Digital Electronics: Principles, Devices and Applications

526 Digital ElectronicsAddress BusMicroprocessorDatabusMemoryandI/OControl BusFigure 13.1Microprocessor-based system.Input/output devices are means through which the microprocessor interacts with the outside world.The commonly used input devices include keyboards, A/D converters, switches, cameras, scanners,microphones and so on. LEDs, seven-segment displays, LCD displays, printers and monitors are someof the commonly used output devices.A bus is basically a communication link between the processing unit and the peripheral devices. Itis a group of wires that carry information in the form of bits. The address bus is unidirectional andis used by the CPU to send out the address of the memory location to be accessed. It is also usedby the CPU to select a particular input or output port. It may consist of 8, 16, 20 or an even greaternumber of parallel lines. The number of bits in the address bus determines the maximum number ofdata locations in the memory that can be accessed. A 16-bit address bus, for instance, can access 2 16data locations. It is labelled as A 0 ,…,A n−1 , where n is the width (in bits) of the address bus.The data bus is bidirectional, that is, data flow occurs both to and from the microprocessor andperipherals. Data bus size has a considerable influence on the computer architecture, as parameterssuch as the word length and the quantum of data that can be manipulated at a time are determinedby the size of the data bus. There is an internal data bus, which may not be of the same width as theexternal data bus that connects the microprocessor to I/O and memory. The size of the internal databus determines the largest number that can be processed by the microprocessor in a single operation.The largest number that can be processed, for instance, by a microprocessor having a 16-bit internaldata bus is 65535. The data bus is labelled as D 0 ,…,D n−1 , where n is the data bus width (in bits).The control bus contains a number of individual lines carrying synchronizing signals. The term ‘bus’would normally imply a group of lines working in unison. The control bus (if we call it a bus) sends outcontrol signals to memory, I/O ports and other peripheral devices to ensure proper operation. It carriescontrol signals such as memory read, memory write, read input port, write output port, hold, interrupt,etc. For instance, if it is desired to read the contents of a particular memory location, the CPU firstsends out the address of that location on the address bus and a ‘memory read’ control signal on thecontrol bus. The memory responds by outputting data stored in the addressed memory location onto thedata bus. ‘Interrupt’ tells the CPU that an external device needs to be read or serviced. ‘Hold’ allowsa device such as the direct memory access (DMA) controller to take over the address and data buses.Figure 13.2 shows the bus interface between the microprocessor and its peripheral devices. Themicroprocessor considered in the diagram is an eight-bit microprocessor such as Intel’s 8085.Microprocessor-based systems can be categorized as general-purpose reprogrammable systems andembedded systems. Reprogrammable systems include microcomputers and mainframe and miniframecomputers where microprocessors are used for computing and data processing. In embedded systems,they perform a specific task and are not available for reprogramming to the end-user. Examples ofthese systems include mobile phones, washing machines, microwave ovens, dish washers and so on.