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 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 9-1 summarizes the control signals for the pin value.
Table 9-1. Port Pin Configurations
PUD
DDxn
PORTxn
(in MCUCR)
I/O
Pull-up
Comment
Default configuration after reset.
Tri-state (Hi-Z)
0
0
X
Input
No
0
0
1
1
1
1
0
1
0
1
Input
Input
Yes
No
No
No
Pxn will source current if ext. pulled low.
Tri-state (Hi-Z)
X
X
Output
Output
Output low (sink)
Output high (source)
9.2.4 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 9-2, 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 9-3 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 9-3. Synchronization when Reading an Externally Applied Pin Value
SYSTEM CLK
INSTRUCTIONS
SYNC LATCH
PINxn
XXX
XXX
in r17, PINx
r17
0x00
0xFF
tpd, max
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 goes low. It is clocked into the PINxn register at the succeeding positive clock edge.
As indicated by the two arrows tpd,max and tpd,min, a single signal transition on the pin will be delayed between ½ and 1½
system clock period depending upon the time of assertion.
ATmega16/32/64/M1/C1 [DATASHEET]
53
7647O–AVR–01/15
MICROCHIP [ MICROCHIP ]
