PaulS:
I really don't understand the LowPower library. It claims that PowerDown() is the lowest current consumption mode, but it leaves the I2C circuit active. It has an idle() method that takes an argument to turn the I2C circuit off. That method calls power_twi_disable() which doesn't seem to be defined anywhere.
In power.h:
#define power_twi_disable() (PRR |= (uint8_t)(1 << PRTWI))
According to the datasheet:
"Bit 7 – PRTWI0: Power Reduction TWI0
Writing a logic one to this bit shuts down the TWI 0 by stopping the clock to the module."
... so I don't know, I will test it but I suspect it won't set the pins in the right state and my little module will still draw current if I only use that.
Franck
[Edit]: I confirmed that power_twi_disable() in itself doesn't help with the sleep current. Maybe it helps when used in addition to enabling the pullups on SDA & SCL, but if so that's in the 1uA range.