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Figure 1.20 Multiple devices can be wired to a microcontroller using individual connections with a wiring scheme that looks like a star.
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are a number of synchronous serial communication protocols (such as I2C and Microwire) that allow point-to-point communication devices to be wired in the common bus format. Synchronous serial is the most basic method of transferring data serially and requires two lines, one for transferring data and another to clock the data to indicate when the transfer is taking place. A sample 8-bit data transfer is shown in Fig. 1.22 with the clock line pulsing when the data bits are valid. There is always a master device that initiates the data transfers and provides the clock for the data transfer (even if it is receiving data from another device). The slave device waits for the clock signal to either receive or send data. While the term clock is typically used to indicate a signal that occurs at a regular interval, the clock in synchronous serial communications is considerably more exible and generally does not have to be timed precisely for the data transfer to take place. When setting up synchronous serial communications between devices there are a number of things to be aware of. The rst is which bit comes rst in the serial data transfer is it the least signi cant bit or the most signi cant bit Next, you have to be aware of when the transfer takes place is it during the rising or falling edge of the clock or some time when the clock is high or low Along with the bit numbering and data valid for the clock, you should also understand whether the data bits are tristate or open collector to allow bidirectional data transfers or allow multiple devices on a common bus.
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Figure 1.21 A single bus scheme can be used to connect multiple devices to a single microcontroller.
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Figure 1.22 Synchronous serial data uses a clock to indicate when the data bits are valid.
While there are some interface standards for synchronous serial communications designed to allow multiple devices on a common synchronous serial bus, you will discover that there really isn t a standard way of implementing synchronous serial communications. Before starting your application development, make sure you have read the various device datasheets and understand how data transfers work. Single data lines can be used for data communications and there are a number of protocols for sending data without a separate clock line. In all of these protocols, the timing of the data bits must be known and the receiver must be able to determine when a bit is coming and whether it is valid. To illustrate the operation of a single data line communications protocol, take a look at Fig. 1.23, which is a diagram of a 5-bit non return to zero (NRZ) data packet. Each bit is a constant amount of time, with the start bit being used to indicate to the receiver that the data is coming with the data bits following (least signi cant bit rst). After the data has been sent, there is a single parity bit, which is a simple error detection code, and a stop bit. The receiver continually polls (reads) the data line and when the signal goes low, it determines the middle of the bit and then waits a full bit period before reading the data. Hardware NRZ receivers built into microcontrollers are very common and provide the start bit detection and bit polling automatically, enabling the processor to simply read the data value from a register. The receiver must be provided with the period of each bit so it can successfully decode the incoming data packet. Before moving on to some of the other single line point-to-point communication protocols, there are few things that you should be aware of about NRZ data packets. First, it is the data protocol used for RS-232 or asynchronous serial communications used in your PC; note that it is not the electrical protocol as this will not be what you expect and requires some specialized circuitry to implement. When used with the PC (and, indeed, most modern applications) the data packets are described as 8-N-1 which means 8 data bits, no parity, and 1 stop bit. The parity bit, as indicated above, is a simple error detection bit and when used will indicate whether the sum of the other bits is odd or even. It is rarely used because modern data transmission protocols provide more elaborate error detection and correction functions, which require less overhead than the
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