27
ATtiny26(L)
1477G–AVR–03/05
External RC Oscillator For timing insensitive applications, the external RC configuration shown in Figure 23
can be used. The frequency is roughly estimated by the equation f = 1/(3RC). C should
be at least 22 pF. By programming the CKOPT Fuse, the user can enable an internal
36 pF capacitor between XTAL1 and GND, thereby removing the need for an external
capacitor.
Figure 23. External RC Configuration
The oscillator can operate in four different modes, each optimized for a specific fre-
quency range. The operating mode is selected by the fuses CKSEL3..0 as shown in
Table 9.
When this oscillator is selected, start-up times are determined by the SUT Fuses as
shown in Table 10.
Notes: 1. This option should not be used when operating close to the maximum frequency of
the device.
Table 9. External RC Oscillator Operating Modes
CKSEL3..0 Frequency Range (MHz)
0101 0.1 - 0.9
0110 0.9 - 3.0
0111 3.0 - 8.0
1000 8.0 - 12.0
Table 10. Start-up Times for the External RC Oscillator Clock Selection
SUT1..0
Start-up Time
from Power-down
Additional Delay from
Reset (V
CC
= 5.0V) Recommended Usage
00 18 CK – BOD enabled
01 18 CK 4.1 ms Fast rising power
10 18 CK 65 ms Slowly rising power
11 6 CK
(1)
4.1 ms Fast rising power or BOD enabled
PB5 (XTAL
XTAL1
GND
V
CC
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