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A8251

Programmable Communications Interface

Altera Corporation 

a8251 Programmable Communications Interface Data Sheet

Transmitter Operation (Asynchronous)

When the a8251 is programmed for asynchronous operation, the

transmitter includes the following functions:

s Transmitter data register write/transfer

s Transmitter start bit

s Transmitter data

s Transmitter parity bit

s Transmitter stop bit

Transmitter Data Register Write/Transfer

After a total state reset operation and when bit 1 (txen) of the command

instruction register is high, a transmit operation begins when the ncts

signal is asserted. At this point, a data byte can be written to the TBR.

However, if no data is written, the txd signal is held in a logic high state.

In the initial write operation, if the shift register is empty, the data is

immediately transferred and the shift operation begins. If a shift operation

is underway, the microprocessor can write to the TBR; however, the data

is not transferred to the shift register until the active shift operation is

finished.

When the TBR contains data that has not been transferred to the shift

register, the txrdy signal and corresponding status bit go low. Once the

data is transferred to the shift register and the TBR is empty, the txrdy

signal and corresponding status bit will again be asserted. If both the shift

register and TBR become empty, the txempty signal and corresponding

status bit will be asserted. See Figure 5.

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Altera Corporation

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