27
8197C–AVR–05/11
ATtiny261A/461A/861A
The fast peripheral clock, clk
PCK, can be selected as the clock source for Timer/Counter1 and a
prescaled version of the PLL output, clk
PLL, can be selected as system clock. See Figure 6-3 for a detailed illustration on the PLL clock system.
Figure 6-3.
PCK Clocking System
The internal PLL is enabled when CKSEL fuse bits are programmed to ‘0001’and the PLLE bit of
PLLCSR is set. The internal oscillator and the PLL are switched off in power down and stand-by
sleep modes.
When the LSM bit of PLLCSR is set, the PLL switches from using the output of the internal 8
MHz oscillator to using the output divided by two. The frequency of the fast peripheral clock is
effectively divided by two, resulting in a clock frequency of 32 MHz. The LSM bit can not be set if
PLL
CLK is used as a system clock.
Since the PLL is locked to the output of the internal 8 MHz oscillator, adjusting the oscillator fre-
quency via the OSCCAL register also changes the frequency of the fast peripheral clock. It is
possible to adjust the frequency of the internal oscillator to well above 8 MHz but the fast periph-
eral clock will saturate and remain oscillating at about 85 MHz. In this case the PLL is no longer
locked to the internal oscillator clock signal. Therefore, in order to keep the PLL in the correct
operating range, it is recommended to program the OSCCAL registers such that the oscillator
frequency does not exceed 8 MHz.
The PLOCK bit in PLLCSR is set when PLL is locked.
Programming CKSEL fuse bits to ‘0001’, the PLL output divided by four will be used as a system
Table 6-4.
PLLCK Operating Modes
CKSEL[3:0]
Nominal Frequency
0001
16 MHz
1/2
8 MHz
LSM
8 MHz
OSCILLATOR
PLL
8x
CKSEL3:0
PLLE
OSCCAL
4 MHz
1/4
LOCK
DETECTOR
PRESCALER
CLKPS3:0
clkPLL
PLOCK
clkPCK
OSCILLATORS
XTAL1
XTAL2
64 / 32 MHz
8 MHz
16 MHz