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ESTIMATING APPLICATION POWER REQUIREMENTS
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Accurate power estimating for your applications is important because the expected power consumption affects how much heat is produced by the circuit and what kind of power supply it requires. With a bit of practice and study of datasheets, you usually can estimate the power consumption of the PIC microcontroller and application circuitry very accurately, but for most applications only an order of magnitude estimation is needed to specify and select a power supply that won t overheat or be terribly inef cient, and you also should be able to specify batteries that will not run out before the speci ed period of time has passed. For the PIC microcontroller itself, the intrinsic current, what I call the current consumed by the microcontroller with nothing connected to it, is available from Microchip in the datasheets. For the PIC16F87x, rated at 4 MHz, you will nd a table in the datasheet that lists the IDD (the supply current or intrinsic current) according to the
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TABLE 15.1 PIC MICROCONTROLLER OSCILLATOR CURRENT CONSUMPTION COMPARISON OSCILLATOR IDD FREQUENCY
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52.5 A 5 mA 5 mA 13.5 mA
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oscillator type. Table 15.1 shows that different clock drivers will produce different current draws, and this is due to how the circuitry is con gured for the different clocking modes. Next, select the current requirements for the devices that connect directly to the PIC microcontroller. Depending on their operation, the current requirements can change drastically. For example, an LED that is off consumes just about no current, whereas one that is on can consume from 5 to 10 mA. As you are starting out, use the worst-case values for your current consumption estimates. Also note that different devices will respond differently depending on how they are used. A very good example of this is an LCD display. Many modern LCDs have builtin pull-ups to make interfacing easier for electronic devices that have open collector outputs (such as the 8051). Typically, the current requirements of these devices will be quoted with the minimum value rather than the maximum. To ensure that you have an accurate estimate, you will have to check the current drain with the LCD connected and operating with the PIC microcontroller. Lastly, the power consumption of other devices connected to the circuit (but not the PIC microcontroller) will have to be determined through the device s datasheets. Again, the worst-case situation should be taken into account. Once these three current values have been found, they can be summed together to get the total application power and then multiplied by the voltage applied to get the power consumed by the application. Once I have this value, I normally multiply it by a 25 to 50 percent derater to make sure that I have the absolute worst case. In the applications in this book where I have speci ed the current, I have continually sought out the worst case and then derated the power to make it seem even worse. The reason for this is to ensure that you will not have any problems with your application power supply. Power really can make or break an application, and incorrectly specifying a supply can lead to problems with the application not powering up properly, failing intermittently, or not running as long on batteries as expected. Marginal power supply problems can be an absolute bear to nd as well. By going with a derated worst case for my application power requirements, I have eliminated one possible point in the application from going bad.
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As I worked through the designs of Harmony remotes, one of the things that I learned is that the secret of long battery life is not keeping the active current low but working at keeping the standby current at an absolute minimum and then only running the circuitry at full power at as short a time as possible. Much of this work has been made easier by Microchip and the introduction of nanowatt devices (which dissipate as little as 100 nW of power when they are in standby mode). Having low standby current is not dif cult, but it does require you to provide a great deal of attention to detail in making sure that you have checked over the operation of the components and how they are placed in very low current draw modes while still being able to work quickly. Standby is the term I use to describe the situation where the PIC microcontroller is in sleep mode, and (if it is a nanowatt device) it is consuming 100 nW of power when it is not driving any other devices. There are a couple of subtleties here. The rst is that it is easy to keep an LED powered while the PIC microcontroller is at sleep. I should have noted that the PIC microcontroller dissipates 100 nW of power (which is 30 nA when powered in a 3.3-V system) plus what else it is powering an LED will require at least 5 mA to drive it, which means that in sleep mode a nanowatt device will be consuming roughtly 5.00003 mA of current, not the 0.00003 mA of current that would be expected by the nanowatt device. The solution to this problem is to stop driving any LEDs or other devices via the PIC microcontroller (which is often easier to say than do). Driving LEDs is an obvious area where current is being drawn, but when other components are involved, the current draw is much harder to detect, and it is more dif cult to understand how to resolve the issue. To address the situation properly, you will have to have a clear understanding of how the circuitry is wired and how the different chips work and the expectations they have for going into a low-current standby mode. For example, you might think that simply putting all the I/O pins into input mode would solve the problem of current ow between the PIC microcontroller and external devices. This is simply not the case; placing all the pins in input mode could result in lower than switching threshold voltages on the peripheral component, causing them to remain active. If there are pull-ups external to the PIC microcontroller, you may nd that there are current ows from these pull-ups through the clamping diodes on the I/O pin circuitry. Finally, there may be de nite power-down sequences that the external chips require, and simply putting the I/O pins in input mode will not be suf cient. To achieve a very low power standby state, you must spend some time reading and clearly planning how you expect the various chips to behave. It should almost go without saying, but when you decide on the components for your application, you should make sure that you select parts that are designed for low-power operation. This clearly rules out any bipolar (i.e., 74xxx logic) components and analog chips that do not have speci c low-power modes. When selecting parts, not only must you understand how they go into a low-power mode, but you also have to make sure that you understand how to bring these components out of low-power mode. Some chips will retain their state information, whereas others will require reloading and reinitialization. When choosing parts, you need to go beyond the rst page of the datasheet
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