Digital Electronics: Principles, Devices and Applications

Microprocessors 56113.14.1 Programmable Timer/CounterThe programmable timer/counter is used for the generation of an accurate time delay for event counting,rate generation, complex waveform generation applications and so on. Examples of programmabletimer/counter devices include Intel’s 8254 and 8253 family of devices. Intel 8254 contains three 16-bitcounters that can be programmed to operate in several different modes. Some of the functions commonto microcomputers and implementable with 8254 are a real-time clock, an event counter, a digitalone-shot, programmable rate generator, a square-wave generator, a binary rate multiplier, a complexwaveform generator and a complex motor controller. It is available in 24-pin CERDIP and plastic DIPpackages.13.14.2 Programmable Peripheral InterfaceProgrammable peripheral interface (PPI) devices are used to interface the peripheral devices withthe microprocessors. 8255 PPI is a widely used programmable parallel I/O device. It is available inPDIP, CerDIP, PLCC and MQPF packages. 8255 can be programmed to transfer data under variousconditions, from simple I/O to interrupt I/O. It can function in bit reset (BSR) mode or I/O mode. InI/O mode it has three ports, namely port A, port B and port C. The I/O mode is further divided intothree different modes, namely mode 0, mode 1 and mode 2. In mode 0, all ports function as simpleI/O ports. Mode 1 is a handshake mode whereby port A and/or B use bits from port C as handshakesignals. In mode 2, port A can be set up for bidirectional data transfer using handshake signals fromport C, and port B can be set up either in mode 0 or in mode 1. In BSR mode, individual bits in portC can be set or reset.13.14.3 Programmable Interrupt ControllerA programmable interrupt controller (PIC) is a device that allows priority levels to be assigned to itsinterrupt outputs. It functions as an overall manager in an interrupt-driven system environment. Whenthe device has multiple interrupt outputs, it will assert them in the order of their relative priority.Common modes of a PIC include hard priorities, rotating priorities and cascading priorities. Intel 8259is a family of programmable interrupt controllers (PICs) designed and developed for use with the Intel8085 and Intel 8086 microprocessors. The family originally consisted of the 8259, 8259A, and 8259BPICs, although a number of manufacturers make a wide range of compatible chips today.It handles up to eight vectored priority interrupts for the CPU. It is designed to minimize the softwareand real-time overhead in handling multi-level priority interrupts. It accepts requests from peripheralequipment, determines which of the incoming requests is of the highest priority, ascertains whetheran incoming request has a higher priority value than the level currently being serviced and issues aninterrupt to the CPU on the basis of this determination.13.14.4 DMA ControllerIn a direct memory access (DMA) data transfer scheme, data are transferred directly from an I/O deviceto memory, or vice versa, without going through the CPU. The DMA controller is used to control theprocess of data transfer. Its primary function is to generate, upon a peripheral request, a sequentialmemory address that will allow the peripheral to read or write data directly to or from memory. One ofthe popular known programmable DMA controllers is Intel’s 8257. It is a four-channel direct memoryaccess (DMA) controller. It is specifically designed to simplify the transfer of data at high speeds