ATMEGA8535 datasheet(53/321 Pages) ATMEL

53 / 321 page

ATmega8535(L)

2502K–AVR–10/06

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 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} = 0b10) as an intermediate step.

Table 21 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 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

pd,max and tpd,min

respectively.

Figure 24. Synchronization when Reading an Externally Applied Pin Value

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 suc-

Table 21. Port Pin Configurations

DDxn

PORTxn

PUD

(in SFIOR)

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

tpd, min

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