AT90USB64/128
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).
10.2.2
10.2.3
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.
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) occurs. Normally, the pull-up enabled state is fully acceptable, 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 10-1 summarizes the control signals for the pin value.
Table 10-1. Port Pin Configurations
PUD
DDxn PORTxn (in MCUCR)
I/O
Pull-up Comment
0
0
0
1
X
0
Input
No
Tri-state (Hi-Z)
Pxn will source current if ext. pulled
low.
Input
Yes
0
1
1
1
0
1
1
X
X
Input
Output
Output
No
No
No
Tri-state (Hi-Z)
Output Low (Sink)
Output High (Source)
10.2.4
Reading the Pin Value
Independent of the setting of Data Direction bit DDxn, the port pin can be read through the
PINxn Register bit. As shown in Figure 10-2, the PINxn Register bit and the preceding latch con-
stitute a synchronizer. This is needed to avoid metastability if the physical pin changes value
near the edge of the internal clock, but it also introduces a delay. Figure 10-3 shows a timing dia-
gram of the synchronization when reading an externally applied pin value. The maximum and
minimum propagation delays are denoted tpd,max and tpd,min respectively.
75
7593A–AVR–02/06
ATMEL [ ATMEL ]
