Rainbow-electronics ATmega8515L Uživatelský manuál stáhnout pdf (Strana 146)

146

ATmega8515(L)

2512A–AVR–04/02

Figure 69 shows the sampling of the stopbit and theearliest possible beginning of the

start bit of thenext frame.

Figure 69. StopBit Sampling andNext Start Bit Sampling

The same majority voting isdonetothe stopbit asdone for theotherbits in the frame. If

the stopbit isregistered to have a logic 0value, the Frame Error(FE)flag will be set.

Anewhigh to low transition indicating the start bit of anewframe can come rightafter

the lastof the bits usedfor majority voting. ForNormalSpeed mode, the first lowlevel

sample can beatpoint marked(A) in Figure 69. ForDouble Speed modethe first low

level must be delayed to (B). (C) marks a stopbit offull length.Theearly start bit detec-

tion influences theoperationalrange of the receiver.

Asynchronous Operational

Range

Theoperationalrange of the receiver isdependent on themismatch between the

receivedbit rate and theinternally generatedbaudrate. If theTransmitter issending

frames at too fastor too slowbit rates, or theinternally generatedbaudrate of the

Receiverdoes not have a similar(see Table 61)base frequency, theReceiverwill not

beabletosynchronizethe frames to the start bit.

The following equationscan beused to calculate the ratio of theincoming data rate and

internalreceiverbaudrate.

DSum ofcharactersizeandparitysize (D = 5- to 10-bit).

SSamplesperbit. S=16forNormalSpeed modeandS=8 forDouble Speed

mode.

First sample number usedfor majority voting. S

= 8 forNormalSpeed andS

= 4

forDouble Speed mode.

Middle sample number usedfor majority voting. S

=9forNormalSpeed and

= 5 forDouble Speed mode.

slow

is the ratio of the slowestincoming data rate that can beaccepted in relationtothe

receiverbaudrate. R

fast

is the ratio of the fastestincoming data rate that can be

accepted in relation to the receiverbaudrate.

Table 61and Table 62 listthemaximum receiverbaudrate error that can betolerated.

Note that NormalSpeed mode hashigher toleration ofbaudrate variations.

12345678 9 10 0/1 0/1 0/1

STOP 1

1234 5 6 0/1

RxD

Sample

(U2X = 0)

Sample

(U2X = 1)

(A) (B) (C)

slow

D 1+()S

S 1– DS⋅ S

-------------------------------------------= R

fast

D 2+()S

D 1+()SS

-----------------------------------=

1 2 ... 141 142 143 144 145 146 147 148 149 150 151 ... 222 223

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