Datasheet

29
8197C–AVR–05/11
ATtiny261A/461A/861A
It is possible to reach a higher accuracy than factory calibration by changing the OSCCAL regis-
ter from software. See “OSCCAL – Oscillator Calibration Register” on page 32. The accuracy of
this calibration is shown as User calibration in Table 19-2 on page 187.
When this oscillator is used as device clock, the Watchdog Oscillator will still be used for the
Watchdog Timer and for the Reset Time-out. For more information on the pre-programmed cali-
bration value, see section “Calibration Byte” on page 170.
6.2.4 Internal 128 kHz Oscillator
The 128 kHz internal oscillator is a low power oscillator providing a clock of 128 kHz. The fre-
quency depends on supply voltage, temperature and batch variations. 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 6-8.
Note: 1. If the RSTDISBL fuse is programmed, this start-up time will be increased to 14CK + 4 ms to
ensure programming mode can be entered.
6.2.5 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 6-4. To find suitable capacitors please consult the manufacturer’s datasheet.
When this oscillator is selected, start-up times are determined by the SUT fuses as shown in
Table 6-9.
Notes: 1. These options should be used only if frequency stability at start-up is not important.
The Low-frequency Crystal Oscillator provides an internal load capacitance, see Table 6-10 at
each TOSC pin.
Table 6-8. Start-up Times for the 128 kHz Internal Oscillator
SUT[1:0]
Start-up Time from Power-
down and Power-save
Additional Delay from
Reset Recommended Usage
00 6 CK 14CK
(1)
BOD enabled
01 6 CK 14CK + 4 ms Fast rising power
10 6 CK 14CK + 64 ms Slowly rising power
11 Reserved
Table 6-9. Start-up Times for the Low Frequency Crystal Oscillator Clock Selection
SUT[1:0]
Start-up Time
from Power Down
Additional Delay
from Reset Recommended usage
00 1K (1024) CK
(1)
4 ms Fast rising power or BOD enabled
01 1K (1024) CK
(1)
64 ms Slowly rising power
10 32K (32768) CK 64 ms Stable frequency at start-up
11 Reserved
Table 6-10. Capacitance of Low-Frequency Crystal Oscillator
Device 32 kHz Osc. Type Cap (Xtal1/Tosc1) Cap (Xtal2/Tosc2)
ATtiny261A/461A/861A System Osc. 16 pF 6 pF