28
7734Q–AVR–02/12
AT90PWM81/161
Data memory holding the Stack Pointer. Halting the CPU clock inhibits the core from performing
general operations and calculations.
5.1.2
clk
I/O - I/O Clock
The I/O clock is used by the majority of the I/O modules, like Timer/Counter. The I/O clock is
also used by the External Interrupt module, but note that some external interrupts are detected
by asynchronous logic, allowing such interrupts to be detected even if the I/O clock is halted.
5.1.3
clk
FLASH - Flash Clock
The Flash clock controls operation of the Flash interface. The Flash clock is usually active simul-
taneously with the CPU clock.
5.1.4
clk
PLL - PLL Clock
The PLL clock allows the PSC modules to be clocked directly from a 64/32MHz clock. A 16MHz
clock is also derived for the CPU.
5.1.5
clk
ADC - ADC Clock
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.
5.2
Clock Sources
The device has the following clock source options, selectable by Flash Fuse bits (default) or by
the CLKSELR register (dynamic clock switch circuit) as shown below. The clock from the
selected source is input to the AVR clock generator, and routed to the appropriate modules.
Table 5-1.
Device clocking options select
(1) , PLL source and PE1 and PE2 functionality.
Device clocking option
System
clock
PE1
PE2
External clock
0000
CLKI
I/O
PLL output divided by 4 : 16MHz driven by internal RC
PLL / 4
0001
I/O
Calibrated internal RC oscillator 8MHz
0010
I/O
Internal 128kHz RC oscillator (WD)
N/A
0011
I/O
PLL output divided by 4 / PLL driven by external
crystal/ceramic resonator
PLL / 4
0100
XTAL1
XTAL2
PLL output divided by 4/ PLL driven by external clock
PLL / 4
0101
CLKI
I/O
Calibrated internal RC oscillator 1MHz
N/A
0110
I/O
External crystal/ceramic resonator (3.0MHz - 8.0MHz)
0111 b
XTAL1
XTAL2
External crystal/ceramic resonator (0.9MHz - 3.0MHz)
1000
b
XTAL1
XTAL2
External crystal/ceramic resonator (0.9MHz - 3.0MHz)
1001
b
XTAL1
XTAL2
External crystal/ceramic resonator (3.0MHz - 8.0MHz)
1010 b
XTAL1
XTAL2
External crystal/ceramic resonator (3.0MHz - 8.0MHz)
1011
b
XTAL1
XTAL2
External crystal/ceramic resonator (3.0MHz - 8.0MHz)
1100 b
XTAL1
XTAL2