c# create barcode image 10: Hardware Design in Objective-C

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CHAPTER 10: Hardware Design
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Figure 10 5. Intel 8051 architecture
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While each product line is likely to have its own set of fanboys, I ve found that pretty much any one of these can be used for what you re trying to accomplish. Taking a maturity approach, I wound up selecting Microchip s PIC16 line of products for my own development. The PIC16 is an 8-bit microprocessor line that has plenty of I/O including analog-to-digital and digital-to-analog conversion, built in oscillators, built-in serial
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CHAPTER 10: Hardware Design
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functions, internal and external interrupts, and very low power requirements. Microchip has several parts in their nanoWatt line that use less than a microamp of power in lowpower mode. What I found was a significant PIC community with lots of available design examples and help. Microchip also provides the MPLAB development environment and HiTech C optimizing compiler. For the adventurous, there are even several realtime operating systems available such as Salvo from Pumkin Inc. Every designer will likely have or develop his go-to part. For me, that part is the PIC16F690, as shown in Figure 10 6. Able to fit probably ten of these on top of a dime, the part sports an internal oscillator that meets all my timing needs with enough memory and I/O to tackle most jobs at very low power consumption. While the Apple NDA again prohibits getting into specifics, let s just say that this was the central part in my first Apple approved accessory. With all that, I still have over 45% of the program memory free for future growth.
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Figure 10 6. PIC16F690 microcontroller
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Choosing to use a Microchip part is in no way an endorsement of this part over any other. Practically speaking, a designer should, if time and money permit, have a large toolset that encompasses various processors. Personally, while I am currently using the PIC16 line in my projects today, I also have an effort underway to migrate several designs to the ARM architecture. But let s get back to power. All in all, power is going to be your number one adversary when developing the hardware design for your accessory. Power is expressed in watts and is defined in one formula as P = IE, where P is power in watts and I is current in amps, and E represents voltage. Power for the types of parts you are considering generally center on the milliwatt region. While watts are the defined units for power, as a new board designer you ll almost never use them. More often than not, you will be measuring your power consumption by looking at the amount of current a device draws measured in milliamps. More specifically, in the general section of the product s specification sheet, you re going to see something that refers to normal operating current at a specific voltage and maybe a frequency.
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CHAPTER 10: Hardware Design
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A couple of the design parameters you will have to decide upon when developing your processor-driven accessory will be what speed to operate the processor chip and what voltage level to provide. Modern processors can operate on a range of voltages typically from something as low as 2.0 volts up to 5.0 volts. Most often, you will choose something around the 3.0-volt level that is normal for these types of designs. There are other considerations as well. Have you ever looked up the voltage of an iPhone battery Nuf said. Another consideration is at what clock speed you will run your processor chip. The off-thetongue answer is as fast as possible. That answer lasts for about a minute until you realize that the faster the speed the more power consumed. Processor speed selection isn t an arbitrary decision. You have to consider other clock speeds within your system. For example, take an RS232 serial connection to some device. Unlike a desktop computer, where you can pretty much set any speed on your modem, a microcontroller is different. You are generally limited to a small set of connection speeds. What s more, some of the speeds will be more accurate than others. For example, you might run 19.2 kbps with a 2.5% error but have an error of 5.5% if you run the connection at 9600 bps. The serial connection speed is derived from multiples (or subdivisions) of the processor s clock speed. This means that you may have to work backwards from your specific rate requirements (e.g., serial speed) to get to your clock speed. If you need a 19.2 kbps clock at less than 2.0%, you look up in the processor s specification which processor clock speed can provide that accuracy. So it s not an arbitrary decision. Also, be sure to check all processor I/O that requires deterministic clock speeds. For example, you could have two serial connections, maybe an RS232 and a SPI interface. Once you find a set of rates and accuracies that work, you ll have your processor s clock. To get the processor s clock, you had to derive it from the requirements for any other clock derived interfaces. Your purpose in deriving the clock speed was to determine power requirements. Now that you have selected a voltage source (level) and a clock speed, you can determine the amount of current your accessory will draw. NOTE: Clock speed of the processor can really drive your design, even going as far as forcing you to switch to a different model or even a different family of chips. Some processors have an internal oscillator that provides a clock you can use without adding parts such as external RC networks or crystals that drive up power and cost. On the other hand, internal oscillators tend to be less accurate. That loss of accuracy can trickle down to parts of your system where a specific speed must be met. Like anything, it s a trade-off. Now that power and clock speed have been dealt with, the next thing to consider is I/O. Processors generally provide increasing amounts of digital I/O along their family lines. As I/O pins increase, so does the number of pins on the device that causes it to get larger. The PIC16F690 I use comes in a 20-pin package, and you saw how small it was by looking at Figure 10 6. Basically, every pin except power and ground is (or can be
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