
25
1477K–AVR–08/10
ATtiny26(L)
Clock Sources
The device has the following clock source options, selectable by Flash Fuse bits as shown
below on
Table 3. The clock from the selected source is input to the AVR clock generator, and
routed to the appropriate modules.The use of pins PB5 (XTAL2), and PB4 (XTAL1) as I/O pins is
limited depending on clock settings, as shown below in
Table 4.Note:
1. For all fuses “1” means unprogrammed while “0” means programmed.
The various choices for each clocking option is given in the following sections. When the CPU
wakes up from Power-down, the selected clock source is used to time the start-up, ensuring sta-
ble oscillator operation before instruction execution starts. When the CPU starts from Reset,
there is as an additional delay allowing the power to reach a stable level before commencing
normal operation. The Watchdog Oscillator is used for timing this real-time part of the start-up
time. The number of WDT Oscillator cycles used for each time-out is shown in
Table 5. The fre-
Table 3. Device Clocking Options Select
Device Clocking Option
PLLCK
CKSEL3..0
External Crystal/Ceramic Resonator
1
1111 - 1010
External Low-frequency Crystal
1
1001
External RC Oscillator
1
1000 - 0101
Calibrated Internal RC Oscillator
1
0100 - 0001
External Clock
1
0000
PLL Clock
0
0001
Table 4. PB5, and PB4 Functionality vs. Device Clocking Options
(1) Device Clocking Option
PLLCK
CKSEL [3:0]
PB4
PB5
External Clock
1
0000
XTAL1
I/O
Internal RC Oscillator
1
0001
I/O
Internal RC Oscillator
1
0010
I/O
Internal RC Oscillator
1
0011
I/O
Internal RC Oscillator
1
0100
I/O
External RC Oscillator
1
0101
XTAL1
I/O
External RC Oscillator
1
0110
XTAL1
I/O
External RC Oscillator
1
0111
XTAL1
I/O
External RC Oscillator
1
1000
XTAL1
I/O
External Low-frequency Oscillator
1
1001
XTAL1
XTAL2
External Crystal/Resonator Oscillator
1
1010
XTAL1
XTAL2
External Crystal/Resonator Oscillator
1
1011
XTAL1
XTAL2
External Crystal/Resonator Oscillator
1
1100
XTAL1
XTAL2
External Crystal/Resonator Oscillator
1
1101
XTAL1
XTAL2
External Crystal/Resonator Oscillator
1
1110
XTAL1
XTAL2
External Crystal/Resonator Oscillator
1
1111
XTAL1
XTAL2
PLL
0
0001
I/O