Datasheet

• Ce - is optional external capacitors as described in Figure 13-2.
• Ci - is the pin capacitance in the above table.
• CL - is the load capacitance for a 32.768 kHz crystal specified by the crystal vendor.
• C
S
- is the total stray capacitance for one TOSC pin.
Crystals specifying a load capacitance (CL) higher than 6pF require external capacitors applied as
described in 13.3 Low-Power Crystal Oscillator.
The low-frequency crystal oscillator must be selected by setting the CKSEL fuses to 0110 or 0111.
Table 13-9. Start-up Times for the Low-frequency Crystal Oscillator Clock Selection
CKSEL[3:0] Start-up Time from Power-down and Power-save Recommended Usage
0100
(1)
1K CK
0101 32K CK Stable frequency at start-up
Note:
1. This option should only be used if frequency stability at start-up is not important for the application.
Start-up times are determined by the SUT Fuses as shown in the following table.
Table 13-10. Start-up Times for the Low-Frequency Crystal Oscillator Clock Selection
SUT[1:0] Additional Delay from Reset (V
CC
= 5.0V) Power Conditions
00 14 CK BOD enabled
01 14 CK + 4.1 ms Fast rising power
10 14 CK + 65 ms Slowly rising power
11 Reserved
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13.2.3 Clock Source Connections
13.9 Timer/Counter Oscillator
13.6 Calibrated Internal RC Oscillator
By default, the internal RC oscillator provides an 8.0 MHz clock. Though voltage and temperature
dependent, this clock can be very accurately calibrated by the user. The device is shipped with the
CKDIV8 fuse unprogrammed.
This clock may be selected as the system clock by programming the CKSEL fuses as shown in the
following table. During Reset, hardware loads the pre-programmed calibration value into the OSCCAL
register and thereby automatically calibrates the RC oscillator.
By changing the OSCCAL register from SW, it is possible to get a higher calibration accuracy than by
using the factory calibration.
When this oscillator is used as the chip clock, the Watchdog oscillator will still be used for the watchdog
timer and for the Reset time out.
ATmega48PA/88PA/168PA
System Clock and Clock Options
© 2018 Microchip Technology Inc.
Datasheet Complete
DS40002011A-page 60