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ST7285C Datasheet(PDF) 28 Page - STMicroelectronics |
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ST7285C Datasheet(HTML) 28 Page - STMicroelectronics |
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28 / 117 page  28/117 ST7285C 4.2 SERIAL COMMUNICATIONS INTERFACE 4.2.1 Introduction The Serial Communications Interface (SCI) offers a flexible means of full-duplex data exchange with external equipment requiring an industry standard NRZ asynchronous serial data format. The SCI of- fers a very wide range of Baud rates thanks to the presence of two Baud rate generator systems: the first is of conventional type and yields common communications Baud rates with standard oscilla- tor frequencies; the second features a program- mable prescaler capable of dividing the input fre- quency by any factor from 1 to 255, thus offering a very wide range of Baud rates even with non- standard oscillator frequencies. Transmitter and Receiver circuits are independent and can operate at different Baud rates; indeed, each can select ei- ther type of Baud rate generator. External connec- tions are by means of two I/O pins: TDO (Port PB0) for the Transmit Data output and RDI (Port PB1) for the Receive Data input. 4.2.2 Features – Full duplex, asynchronous communications – NRZ standard format (Mark/Space) – Dual Baud rate generator systems – Independently programmable transmission and reception Baud rates – Separate Transmit and Receive Baud rates – Programmable word length (8 or 9 bits) – Receive buffer full, Transmit buffer empty and End of Transmission flags – Receiver wake-up function by the most signifi- cant bit or by idle line – Muting function for multiprocessor configurations – Separate enable bits for Transmitter and Receiver – Noise, Overrun and Frame Error detection – Four interrupt sources with flags – Overall accuracy better than 1% of Baud rate. 4.2.3 Serial Data Format Serial data is transmitted and received as frames comprising the following elements: – An Idle Line in the ”high” state prior to transmis- sion or reception. – A Start bit in the ”low” state, denoting the start of each character. – Character data word (8 or 9 bits), least significant bit first. – A Stop bit in the ”high” state, indicating that the frame is complete. Word length may be selected as being either 8 or 9 bits by programming the M bit in the SCCR1 con- trol register. An Idle Line condition is interpreted on receiving an entire frame of ”ones”. A Break is interpreted on receiving ”zeros” for some multiple of the frame period. 4.2.4 Data Reception and Transmission The following description is best read with refer- ence to the SCI Block Diagram illustrated in Figure 1, where it will be noted that the SCDR data regis- ter is shown as two separate registers, one for transmitted data and the other for received data. The Serial Communications Data Register (SCDR) performs a dual function (Read And Write), since it accesses two separate registers, one for transmis- sion (TDR) and one for reception (RDR). The TDR register provides the data interface between the in- ternal bus and the output shift register for data to be transmitted, while the RDR register provides an in- terface between the input shift register and the in- ternal bus for incoming data. When the SCDR is read, the RDR is accessed and its contents are transferred to the data bus. The RDRF (RDR Full Flag) in the SCSR register is set to ”1” as soon as the word in the receiver shift reg- ister is transferred to the RDR register. When the SCDR is written to, the data word is transferred to the TDR register. The TDRE flag (TDR empty) in the SCSR register is set to ”1” as soon as the word in the TDR is transferred to the transmit shift register. Incoming data is received in a serial shift register and then transferred to a parallel Receive Data Register (RDR) as a complete word, thus allowing the next incoming character to be received in the shift register while the current character is still in the RDR. Oversampling techniques are used for data recov- ery by discriminating between valid incoming data and noise. 4.2.5 Receiver Muting and Wake-up Feature In multiprocessor configurations it is often desira- ble that only the intended message recipient should actively receive the full message contents, thus reducing redundant SCI service overheads for all non addressed receivers. Communications protocols in such configurations generally issue the recipient address as a message header. |
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