AT90USB646_14 Datasheet, PDF (73/456 Pages) ATMEL Corporation – USB boot loader programmed by default in the factory

AT90USB64/128
11.2.2
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.
11.2.3
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) 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 11-1 summarizes the control signals for the pin value.
Table 11-1. Port pin configurations.
PUD
DDxn PORTxn (in MCUCR) I/O Pull-up Comment
0
0
X
Input
No Tri-state (Hi-Z)
0
1
0
Input
Yes Pxn will source current if ext. pulled low
0
1
1
Input
No Tri-state (Hi-Z)
1
0
X
Output No Output Low (Sink)
1
1
X
Output No Output High (Source)
11.2.4
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 11-2 on page 72, the PINxn Register bit and the preced-
ing latch constitute 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 11-3 on
page 74 shows a timing diagram of the synchronization when reading an externally applied pin
value. The maximum and minimum propagation delays are denoted t_pd,max and t_pd,min
respectively.
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