at90pwm81-16se ATMEL Corporation, at90pwm81-16se Datasheet - Page 27

• Manufacturer Part Number: at90pwm81-16se
• Description: 8-bit Avr Microcontroller With 8k Bytes In- System Programmable Flash
• Manufacturer: ATMEL Corporation

6.
6.1
7734M–AVR–03/10
System Clock and Clock Options
Clock Systems and their Distribution
The AT90PWM81 provides a large number of clock sources. Those can be divided in two categories:
internal and external.
After reset, CKSEL Fuses select one clock source. Once the device is running, software clock switching is
available on any other clock sources.
Some hardware controls are provided for clock switching management but some specific procedures must
be observed. Some settings may lead the user to program the device in an inadequate configuration.
Figure 6-1
not be active at a given time. In order to reduce power consumption, the clocks from modules not being
used can be halted by using different sleep modes or by using features of the dynamic clock switch
(“Power Management and Sleep Modes” on page 44
systems are detailed below.
Figure 6-1.
CLKO
CKOUT
Fuse
PSC2/PSCR
PLL Input
Multiplexer
PLL
presents the principal clock systems in the AVR and their distribution. All of the clocks may
CLK
PLL
Clock Distribution
General I/O
Modules
CLK
PLL
clk
/4
I/O
External Clock
Clock switch
ADC
Control Unit
AVR Clock
Prescaler
clk
ADC
Source Clock
XTAL1
or
Oscillator)
(Crystal
“Dynamic Clock Switch” on page
CPU Core
clk
clk
XTAL2
CPU
FLASH
Reset Logic
Watchdog Clock
RAM
Oscillator
Watchdog
Watchdog Timer
AT90PWM81
Flash and
EEPROM
35). The clock
Calibrated RC
Oscillator
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