Datasheet

28
8197C–AVR–05/11
ATtiny261A/461A/861A
When the PLL output is selected as clock source, the start-up times are determined by SUT fuse
bits as shown in Table 6-5.
6.2.3 Calibrated Internal 8 MHz Oscillator
By default, the Internal Oscillator provides an approximately 8 MHz clock signal. Though voltage
and temperature dependent, this clock can be very accurately calibrated by the user. See Table
19-2 on page 187 and “Internal Oscillators” on page 222 for more details. The device is shipped
with the CKDIV8 Fuse programmed. See “System Clock Prescaler” on page 31 for more details.
This clock may be selected as the system clock by programming the CKSEL Fuses as shown in
Table 6-6. If selected, it will operate with no external components. During reset, hardware loads
the pre-programmed calibration value into the OSCCAL Register and thereby automatically cal-
ibrates the internal oscillator. The accuracy of this calibration is shown as Factory calibration in
Table 19-2 on page 187.
Notes: 1. The device is shipped with this option selected.
2. If the oscillator frequency exceeds the specification of the device (depends on V
CC
), the
CKDIV8 Fuse can be programmed to divide the internal frequency by 8.
When this oscillator is selected, start-up times are determined by SUT fuses as shown in Table
6-7.
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.
2. The device is shipped with this option selected.
Table 6-5. Start-up Times for the PLLCK
SUT[1:0]
Start-up Time from
Power Down
Additional Delay from
Power-On-Reset (V
CC
= 5.0V)
Recommended
usage
00 14CK + 1K (1024) + 4 ms 4 ms BOD enabled
01 14CK + 16K (16384) + 4 ms 4 ms Fast rising power
10 14CK + 1K (1024) + 64 ms 4 ms Slowly rising power
11 14CK + 16K (16384) + 64 ms 4 ms Slowly rising power
Table 6-6. Internal Calibrated Oscillator Operating Modes
CKSEL[3:0] Nominal Frequency
0010
(1)
8.0 MHz
(2)
Table 6-7. Start-up Times for the Internal Calibrated Oscillator Clock Selection
SUT[1:0]
Start-up Time
from Power-down
Additional Delay from
Reset (V
CC
= 5.0V)
Recommended
Usage
00 6 CK 14CK
(1)
BOD enabled
01 6 CK 14CK + 4 ms Fast rising power
10
(2)
6 CK 14CK + 64 ms Slowly rising power
11 Reserved