19.2.3 Switching Between Input and Output

When switching between tri-state ({DDxn, PORTxn} = 0b00) and output high ({DDxn, PORTxn} = 0b11), an intermediate state with either pull-up enabled {DDxn, PORTxn} = 0b01) or output low ({DDxn, PORTxn} = 0b10) must occur. Normally, the pull-up enabled state is fully acceptable, as a high-impedance environment will not notice the difference between a strong high driver and a pull-up. If this is not the case, the PUD bit in the MCUCR Register can be set to disable all pull-ups in all ports.

Switching between input with pull-up and output low generates the same problem. The user must use either the tri-state ({DDxn, PORTxn} = 0b00) or the output high state ({DDxn, PORTxn} = 0b11) as an intermediate step.

The following table summarizes the control signals for the pin value.

Table 19-1. Port Pin Configurations
DDxnPORTxnPUD
(in MCUCR)I/OPull-upComment
00XInputNoTri-state (Hi-Z)
010InputYesPxn will source current if ext. pulled low
011InputNoTri-state (Hi-Z)
10XOutputNoOutput Low (Sink)
11XOutputNoOutput High (Source)