ATMEGA261 View Datasheet(PDF) - Atmel Corporation
Part Name
Description
MFG CO.
'ATMEGA261' PDF : 226 Pages View PDF
7.7 128 kHz Internal Oscillator
The 128 kHz internal Oscillator is a low power Oscillator providing a clock of 128 kHz. The fre-
quency is nominal at 3V and 25°C. This clock may be select as the system clock by
programming the CKSEL Fuses to “0011”.
When this clock source is selected, start-up times are determined by the SUT Fuses as shown in
Table 7-8.
Table 7-8.
SUT1..0
00
01
10
11
Start-up Times for the 128 kHz Internal Oscillator
Start-up Time from
Power-down and Power-save
Additional Delay from
Reset
6 CK
14CK
6 CK
14CK + 4 ms
6 CK
14CK + 64 ms
Reserved
Recommended Usage
BOD enabled
Fast rising power
Slowly rising power
7.8 Low-frequency Crystal Oscillator
To use a 32.768 kHz watch crystal as the clock source for the device, the low-frequency crystal
oscillator must be selected by setting CKSEL fuses to ‘0100’. The crystal should be connected
as shown in Figure 7-4. Refer to the 32 kHz Crystal Oscillator Application Note for details on
oscillator operation and how to choose appropriate values for C1 and C2.
When this oscillator is selected, start-up times are determined by the SUT fuses as shown in
Table 7-9.
Table 7-9. Start-up Times for the Low Frequency Crystal Oscillator Clock Selection
SUT1..0
00
01
Start-up Time from Power
Down and Power Save
1K (1024) CK(1)
1K (1024) CK(1)
Additional Delay from
Reset (VCC = 5.0V)
4 ms
64 ms
Recommended usage
Fast rising power or BOD enabled
Slowly rising power
10
32K (32768) CK
64 ms
Stable frequency at start-up
11
Reserved
Notes: 1. These options should only be used if frequency stability at start-up is not important for the
application.
7.9 Crystal Oscillator
XTAL1 and XTAL2 are input and output, respectively, of an inverting amplifier which can be con-
figured for use as an On-chip Oscillator, as shown in Figure 7-4. Either a quartz crystal or a
ceramic resonator may be used.
C1 and C2 should always be equal for both crystals and resonators. The optimal value of the
capacitors depends on the crystal or resonator in use, the amount of stray capacitance, and the
electromagnetic noise of the environment. Some initial guidelines for choosing capacitors for
use with crystals are given in Table 7-10. For ceramic resonators, the capacitor values given by
the manufacturer should be used.
30 ATtiny261/ATtiny461/ATtiny861
7753E–AVR–06/10
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