Embedded Systems Design with the Atmel AVR Microcontroller Part II

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182 CHAPTER 7. ATMEL AVR OPERATING PARAMETERS AND INTERFACING must be carefully analyzed and designed.This will ensure the microcontroller will continue to operate within specified parameters. Should the microcontroller be operated outside its operational envelope, erratic, unpredictable, and unreliable system may result. 7.1 OPERATING PARAMETERS Any time a device is connected to a microcontroller, careful interface analysis must be performed. Most microcontrollers are members of the “HC,” or high-speed CMOS, family of chips. As long as all components in a system are also of the “HC” family, as is the case for the Atmel AVR line of microcontrollers, electrical interface issues are minimal. If the microcontroller is connected to some component not in the “HC” family, electrical interface analysis must be completed. Manufacturers readily provide the electrical characteristic data necessary to complete this analysis in their support documentation. To perform the interface analysis, there are eight different electrical specifications required for electrical interface analysis. The electrical parameters are: VOH: the lowest guaranteed output voltage for a logic high, VOL: the highest guaranteed output voltage for a logic low, IOH: the output current for a VOH logic high, IOL: the output current for a VOL logic low, VIH: the lowest input voltage guaranteed to be recognized as a logic high, VIL: the highest input voltage guaranteed to be recognized as a logic low, IIH: the input current for a VIH logic high, and IIL: the input current for a VIL logic low. These electrical characteristics are required for both the microcontroller and the external components. Typical values for a microcontroller in the HC CMOS family assuming VDD = 5.0 volts and VSS = 0 volts are provided below. The minus sign on several of the currents indicates a current flow out of the device. A positive current indicates current flow into the device. VOH = 4.2 volts, VOL = 0.4 volts, IOH =−0.8 milliamps, IOL = 1.6 milliamps, VIH = 3.5 volts,
VIL = 1.0 volt, IIH = 10 microamps, and IIL =−10 microamps. 7.1. OPERATING PARAMETERS 183 It is important to realize that these are static values taken under very specific operating conditions. If external circuitry is connected such that the microcontroller acts as a current source (current leaving the microcontroller) or current sink (current entering the microcontroller), the voltage parameters listed above will also be affected. In the current source case, an output voltage VOH is provided at the output pin of the microcontroller when the load connected to this pin draws a current of IOH. If a load draws more current from the output pin than the IOH specification, the value of VOH is reduced. If the load current becomes too high, the value of VOH falls below the value of VIH for the subsequent logic circuit stage and not be recognized as an acceptable logic high signal. When this situation occurs, erratic and unpredictable circuit behavior results. In the sink case, an output voltage VOL is provided at the output pin of the microcontroller when the load connected to this pin delivers a current of IOL to this logic pin. If a load delivers more current to the output pin of the microcontroller than the IOL specification, the value of VOL increases. If the load current becomes too high, the value of VOL rises above the value of VIL for the subsequent logic circuit stage and not be recognized as an acceptable logic low signal. As before, when this situation occurs, erratic and unpredictable circuit behavior results. For convenience, this information is illustrated in Figure 7.1. In (a), we provided an illustration of the direction of current flow from the HC device and also a comparison of voltage levels. As a reminder, current flowing out of a device is considered a negative current (source case) while current flowing into the device is considered positive current (sink case). The magnitude of the voltage and current for HC CMOS devices are shown in (b). As more current is sinked or sourced from a microcontroller pin, the voltage will be pulled up or pulled down, respectively, as shown in (c). If input and output devices are improperly interfaced to the microcontroller, these loading conditions may become excessive and voltages will not be properly interpreted as the correct logic levels. You must also ensure that total current limits for an entire microcontroller port and overall bulk port specifications are complied with. For planning purposes, the sum of current sourced or sinked from a port should not exceed 100 mA. Furthermore, the sum of currents for all ports should not exceed 200 mA. As before, if these guidelines are not followed, erratic microcontroller behavior may result. The procedures presented in the following sections, when followed carefully, will ensure the microcontroller will operate within its designed envelope.The remainder of the chapter is divided into input device interface analysis followed by output device interface analysis. Since many embedded systems operate from a DC battery source, we begin by examining several basic battery supply circuits.
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Embedded Systems Design with the At
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iv An Introduction to Logic Circuit
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Copyright © 2010 by Morgan & Clayp
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ABSTRACT This textbook provides pra
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Author’s Biography STEVEN F. BARR
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296 INDEX switch debouncing, 187 sw
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