ATMEGA88 datasheet(64/349 Pages) ATMEL

64 / 349 page

ATmega48/88/168

2545D–AVR–07/04

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

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 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.

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 t

pd,max and tpd,min

respectively.

Figure 25. Synchronization when Reading an Externally Applied Pin value

Table 31. Port Pin Configurations

DDxn

PORTxn

PUD

(in MCUCR)

I/O

Pull-up

Comment

Input

Tri-state (Hi-Z)

Input

Yes

Pxn will source current if ext. pulled

low.

Input

Tri-state (Hi-Z)

Output

Output Low (Sink)

Output

Output High (Source)

XXX

in r17, PINx

0x00

0xFF

INSTRUCTIONS

SYNC LATCH

PINxn

r17

XXX

SYSTEM CLK

pd, max

t pd, min

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