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Some real-life devices that you may have to control by a microcontroller are electromagnetic relays, solenoids, and motors. These devices cannot be driven directly by a microcontroller because of the current required and the noise they generate. This means that special interfaces must be used to control electromagnetic devices. The simplest method of controlling these devices is to just switch them on and off and by supplying power to the coil in the device. The circuit shown in Fig. 17.25 is true for relays (as is shown), solenoids (which are coils that draw an iron bar into them when they are energized), and dc motors (which will only turn in one direction). In this circuit, the microcontroller turns on the Darlington transistor pair, causing current to pass through the relay coils, closing the contacts. To open the relay, the output is turned off (or a 0 is output). The shunt diode across the coil is used as a kickback suppressor. When the current is turned off, the magnetic ux in the coil will induce a large back EMF (voltage) that has to be absorbed by the circuit or there may be a voltage spike that can damage the relay power supply and even the microcontroller. This diode never must be
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ASYNCHRONOUS (NRZ) SERIAL INTERFACES
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Figure 17.25 A relay can be controlled using a high-current transistor and a kickback-suppression diode.
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forgotten in a circuit that controls an electromagnetic device. The kickback voltage is usually on the order of several hundred volts for a few nanoseconds. This voltage causes the diode to break down and allows current to ow, attenuating the induced voltage. Rather than designing discrete circuits to carry out this function, I like to use integrated chips for the task. One of the most useful devices is the ULN2003A (Fig. 17.26) or the ULN2803 series of chips, which have Darlington transistor pairs and shunt diodes built in for multiple drivers.
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Asynchronous long-distance communications came about as a result of the Baudot teletype. This device mechanically (and, later, electronically) sent a string of electrical signals (which we would call a packet of bits, shown in Fig. 17.27) to a receiving printer.
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1 2 3 4 5 6 7 Gnd 8 16 15 14 13 12 11 10 9 Common
Figure 17.26 The ULN2003A (along with modern variants) is an ef cient way to control multiple relays.
PIC MCU INPUT AND OUTPUT DEVICE INTERFACING
Bit 0 Bit 1 Start Bit
Bit 0 Bit 3 Bit 4 Parity Stop Bit Bit
Data
Figure 17.27 Every NRZ asynchronous packet consists of a start bit, a stop bit, and data bits.
With the invention of the teletype, data could be sent and retrieved automatically without having to have an operator sitting by the teletype all night unless an urgent message was expected. This data-packet format is still used today for the electrical asynchronous transmission protocols described below. Before going on, there is one point that some people get unreasonably angry about, and that s the de nition and use of the terms data rate and baud rate. The baud rate is the maximum number of possible data-bit transitions per second. This includes the start, parity, and stop bits at the ends of the data packet shown in the gure, as well as the 5 data bits in the middle. I use the term packet because we are including more than just data (there is also some additional information in there as well), so character and byte (if there were 8 bits of data) are not appropriate terms. This means that for every 5 data bits transmitted, 8 bits in total are transmitted (which means that nearly 40 percent of the data transmission bandwidth is lost in teletype asynchronous serial communications). The data rate is the number of data bits that are transmitted per second. For this example, if you were transmitting at 110 baud (which is a common teletype data speed), the actual data rate would be 68.75 bps (or assuming 5 bits per character, 13.75 characters per second). I tend to use the term data rate to describe the baud rate. This means that when I say data rate, I am specifying the number of bits of all types that can be transmitted in a given period of time (usually 1 second). I realize that this is not absolutely correct, but it makes sense to me to use it in this form, and I have used it consistently throughout this book (and I have not used the term baud rate). With only 5 data bits, the Baudot code could transmit only up to 32 distinct characters. To handle a complete character set, a speci c ve-digit code was used to notify the receiving teletype that the next 5-bit character would be an extended character. With the alphabet and most common punctuation characters in the primary 32 characters, this second data packet wasn t required very often. As discussed in Chap. 16, when waiting for a character, the PIC microcontroller USART receiver polls the line repeatedly at 1/16 bit period intervals until a 0 (space) is detected. The receiver then waits half a cycle before polling the line again to see if a glitch was detected and not a start bit. Once the start bit is validated, the receiver hardware polls the incoming data once every bit period multiple times (again, to ensure that glitches are not read as incorrect data). The stop bit was provided originally to give both the receiver and the transmitter some time before the next packet is transferred (in early computers, the serial datastream was created and processed by the computers and not by custom hardware, as in modern
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