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A microcontroller has on-chip peripherals that dramatically decrease the amount of external components needed in a design. It may have general purpose IO, serial IO,ADC and sometimes even special purpose IO pins that support protocol such as the I2C bus, all built into the chip itself. Typically these peripherals present themselves as IO registers to the CPU – for example, to generate a high signal on an output pin, one usually only requires the CPU to write a “1” to the corresponding IO register bits. One often needs to access individual bits of an IO register. For example, PORT A may be an 8-bit output register, and each bit of the IO register corresponds to a hardware output pin of the PORT. There are several methods of accessing bits in C, with advantages and disadvantages to each approach: Bitwise Operations Plain C is powerful enough to perform any needed bit operations on IO registers (or any other integer objects). (After all, one of the first major tasks for the original C compiler was to rewrite the nascent Unix operating system in C!) Note the following bitwise operation example: Const Qualifier and Strings in Harvard Architecture If you have read-only tables or “variables”, then you should declare them with the “const” modifier. In most cases, the compiler will allocate them in the program memory and not take away precious SRAM space. Some microcontrollers have what is known as the “Harvard Architecture” – the program and data spaces are separate and different instructions are needed to access items in the separate spaces.

Tags : harvard architecture, unix operating system, data spaces, output pin, program memory, plain c, external components, microcontrollers, c language, c compiler, special purpose, adc, general purpose, registers, peripherals

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