Datasheet

25
8197C–AVR–05/11
ATtiny261A/461A/861A
6.1.3 Flash Clock – clk
FLASH
The Flash clock controls operation of the Flash interface. The Flash clock is usually active simul-
taneously with the CPU clock.
6.1.4 ADC Clock – clk
ADC
The ADC is provided with a dedicated clock domain. This allows halting the CPU and I/O clocks
in order to reduce noise generated by digital circuitry. This gives more accurate ADC conversion
results.
6.1.5 Fast Peripheral Clock – clk
PCK
Selected peripherals can be clocked at a frequency higher than the CPU core. The fast periph-
eral clock is generated by an on-chip PLL circuit.
6.1.6 PLL System Clock – clk
ADC
The PLL can also be used to generate a system clock. The clock signal can be prescaled to
avoid overclocking the CPU.
6.2 Clock Sources
The device has the following clock source options, selectable by Flash Fuse bits as shown
below. The clock from the selected source is input to the AVR clock generator, and routed to the
appropriate modules.
Note: 1. For all fuses “1” means unprogrammed and “0” means programmed.
The various choices for each clocking option is given in the following sections. When the CPU
wakes up from Power-down or Power-save, the selected clock source is used to time the start-
up, ensuring stable oscillator operation before instruction execution starts. When the CPU starts
from reset, there is an additional delay allowing the power to reach a stable level before com-
Table 6-1. Device Clocking Options Select
(1)
vs. PB4 and PB5 Functionality
Device Clocking Option CKSEL[3:0] PB4 PB5
External Clock (see page 26) 0000 XTAL1 I/O
High-Frequency PLL Clock (see page 26) 0001 I/O I/O
Calibrated Internal 8 MHz Oscillator (see page 28) 0010 I/O I/O
Internal 128 kHz Oscillator (see page 29) 0011 I/O I/O
Low-Frequency Crystal Oscillator (see page 29) 01xx XTAL1 XTAL2
Crystal Oscillator / Ceramic Resonator
0.4...0.9 MHz (see page 30)
1000
1001
XTAL1 XTAL2
Crystal Oscillator / Ceramic Resonator
0.9...3.0 MHz (see page 30)
1010
1011
XTAL1 XTAL2
Crystal Oscillator / Ceramic Resonator
3...8 MHz (see page 30)
1100
1101
XTAL1 XTAL2
Crystal Oscillator / Ceramic Resonator
8...20 MHz (see page 30)
1110
1111
XTAL1 XTAL2