Datasheet

57
ATtiny25/45/85 [DATASHEET]
2586Q–AVR–08/2013
10.2.5 Digital Input Enable and Sleep Modes
As shown in Figure 10-2, the digital input signal can be clamped to ground at the input of the schmitt-trigger. The
signal denoted SLEEP in the figure, is set by the MCU Sleep Controller in Power-down mode to avoid high power
consumption if some input signals are left floating, or have an analog signal level close to V
CC
/2.
SLEEP is overridden for port pins enabled as external interrupt pins. If the external interrupt request is not enabled,
SLEEP is active also for these pins. SLEEP is also overridden by various other alternate functions as described in
“Alternate Port Functions” on page 57.
If a logic high level (“one”) is present on an asynchronous external interrupt pin configured as “Interrupt on Rising
Edge, Falling Edge, or Any Logic Change on Pin” while the external interrupt is not enabled, the corresponding
External Interrupt Flag will be set when resuming from the above mentioned Sleep mode, as the clamping in these
sleep mode produces the requested logic change.
10.2.6 Unconnected Pins
If some pins are unused, it is recommended to ensure that these pins have a defined level. Even though most of
the digital inputs are disabled in the deep sleep modes as described above, floating inputs should be avoided to
reduce current consumption in all other modes where the digital inputs are enabled (Reset, Active mode and Idle
mode).
The simplest method to ensure a defined level of an unused pin, is to enable the internal pull-up. In this case, the
pull-up will be disabled during reset. If low power consumption during reset is important, it is recommended to use
an external pull-up or pulldown. Connecting unused pins directly to V
CC
or GND is not recommended, since this
may cause excessive currents if the pin is accidentally configured as an output.
10.3 Alternate Port Functions
Most port pins have alternate functions in addition to being general digital I/Os. Figure 10-5 shows how the port pin
control signals from the simplified Figure 10-2 can be overridden by alternate functions. The overriding signals may
not be present in all port pins, but the figure serves as a generic description applicable to all port pins in the AVR
microcontroller family.