ATmega8(L)
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 SFIOR 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 20 summarizes the control signals for the pin value.
Table 20. Port Pin Configurations
PUD
DDxn PORTxn (in SFIOR)
I/O
Pull-up Comment
0
0
0
1
X
0
Input
No
Tri-state (Hi-Z)
Pxn will source current if external
pulled low.
Input
Yes
0
1
1
1
0
1
1
X
X
Input
No
No
No
Tri-state (Hi-Z)
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 22 on page 52, 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 23 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 23. 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
Consider the clock period starting shortly after the first falling edge of the system clock. The latch
is closed when the clock is low, and goes transparent when the clock is high, as indicated by the
shaded region of the “SYNC LATCH” signal. The signal value is latched when the system clock
53
2486AA–AVR–02/2013
ATMEL [ ATMEL ]
