lab7_serialIO

lab7_serialIO - Microprocessor Lab Asynchronous Serial I/O...

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Microprocessor Lab Asynchronous Serial I/O Serial communication is the transfer of data performed by sending one bit at a time over a communication channel. Serial communication can be either synchronous or asynchronous. Synchronous communication uses a clock signal, which is common to the receiver and transmitter, to indicate when each bit is being transmitted. Asynchronous communication does not use a common clock. The 68HC12 microcontroller contains on chip support for both synchronous and asynchronous communications. In this course, we will only be using the standard asynchronous mode (e.g., what is used when you connect a serial port to a printer). Serial I/O is performed by Serial Communication Interface (SCI) chip contained in the 68HC12. The SCI is controlled, through software, by registers. The registers are special memory locations that transfer data to hardware. The registers on the 68HC12 are mapped into memory locations internally within the microcontroller. Typically, these addresses cannot be changed (but may differ among models of a given microprocessor since different address decoding functions can be used within the IC for different models). The three general types of register are (i) control registers "controlling" the operation of the peripheral, (ii) status registers where you can find the current state of the peripheral, and (iii) data registers through which you can send and receive data. Figure 1 illustrates the signals corresponding to the data being sent across a serial RS232 connection. When no data is being transmitted on a data line, it is held in the high state. When data is transferred, a start bit must precede the data, to insure that the receiver can find the beginning of the data. The start bit is low, so that it differs from the idle data
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lab7_serialIO - Microprocessor Lab Asynchronous Serial I/O...

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