ATmega88PA Automotive Atmel Corporation, ATmega88PA Automotive Datasheet - Page 26

• Manufacturer Part Number: ATmega88PA Automotive
• Manufacturer: Atmel Corporation

9. System Clock and Clock Options
9.1
9.1.1
9.1.2
26
Clock Systems and their Distribution
Atmel ATmega48PA/88PA/168PA [Preliminary]
CPU Clock – clk
I/O Clock – clk
I/O
CPU
Figure 9-1
clocks need not be active at a given time. In order to reduce power consumption, the clocks to
modules not being used can be halted by using different sleep modes, as described in
Management and Sleep Modes” on page
Figure 9-1.
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 Register and the
data memory holding the Stack Pointer. Halting the CPU clock inhibits the core from perform-
ing general operations and calculations.
The I/O clock is used by the majority of the I/O modules, like Timer/Counters, SPI, and
USART. The I/O clock is also used by the External Interrupt module, but note that start condi-
tion detection in the USI module is carried out asynchronously when clk
address recognition in all sleep modes.
Note:
Asynchronous
Timer/Counter
Timer/Counter
Note that if a level triggered interrupt is used for wake-up from Power-down, the required level
must be held long enough for the MCU to complete the wake-up to trigger the level interrupt. If
the level disappears before the end of the Start-up Time, the MCU will still wake up, but no inter-
rupt will be generated. The start-up time is defined by the SUT and CKSEL Fuses as described
in
Oscillator
“System Clock and Clock Options” on page
presents the principal clock systems in the AVR and their distribution. All of the
Clock Distribution
General I/O
Modules
External Clock
clk
clk
ASY
I/O
System Clock
Control Unit
AVR Clock
Multiplexer
Prescaler
Clock
ADC
clk
Source clock
39. The clock systems are detailed below.
Oscillator
ADC
Crystal
26.
CPU Core
clk
clk
Reset Logic
CPU
FLASH
Crystal Oscillator
Watchdog clock
Low-frequency
Watchdog Timer
RAM
Watchdog
Oscillator
I/O
Calibrated RC
Flash and
EEPROM
Oscillator
is halted, TWI
9223B–AVR–09/11
“Power