Rainbow Electronics ATtiny28L - Manual (Page 22)

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ATtiny28L/V
1062E–10/01
Power-down Mode When the SM bit is set (one), the SLEEP instruction forces the MCU into the Power-
down mode. In this mode, the external oscillator is stopped, while the external interrupts
and the Watchdog (if enabled) continue operating. Only an external reset, a Watchdog
reset (if enabled), or an external level interrupt can wake up the MCU.
Note that if a level-triggered interrupt is used for wake-up from Power-down mode, the
changed level must be held for some time to wake up the MCU. This makes the MCU
less sensitive to noise. The wake-up period is equal to the clock-counting part of the
reset period (see Table 5). The MCU will wake up from power-down if the input has the
required level for two Watchdog oscillator cycles. If the wake-up period is shorter than
two Watchdog oscillator cycles, the MCU will wake up if the input has the required level
for the duration of the wake-up period. If the wake-up condition disappears before the
wake-up period has expired, the MCU will wake up from power-down without executing
the corresponding interrupt. The period of the Watchdog oscillator is 2.7 µs (nominal) at
3.0V and 25
°C. The frequency of the watchdog oscillator is voltage-dependent as
shown in the section “Typical Characteristics” on page 55.
When waking up from the Power-down mode, there is a delay from the wake-up condi-
tion until the wake-up becomes effective. This allows the clock to restart and become
stable after having been stopped.