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Copy bootloader code from existing Flash contents into BootRAM. Con gure BootRAM as program memory. Change execution to BootRAM. Indicate to the host that the system is ready to receive the new application code. Copy new application code into external Flash.
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The PIC16C601 and PIC16C801 require the external Flash to be loaded initially with your bootloader code, but this can be accomplished by using MPLAB ICD or some other ICSP programmer. Built into these chips is code that will execute with the ICD program download, which will accomplish programming of the Flash chip without any additional effort on your part. Once the external Flash has been loaded using ICSP, you can use a bootloader that will allow you to use another port such as the serial port or USB to update the external Flash. There are newer PIC microcontrollers that have internal Flash and can access external parallel Flash (such as the PIC19F87J50). These chips can have their bootloaders stored in the internal Flash, and they can be used to update the external Flash.
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One of the most useful application programming environments that can be used in a microcontroller is the real-time operating system (RTOS). When used properly, an RTOS can simplify application development, and by compartmentalizing the different execution threads of an application, it provides the opportunity to reduce errors significantly. This option has not been available to PIC microcontrollers until the initial availability of the PIC18 MCU family. This PIC microcontroller family allows access to the processor stack, which then can be modi ed with different task data. This feature is not available in other PIC microcontrollers and will be taken advantage of in the PIC18 for the example RTOS that I will present in this chapter. The best de nition I can come up with for a RTOS is . . . a program that controls the execution of multiple threads of an application in a computing system by prioritizing their execution and allowing them to communicate with each other. Thread is the term used to describe the individual subtasks (of just tasks) of an application and is analogous to an individual thread of conversation between two people in a room. There may be multiple threads, but each one is following its own path. An important point about threads is that if the operating system is executing more than one of them simultaneously, then it is known as a multithreaded operating system or a multitasking operating system (often abbreviated to multitasker). There are two primary types of multitasking operating systems: The event-driven operating system changes only the executing thread when some kind of hardware event occurs, whereas the time-sharing operating system stops each thread after it has executed for some length of time and passes execution on to another thread. For the RTOSs that I have written, I like to implement a hybrid of the two: If the currently executing thread is not stopped by a hardware event, a timer event will stop it and start up the next thread in the queue. An event-driven operating system is used primarily in microcontroller interfacing applications, whereas a time-sharing operating system is used in mainframes where CPU time is shared between users. You may have heard the term processes when multitasking operating systems are discussed. In some operating systems, the word process is interchangeable with task, but
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the most common usage is in PC operating systems, in which a process is an application running in the operating system that is composed of multiple threads that perform the necessary operations of the application. When discussing PIC microcontroller RTOSs, in which there is only one application running, it is common to use the terms process, task, and thread interchangeably because there is normally only one application running at a time in the chip. Similarly, the term device driver loses its meaning when applied to an RTOS designed for a chip such as the PIC microcontroller. In a PC, a device driver is a low-level code that provides an interface to hardware peripherals and the threads of the operating system and application processes. The purpose of a device driver is to provide a common set of application programming interfaces (APIs) to the application and operating system that is independent of the hardware. In a much smaller system, the need to provide a common API is essentially not required because the peripherals to be accessed are built into the chip, and there is little need or opportunity to use different peripheral hardware in the system. Unlike processes, tasks, and threads that become synonymous in a singlechip RTOS, the term device driver is not used at all when describing the code that provides an interface to the hardware in the system; instead, single threads are used to provide hardware access for the application. I tend to lump both an operating system and an RTOS together because the central kernel (the part of the operating system that is central to controlling the operation of the executing tasks) is really the same for both cases. The difference comes into play with the processes that are loaded initially along with the kernel. In a PC s operating system, the console input-output (I/O), command interpreter, and le system processes usually are loaded with the kernel, and everything has been optimized to run in a PC environment (which means they they respond to operator requests). In an RTOS, the actual application tasks are loaded in with the kernel, with priority given to tasks that are critical to operation of the application. You might be a bit suspicious of an RTOS after what I ve just written. After all, you probably have a PC running Windows/95 or Windows/NT, and you are probably familiar with problems working with different pieces of hardware or software applications. I would be surprised if somebody reading this book had never had a problem with Windows not coming up properly, crashing when you least expect it, or hanging up and not responding to input or displaying a blue screen of death. Along with these problems, these operating systems require literally hundreds of megabytes on a hard drive to operate. With this background, you re probably wondering, How can a multitasking operating system be implemented in an 8-bit microcontroller with only a few kilobytes of program memory Another question you might be asking is, What features does the PIC18 microcontroller have that would make me feel like I would want to invest the time and effort into developing a multitasking operating system for it To answer these questions, I would look at it from the negative and ask what the PIC18 doesn t have compared with the PC. The smaller system environment of the PIC18 is what is important and makes an RTOS very appropriate for this type of device. The PIC18 does not have a le system, arbitrary amounts of memory (including virtual memory models), or the sophisticated user interface (unless you want to provide one yourself in your application). In addition, in the microcontroller s case, you can
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