Rainbow-electronics ATmega8515L Manuel d'utilisateur télécharger pdf (Page 138)

138

ATmega8515(L)

2512A–AVR–04/02

be placeddirectly in themainroutine, orbe combinedwith initialization code for other

I/O modules.

Data Transmission – The

USART Transmitter

The USART Transmitter is enabledbysetting the

Transmit Enable

(TXEN) bit in the

UCSRB Register. When theTransmitter is enabled, thenormalportoperation of the

TxD pin is overridden by the USART and given the function as thetransmitter’sserial

output. The baudrate, modeof operation andframe format must be set up once before

doing any transmissions. If synchronous operation is used, the clock on the XCK pin will

beoverridden and used as transmission clock.

Sending Frames with 5 to 8

Data Bits

A data transmission is initiatedbyloading thetransmit bufferwith the data to betrans-

mitted.The CPUcan load thetransmit bufferbywriting to the UDR I/Olocation. The

buffereddata in thetransmit bufferwill bemoved to the ShiftRegisterwhen the Shift

in idle state (no ongoing transmission) or immediately after the last stopbit of the previ-

ousframe is transmitted. When the Shift Register isloadedwith newdata, it will transfer

one complete frame at the rate given by the baudregister, U2Xbit orbyXCKdepending

on modeof operation.

The following codeexamplesshow a simple USART transmit function based on polling

of the

Data Register Empty

(UDRE) flag. When using frameswithless than eight bits,

themost significant bitswrittentothe UDRare ignored.The USART has to beinitialized

beforethe function can beused. For theassembly code, the data to be sent is assumed

to be stored in Register R16

Note: 1. Theexample codeassumes that the part specificheaderfileis included.

The function simply waitsfor thetransmit buffer to beemptybychecking the UDREflag,

before loading it with newdata to betransmitted. If the data register empty interruptis

utilized, theinterrupt routine writes the dataintothe buffer.

Assembly Code Example

(1)

USART_Transmit:

;

Wait for empty transmit buffer

sbis UCSRA,UDRE

rjmp USART_Transmit

;

Put data (r16) into buffer, sends the data

out UDR,r16

ret

CCode Example

(1)

void USART_Transmit( unsigned char data )

{

Wait for empty transmit buffer

while ( !( UCSRA & (1<<UDRE)) )

;

Put data into buffer, sends the data

UDR = data;

}

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Rainbow-electronics ATmega8515L Manuel d'utilisateur Page 138

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