Datasheet

Figure 11-2. Crystal Oscillator Connections
XTAL2
XTAL1
GND
C2
C1
11.3 Low-Power Crystal Oscillator
This Crystal Oscillator is a low-power oscillator, with reduced voltage swing on the XTAL2 output. It gives
the lowest power consumption, but is not capable of driving other clock inputs, and may be more
susceptible to noise in noisy environments.
The crystal should be connected as described in Clock Source Connections. When selecting crystals,
load capacitance must be taken into consideration. The capacitance (C
e
+C
i
) needed at each TOSC pin
can be calculated by using:


+ 

= 2

 

where:
• C
e
- is optional external capacitors. (= C
1
, C
2
as shown in the schematics.)
• C
i
- is the pin capacitance in the following table.
• C
L
- is the load capacitance specified by the crystal vendor.
• C
S
- is the total stray capacitance for one XTAL pin.
Table 11-3. Internal Capacitance of Low-Power Oscillator
32 kHz Osc. Type Internal Pad Capacitance
(XTAL1)
Internal Pad Capacitance
(XTAL2)
C
i
of system oscillator (XTAL pins) 18 pF 8 pF
The Low-power Oscillator can operate in three different modes, each optimized for a specific frequency
range. The operating mode is selected by the fuses CKSEL[3:1], as shown in the following table:
Table 11-4. Low-Power Crystal Oscillator Operating Modes
(1)
Frequency Range
[MHz]
CKSEL[3:1]
(2)
Absolute Limits for Total Capacitance of C1 and C2 [pF]
(4)
0.4 - 0.9 100
(3)
–
0.9 - 3.0 101 12 - 22
3.0 - 8.0 110 12 - 22
8.0 - 16.0 111 12 - 22
Note:
1. These are the recommended CKSEL settings for the different frequency ranges.
ATmega328PB
System Clock and Clock Options
© 2018 Microchip Technology Inc.
Datasheet Complete
DS40001906C-page 49