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 the start condition detection in the USI module is carried
out asynchronously when clkI/O
is halted, TWI address recognition in all sleep modes.
Note: 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 interrupt will be generated. The
start-up time is defined by the SUT and CKSEL Fuses.