Rainbow-electronics ATmega8515L Bedienungsanleitung PDF herunterladen (Seite 134)

134

ATmega8515(L)

2512A–AVR–04/02

Signaldescription:

txclk Transmitterclock. (InternalSignal)

rxclk Receiverbase clock. (InternalSignal)

xcki Input from XCK pin (internalSignal). Usedforsynchronousslave operation.

xcko Clock output to XCK pin (InternalSignal). Usedforsynchronous master

operation.

fosc XTALpin frequency (System Clock).

Internal Clock Generation –

The Baud Rate Generator

Internalclock generation is usedfor theasynchronous and the synchronous master

modes of operation. The description in thissection refers to Figure 64.

The USART Baud Rate Register(UBRR) and the down-counterconnected to it function

as a programmable prescaler orbaudrate generator.The down-counter, running at sys-

tem clock (fosc), isloadedwith the UBRR value each time the counterhascounted

downtozeroorwhen the UBRRL Register iswritten. A clock is generated each time the

counterreacheszero. Thisclock i s the baudrate generatorclock output

(= fosc/(UBRR+1)).Thetransmitterdivides the baudrate generatorclock output by 2, 8,

or 16depending on mode. The baudrate generator output is useddirectly by the

receiver’sclock anddata recovery units. However, the recovery units useastate

machine that uses 2, 8, or 16statesdependingonmode set by the state of the UMSEL,

U2X, andDDR_XCK bits.

Table 60 contains equationsforcalculating the baudrate (in bitspersecond) andfor

calculating the UBRR value for each modeof operation usinganinternally generated

clock source.

Note: 1. The baudrate isdefined to bethetransferrate in bit persecond (bps).

BAUD Baudrate (in bitspersecond, bps)

OSC

System Oscillatorclock frequency

UBRR Contents of the UBRRH andUBRRL Registers, (0-4095)

Some examples ofUBRR valuesforsome system clock frequencies are found in Table

68 (see page 156).

Table 60. EquationsforCalculating Baud Rate RegisterSetting

Operating Mode

Equation for Calculating

Baud Rate

(1)

Equation for Calculating

UBRR Value

AsynchronousNormal mode

(U2X=0)

AsynchronousDouble Speed

mode (U2X=1)

SynchronousMaster mode

BAUD

OSC

16 UBRR 1+()

---------------------------------------= UBRR

OSC

16BAUD

------------------------ 1–=

BAUD

OSC

8 UBRR 1+()

-----------------------------------= UBRR

OSC

8BAUD

-------------------- 1–=

BAUD

OSC

2 UBRR 1+()

-----------------------------------= UBRR

OSC

2BAUD

-------------------- 1–=

1 2 ... 129 130 131 132 133 134 135 136 137 138 139 ... 222 223

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