System Clock
Clock Systems and
their Distribution
Figure 19 presents the principal clock systems in the AVR and their distribution. All of
the clocks need not be active at a given time. In order to reduce power consumption, the
clocks to unused modules can be halted by using different sleep modes, as described in
“Power Management and Sleep Modes” on page 43. The clock systems are detailed
below.
Figure 19. Clock Distribution
Asynchronous
Timer/Counter2
CAN
Controller
General I/O
Modules
Flash and
EEPROM
ADC
CPU Core
RAM
clkADC
clkI/O
clkCPU
AVR Clock
Control Unit
CLKO
clkASY
clkFLASH
CKOUT Fuse
Reset Logic
Watchdog Timer
Source clock
Watchdog clock
Prescaler
Watchdog
Oscillator
Clock
Multiplexer
Multiplexer
Timer/Counter2
External Clock
Timer/Counter2
Oscillator
Crystal
Oscillator
Low-frequency
Crystal Oscillator
Calibrated RC
Oscillator
External Clock
TOSC1
TOSC2
XTAL1
XTAL2
CPU Clock – clkCPU
I/O Clock – clkI/O
The CPU clock is routed to parts of the system concerned with operation of the AVR
core. Examples of such modules are the General Purpose Register File, the Status Reg-
ister and the data memory holding the Stack Pointer. Halting the CPU clock inhibits the
core from performing general operations and calculations.
The I/O clock is used by the majority of the I/O modules, like Timer/Counters, SPI, CAN,
USART. 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. Also note that address recognition in the TWI
module is carried out asynchronously when clkI/O is halted, enabling TWI address recep-
tion in all sleep modes.
Flash Clock – clkFLASH
The Flash clock controls operation of the Flash interface. The Flash clock is usually
active simultaneously with the CPU clock.
Asynchronous Timer Clock – The Asynchronous Timer clock allows the Asynchronous Timer/Counter to be clocked
clkASY
directly from an external clock or an external 32 kHz clock crystal. The dedicated clock
34
AT90CAN128
4250E–CAN–12/04
ATMEL [ ATMEL ]
