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A8251

Programmable Communications Interface

Altera Corporation 

a8251 Programmable Communications Interface Data Sheet

Sync Character Two Register

The sync character two register holds the value of the second sync

character. The information is used by the receiver for sync comparison

and by the transmitter for sync character transmission.

Transmitter Buffer Register

The transmitter buffer register (TBR) holds the transmitter data, which the

a8251 formats, serializes, and transmits on the txd output. Once the

existing data bits in the shift register are completely transmitted, the TBR

transfers new data into the shift register.

Operation

Receiver Buffer Register

The receiver buffer register (RBR) holds the data received from the shift

register. After the shift register receives a new data word, it is ready to

transfer the new data word to the RBR. If the existing data in the RBR has

already been read by the microprocessor, then the transfer takes place. If

the existing RBR data has not been read, the overrun error (oe) bit is set.

This section describes the following:

s Programming operation

s Receiver operations: asynchronous and synchronous

s Transmitter operations: asynchronous and synchronous

s Reset operation

Programming Operation

The a8251 must be programmed in a specific order and immediately

following a total state reset or an internal state reset.

First, the microprocessor writes to the MIR. When synchronous operation

is selected in the MIR, the microprocessor writes to the first sync

character. If two sync characters are selected in the MIR, the second

character is written immediately after the first; if only one sync character

is selected, the second character is skipped. However, when

asynchronous operation is selected, both sync characters are skipped.

Once the microprocessor writes to the MIR and sync characters (if

appropriate), the command instruction, status, TBR and RBR can be

randomly accessed.

Table 10 outlines the a8251 programming sequence including the logic

level of control signals.

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Altera Corporation

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