STR720 datasheet(171/401 Pages) STMICROELECTRONICS

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STR720 - UART

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16.3.2

Transmission

Values to be transmitted are written to the transmit fifo, txfifo, by writing to ASCTxBuffer. The

txfifo is implemented as a 16 deep array of 9 bit vectors.

If the fifos are enabled (the ASCControl(FifoEnable) is set), the txfifo is considered full

(ASCStatus(TxFull) is set) when it contains 16 characters. Further writes to ASCTxBuffer in

this situation will fail to overwrite the most recent entry in the txfifo. If the fifos are disabled, the

txfifo is considered full (ASCStatus(TxFull) is set) when it contains 1 character, and a write to

ASCTxBuffer in this situation will overwrite the contents.

If the fifos are enabled, ASCStatus(TxHalfEmpty) is set when the txfifo contains 8 or fewer

characters. If the fifos are disabled, it’s set when the txfifo is empty.

Writing anything to ASCTxReset empties the txfifo.

Values are shifted out of the bottom of the txfifo into a 9-bit txshift register in order to be

transmitted. If the transmitter is idle (the txshift register is empty) and something is written to

the ASCTx-Buffer so that the txfifo becomes non-empty, the txshift register is immediately

loaded from the txfifo and transmission of the data in the txshift register begins at the next

baud rate tick.

At the time the transmitter is just about to transmit the stop bits, then if the txfifo is non-empty,

the txshift register will be immediately loaded from the txfifo, and transmission of this new data

will begin as soon as the current stop bit period is over (i.e. the next start bit will be transmitted

immediately following the current stop bit period). Thus back-to-back transmission of data can

take place. If instead the txfifo is empty at this point, then the txshift register will become

empty. ASC-Status(TxEmpty) indicates whether the txshift register is empty.

After changing the fifoenable bit, it is important to reset the fifo to empty (by writing to the

ASCTxReset register), since the state of the fifo pointer may be garbage.

The loop-back option (selected by the ASCControl(LoopBack) bit) internally connects the

output of the transmitter shift register to the input of the receiver shift register. This may be

used to test serial communication routines at an early stage without having to provide an

external network.

STR720 数据表(PDF) 171 Page - STMicroelectronics