70
7734Q–AVR–02/12
AT90PWM81/161
If PORTxn is written logic one when the pin is configured as an output pin, the port pin is driven
high (one). If PORTxn is written logic zero when the pin is configured as an output pin, the port
pin is driven low (zero).
9.2.2
Toggling the Pin
Writing a logic one to PINxn toggles the value of PORTxn, independent on the value of DDRxn.
Note that the SBI instruction can be used to toggle one single bit in a port.
9.2.3
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 accept-
able, as a high-impedant 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.
Table 9-1 summarizes the control signals for the pin value.
9.2.4
Reading the Pin Value
Independent of the setting of Data Direction bit DDxn, the port pin can be read through the
latch constitute a synchronizer. This is needed to avoid metastability if the physical pin changes
shows a timing diagram of the synchronization when reading an externally applied pin value.
The maximum and minimum propagation delays are denoted t
p
d,max and tp
d,min respectively.
Table 9-1.
Port pin configurations.
DDxn
PORTxn
PUD
(in MCUCR)
I/O
Pull-up
Comment
0
X
Input
No
Default configuration after reset.
Tri-state (Hi-Z)
0
1
0
Input
Yes
Pxn will source current if ext. pulled low
0
1
Input
No
Tri-state (Hi-Z)
1
0
X
Output
No
Output low (Sink)
1
X
Output
No
Output high (Source)