read barcode from image c#.net DRIVING RELAYS WITH AVR 139 in Software

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DRIVING RELAYS WITH AVR 139
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Figure 6.36 LCD character codes.
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The following diagram shows how these drivers are used to drive relay coils. Figure 6.39 shows three relays being driven by the outputs of three drivers from the ULN2003A IC. One end of the relay coil is connected to the output of the driver and the other end is connected to the ve supply voltage. The value of this voltage will depend upon the relay coil voltage ratings. The diode common point is also connected to the ve supply voltage. The inputs to the ULN2003A IC is TTL voltages, say the output of the port pins of the AVR, for example. With this arrangement, the port signals could be used to control each of the relays. The relay terminals labeled NC (Normally Closed), common, and NO (Normally Open) could be used to switch whatever voltage that may need to be switched. Typically, the relay terminals are used to switch the main supply (220 V AC or 115 V AC as may be the case) to the required load (a heater or a lamp, etc.), but, of course, it may be used to switch any voltage (AC or DC) as long as the relay contact can handle the voltage and the current.
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140 HARDWARE AND SOFTWARE INTERFACING WITH THE AVR
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Figure 6.37 Circuit schematic for an AT90S2313 processor interface to a 2-line, 16-character LCD.
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6.12 Stepper Motor Interface for the AVR
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Figure 6.40 illustrates a very popular stepper motor sequencer and driver interface to the AVR processor. The L297/298 sequencer and driver is made by SGS Thomson (us.st.com). L297 is a stepper motor controller IC that generates four-phase drive signals for twophase bipolar and four-phase unipolar step motors in microcontroller-controlled applications. The motor can be driven in half step, normal, and other modes, and on-chip PWM chopper circuits permit switch-mode control of the current in the windings. The IC only requires a mode input, a clock input, and a direction input for its operation. This greatly reduces the software burden of the microcontroller. To drive the stepper or DC motors, a matching driver IC such as the L298 is used. L298 is a dual full-bridge driver. It can be used with power supply voltages up to 48 V and total DC current up to 4 A. For a larger drive, L2603 from SGS Thomson can be used instead of the L298. Figure 6.41 illustrates the circuit schematic of L297 and L298, which can be used with an AVR processor. When moving motors, it is always advisable to gently increase the speed of the motors rather than operate the motor at a fixed speed right at the beginning. The motor is started slowly at a minimum speed and then gradually, the speed is increased till it reaches the
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INTERFACING TO A SERIAL EEPROM 141
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TTL inputs
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Open Collector Outputs
Figure 6.38 ULN2003A darlington array.
maximum operable speed. To bring the motor to a halt, the speed is gradually decreased before stopping. Figure 6.42 illustrates the motor speed ramping.
6.13 Interfacing to a Serial EEPROM
Serial EEPROMs are getting very popular for a variety of reasons. You can store up to 64 Kbytes of data in small 8-pin DIP package. The communication takes only two signals, since most serial EEPROMs are available with IIC interface. These EEPROMs can be written 100,000 times or even more. Even though the data write takes 10 ms, by writing an entire page at a time, the average write rate can be improved. Writing a page of data into the EEPROM also takes almost the same time as writing a byte. The page size can vary between 16 bytes for smaller-capacity EEPROMs to 128 bytes for the larger 64Kbyte capacity EEPROMs. Thus data transfer speeds can be improved by writing in a burst mode. Figure 6.47 (appearing later in this chapter) illustrates the circuit schematic for the AVR to EEPROM interface. A MAX232 RS-232 line translator has been connected so that the user can read and write data to the EEPROM.
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