ATMEGA325-16AJ ATMEL [ATMEL Corporation], ATMEGA325-16AJ Datasheet - Page 54

• Manufacturer Part Number: ATMEGA325-16AJ
• Description: 8-bit Microcontroller with In-System Programmable Flash
• Manufacturer: ATMEL [ATMEL Corporation]

Toggling the Pin
Switching Between Input and
Output
Reading the Pin Value
54
ATmega325/3250/645/6450
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.
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 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 24 summarizes the control signals for the pin value.
Table 24. Port Pin Configurations
Independent of the setting of Data Direction bit DDxn, the port pin can be read through
the PINxn Register bit. As shown in Figure 23, 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
24 shows a timing diagram of the synchronization when reading an externally applied
pin value. The maximum and minimum propagation delays are denoted t
respectively.
Figure 24. Synchronization when Reading an Externally Applied Pin value
DDxn
INSTRUCTIONS
0
0
0
1
1
SYSTEM CLK
SYNC LATCH
PORTxn
0
1
1
0
1
PINxn
r17
(in MCUCR)
PUD
X
0
1
X
X
XXX
Output
Output
Input
Input
Input
I/O
t
pd, max
Pull-up
Yes
0x00
No
No
No
No
XXX
t
pd, min
Comment
Tri-state (Hi-Z)
Pxn will source current if ext. pulled
low.
Tri-state (Hi-Z)
Output Low (Sink)
Output High (Source)
in r17, PINx
pd,max
2570A–AVR–09/04
0xFF
and t
pd,min