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 When switching between tri-state ({DDxn, PORTxn} = 0b00) and output high ({DDxn,
Output
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-impedant environment will not notice the dif-
ference 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 31 summarizes the control signals for the pin value.
Table 31. Port Pin Configurations
PUD
DDxn PORTxn (in MCUCR)
I/O
Pull-up Comment
0
0
X
Input
No
Tri-state (Hi-Z)
Pxn will source current if ext. pulled
low.
0
0
1
1
1
1
0
1
0
1
Input
Input
Yes
No
No
No
Tri-state (Hi-Z)
X
X
Output
Output
Output Low (Sink)
Output High (Source)
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 24, the PINxn Register bit and the preceding
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
25 shows a timing diagram of the synchronization when reading an externally applied
pin value. The maximum and minimum propagation delays are denoted tpd,max and tpd,min
respectively.
Figure 25. Synchronization when Reading an Externally Applied Pin value
SYSTEM CLK
XXX
XXX
in r17, PINx
INSTRUCTIONS
SYNC LATCH
PINxn
0x00
tpd, max
0xFF
r17
tpd, min
64
ATmega48/88/168
2545D–AVR–07/04
ATMEL [ ATMEL ]
