Datasheet

Figure 13-2. Crystal Oscillator Connections
XTAL2
XTAL1
GND
C2
C1
Note: XTALn share the same pins as TOSCn
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13.3 Low-Power Crystal Oscillator
13.4 Full Swing Crystal Oscillator
13.5 Low-Frequency Crystal Oscillator
13.3 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 13.2.3 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.
32kHz 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 13-3. 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
ATmega48PA/88PA/168PA
System Clock and Clock Options
© 2018 Microchip Technology Inc.
Datasheet Complete
DS40002011A-page 56