
The capacitors shown in the external crystal configuration provide the load capacitance required by the crystal for correct oscillation.
These capacitors are “in series” as seen by the crystal and “in parallel” with the stray capacitance of the XTAL1 and XTAL2 pins.
Note:
The recommended load capacitance depends upon the crystal and the manufacturer. Refer to the crystal data sheet when com-
pleting these calculations.
The equation for determining the load capacitance for two capacitors is as follows:
C
L
=
C
A
×
C
B
C
A
+
C
B
+
C
S
Figure 8.2. External Oscillator Load Capacitance
Where:
• C
A
and C
B
are the capacitors connected to the crystal leads.
• C
S
is the total stray capacitance of the PCB.
• The stray capacitance for a typical layout where the crystal is as close as possible to the pins is 2-5 pF per pin.
If C
A
and C
B
are the same (C), then the equation becomes the following:
C
L
=
C
2
+
C
S
Figure 8.3. External Oscillator Load Capacitance with Equal Capacitors
For example, a tuning-fork crystal of 25 MHz has a recommended load capacitance of 12.5 pF. With a stray capacitance of 3 pF per pin
(6 pF total), the 13 pF capacitors yield an equivalent capacitance of 12.5 pF across the crystal.
15 pF
15 pF
25 MHz
10 M
XTAL1
XTAL2
Figure 8.4. 25 MHz External Crystal Example
Crystal oscillator circuits are quite sensitive to PCB layout. The crystal should be placed as close as possible to the XTAL pins on the
device. The traces should be as short as possible and shielded with ground plane from any other traces which could introduce noise or
interference. When using an external crystal, the external oscillator drive circuit must be configured by firmware for Crystal Oscillator
Mode or Crystal Oscillator Mode with divide by 2 stage. The divide by 2 stage ensures that the clock derived from the external oscillator
has a duty cycle of 50%. The External Oscillator Frequency Control value (XFCN) must also be specified based on the crystal frequen-
cy. For example, a 25 MHz crystal requires an XFCN setting of 111b.
EFM8SB2 Reference Manual
Clocking and Oscillators
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