Datasheet

28
7766F–AVR–11/10
ATmega16/32U4
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 PLL Prescaler Clock – clk
PllPresc
The PLL requires a 8 MHz input. A prescaler allows user to use either a 8MHz or a 16MHz
source (from a crystal or an external source), using a divider (by 2) if necessary. The output of
the prescaler goes into the PLL Input multiplexer, that allows the user to select either the pres-
caler output of the System Clock Multiplexer, or the Internal 8MHz Calibrated Oscillator.
6.1.6 PLL Output Clock – clk
Pll
When enabled, the PLL outputs one frequency among numerous choices between 32MHz and
96 MHz. The output frequency is determined by the PLL clock register. The frequency is inde-
pendent of the power supply voltage. The PLL Output is connected to a postcaler that allows
user to generate two different frequencies (clk
USB
and clk
TMR
) from the common PLL signal,
each on them resulting of a selected division ratio (/1, /1.5, /2).
6.1.7 High-Speed Timer Clock– clk
TMR
When enabled, the PLL outputs one frequency among numerous choices between 32MHz and
96 MHz, that goes into the PLL Postcaler. The High Speed Timer frequency input is generated
from the PLL Postcaler, that proposes /1, /1.5 and /2 ratios. That can be determined from the
PLL clock register. The High Speed Timer maximum frequency input depends on the power sup-
ply voltage and reaches its maximum of 64 MHz at 5V.
6.1.8 USB Clock – clk
USB
The USB hardware module needs for a 48 MHz clock. This clock is generated from the on-chip
PLL. The output of the PLL passes through the PLL Postcaler where the frequency can be either
divided by 2 or directly connected to the clk
USB
signal.
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 while “0” means programmed.
Table 6-1. Device Clocking Options Select
(1)
Device Clocking Option CKSEL[3:0]
(or EXCKSEL[3:0])
Low Power Crystal Oscillator 1111 - 1000
Reserved 0111 - 0110
Low Frequency Crystal Oscillator 0101 - 0100
Reserved 0011
Calibrated Internal RC Oscillator 0010
External Clock 0000
Reserved 0001