AT90S/LS4434 and AT90S/LS8535
29
Power-down Mode
When the SM1/SM0 bits are set to 10, the SLEEP instruction makes the MCU enter 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 when a level-triggered interrupt is used for wake-up from power-down, the low level must be held for a time
longer than the reset delay Time-out period t
TOUT
.
When waking up from Power-down Mode, a delay from the wake-up condition occurs until the wake-up becomes effective.
This allows the clock to restart and become stable after having been stopped. The wake-up period is equal to the reset
period, as shown in Table 4 on page 21.
If the wake-up condition disappears before the MCU wakes up and starts to execute, e.g., a low-level on is not held long
enough, the interrupt causing the wake-up will not be executed.
Power Save Mode
When the SM1/SM0 bits are 11, the SLEEP instruction makes the MCU enter the Power Save Mode. This mode is identical
to Power-down, with one exception: If Timer/Counter2 is clocked asynchronously, i.e., the AS2 bit in ASSR is set,
Timer/Counter2 will run during sleep. In addition to the power-down wake-up sources, the device can also wake up from
either a Timer Overflow or Output Compare event from Timer/Counter2 if the corresponding Timer/Counter2 interrupt
enable bits are set in TIMSK and the global interrupt enable bit in SREG is set.
When waking up from Power Save Mode by an external interrupt, two instruction cycles are executed before the interrupt
flags are updated. When waking up by the asynchronous timer, three instruction cycles are executed before the flags are
updated. During these cycles, the processor executes instructions, but the interrupt condition is not readable and the inter-
rupt routine has not started yet.
When waking up from Power Save Mode by an asynchronous timer interrupt, the part will wake up even if global interrupts
are disabled.
If the asynchronous timer is not clocked asynchronously, Power-down Mode is recommended instead of Power Save Mode
because the contents of the registers in the asynchronous timer should be considered undefined after wake-up in Power
Save Mode, even if AS2 is 0.
