Rainbow Electronics AT90LS4433 - Manual (Page 32)

32
AT90S/LS4433
1042G–AVR–09/02
Idle Mode When the SM bit is cleared (zero), the SLEEP instruction forces the MCU into the Idle
mode stopping the CPU but allowing Timer/Counters, Watchdog and the interrupt sys-
tem to continue operating. This enables the MCU to wake up from external triggered
interrupts as well as internal ones like Timer Overflow interrupt and Watchdog Reset. If
wake-up from the Analog Comparator interrupt is not required, the Analog Comparator
can be powered down by setting the ACD bit in the Analog Comparator Control and Sta-
tus Register (ACSR). This will reduce power consumption in Idle mode.
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 a 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 fre-
quency of the Watchdog Oscillator is voltage dependent as shown in the Electrical
Characteristics section.
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 defined by the same CKSEL Fuses
that define the Reset Time-out period.