Rainbow-electronics ATmega8515L Manual do Utilizador descarregar pdf (Página 58)

ATmega8515(L)

2512A–AVR–04/02

enabledstate isfully acceptable, as a high-impedant environment will not noticethe dif-

ference between a strong highdriver and a pull-up. If this is not the case, thePUD bit in

the SFIOR Registercan be set to disableall pull-ups in all ports.

Switching between input withpull-up and output low generates the same problem. The

user mustuseeither thetri-state ({DDxn, PORTxn}=0b00) or the output highstate

({DDxn, PORTxn}=0b10) as an intermediate step.

Table24summarizes the controlsignals for the pin value.

ReadingthePinValue Independent of the setting ofData Direction bit DDxn, the port pin can be read through

thePINxn Registerbit. AsshowninFigure29, thePINxn Registerbit and the preceding

latch constitute a synchronizer.This is needed to avoid metastability if the physicalpin

changes value near theedge of theinternalclock, but it alsointroduces a delay. Figure

30 shows atimingdiagram of the synchronization when readinganexternally applied

pin value. Themaximum and minimum propagation delays are denoted t

pd,max

and t

pd,min

respectively.

Figure 30. Synchronization when Reading an Externally Applied Pin Value

Consider the clock periodstarting shortly

after

the first falling edge of the system clock.

The latch isclosedwhen the clock islow, and goes transparent when the clock ishigh,

as indicatedbythe shadedregion of the“SYNC LATCH” signal.The signal value is

latchedwhen the system clock goeslow. Itisclocked into thePINxn Register at the suc-

ceeding positive clock edge. As indicatedbythetwoarrows t

pd,max

and t

pd,min

, a single

Table 24. PortPinConfigurations

DDxn PORTxn

PUD

(in SFIOR) I/O Pull-up Comment

00 XInput NoTri-state (Hi-Z)

01 0Input Yes

Pxn will source current if ext. pulled

low.

01 1Input NoTri-state (Hi-Z)

10 XOutput No Output Low(Sink)

11 XOutput No Output High(Source)

XXX in r17, PINx

0x00 0xFF

INSTRUCTIONS

SYNC LATCH

PINxn

r17

XXX

SYSTEM CLK

pd, max

pd, min

1 2 ... 53 54 55 56 57 58 59 60 61 62 63 ... 222 223

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