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

Draw QR-Code in Objective-C 10: Hardware Design

CHAPTER 10: Hardware Design
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used as) an I/O pin, so the PIC16F690 has 18 pins available for I/O. These must be shared across any other function that you need to use. For example, if you want to use the built-in serial I/O function of the part, you lose two I/O pins to transmit and receive. Decide to add an oscillator to create a super-stable clock, another two pins. Use a part-to-part link such as SPI or I2C and there go another three or four pins. Therefore, pincount is a critical part of your design parameters as well. NOTE: Many chips provide both on-chip and off-chip capability. First time designers should consider using as much as possible provided by the chip, such is the on-chip oscillator. Oscillator design is one of those areas that looks simple at first glance but can really eat up your time trying to tweak things to get them correct. Never go into the process targeting 100% utilization, that is, have every I/O pin accounted for in the critical elements of your design. Give yourself some wiggle room. I always use two I/O pins on all of my designs to drive diagnostic LEDs. While there are debuggers and single-step tools available, once your part is soldered onto your board, you need a quick-and-dirty way to tell what s going on. LEDs are the embedded engineer s equivalent of printf() statements. Another very critical parameter you need to consider is memory. Microcontrollers such as the PIC16F690 have three main types of memory. Program memory is generally how a part is identified. The PIC16F690 is a 4K-part meaning that it contains 4K (4096) program words. It also contains 256 bytes of static RAM (SRAM) and 256 bytes of EEPROM. Look again at those numbers; that s 4K of program space and 256 bytes of space for any variables you need, loop counters, program variables, etc. The Mac I m writing this book with has 4 GB of memory. Program memory is where the machine code generated by your compiler is kept. It s actually executed from there as well so there s no swapping to RAM. Of the 256 SRAM bytes, some of those are used for other things so you lose a little bit there as well. EEPROM memory can be used to store data elements that may change from assembly to assembly; serial numbers are a good thing to store in EEPROM. To a seasoned programmer that 4K number has got to be pretty frightening. But remember what I said earlier, in my first Apple approved design, a credit card reader, I had 45% free program memory. Looking at it another way, my software used 55% of the available program memory to do all functions including identifying itself to the iPhone, handling communications with the iPhone, data transfer, and processing stripe data from a credit card. For the game controller accessory the numbers were closer to 50% utilization as well as 50% free space. How do you know how much memory to select The short answer is to look at what I did. I am no expert in programming intricate, tight loops in PIC assembly code. In fact, I used C and made the structure of my code as reusable and understandable as I could.
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CHAPTER 10: Hardware Design
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In other words, seasoned embedded programmers would call me sloppy. I leaned heavily on the optimization features of my compiler to get me where I am. That brings me to another point; the compiler you choose can have serious make-orbreak consequences for your design. I actually tried a couple of compilers when I was programming my accessory firmware. I ll get more into firmware in the next chapter, but another reason I chose the PIC16 line of processors was that there were several compiler options. I could start out with a free version that came with my evaluation kit, but because of licensing agreements couldn t distribute the code. From there, I progressed to a low-cost version that I could use for code distribution and verify that my design was workable. Later, after becoming familiar with the PIC16F690 after several projects, I felt secure enough in my decision to go all out and invest in professional optimizing compiler. Most companies will let you try out even their high-end versions for a limited time. You ll need about a week to convert from one compiler to another, but after that you can see how much an optimizing compiler might buy you. For my credit card reader, I went from a 99% utilization of program memory (meaning I had very little space left) to my 55% number. The increase in free memory made the significant investment in an optimizing compiler worthwhile. Why Because now I can use this part with which I have become very familiar, in even more complicated designs. I don t have to learn a whole new part; I can continue to use the PIC16F690 for new designs. In reality, any of the four microcontroller lines that were mentioned earlier should be suitable for anything you re looking to build. Your goal should be to pick a line that you feel comfortable with because you ll probably wind up sticking with it for a while. Derive the requirements for what your accessory needs to accomplish. Look at the product line you ve selected and make sure that the part that works to fulfill your needs isn t at the high end of the list of parts. Give yourself some room to grow. Although I called this section Parts Selection, I ve focused only on the processor as that is the key component of your accessory. Although there are other parts to the system, LEDs, resistors, chip capacitors, switches, potentiometers, etc., you ll tend to purchase these based on distributor and availability rather than manufacturer. Like the processor you will need to specify all these other components. For example, to add diagnostic LEDs to a system you need to consider forward voltage (the voltage at which the LED illuminates) and current draw. You can t put an LED with a forward voltage of 5 volts in a 3-volt system.
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