ATmega128 datasheet(68/386 Pages) ATMEL

68 / 386 page

2467S–AVR–07/09

ATmega128

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 written to one 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 25 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 30, the PINxn Register bit and the preceding latch consti-

tute 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 31 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 31. 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

Table 25. 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)

SYSTEM CLK

INSTRUCTIONS

SYNC LATCH

PINxn

r17

in r17, PINx

0xFF

0x00

tpd, max

XXX

tpd, min

ATmega128 датащи(PDF) 68 Page - ATMEL Corporation

ATmega128 датащи(HTML) 68 Page - ATMEL Corporation

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 ...More

датащи скачать

ссылки URL