Programmable Peripheral Interface in Microprocessor

YASH PAL,

Programmable Peripheral Interface in Microprocessor – A programmable Peripheral Interface device is implemented to perform various input/output functions. Such a device can be set up to perform a specific function by writing an instruction (or instructions) in its internal register, called the control register. Furthermore, functions can be changed anytime during the execution of the program by writing a new instruction in the control register. These devices are flexible, versatile, and economical; they are widely used in microprocessor-based products.

In a programmable device, functions are determined through software interactions. A programmable interface device can be viewed as multiple input/output devices, but it also performs many other functions. Such as time delays, counting, and interrupts. It consists of many devices on a single chip, interconnected through a common bus. This is a hardware approach through software control to perform the input/output functions. This approach is a trade-off between hardware and software, which should reduce programming.

For interfacing with a programmable device, a microprocessor needs to check whether a peripheral is ready before it reads from or writes into a device. This is because the execution speed of the microprocessor is much faster than the response of a peripheral such as a printer. For example, when the microprocessor sends data bytes (characters) to a printer, the microprocessor can execute the instructions to transfer a byte in microseconds; on the other hand, the printer can take 10 to 25ms to print a character. After transferring a character to the printer, the microprocessor should wait until the printer is ready for the next character; otherwise, data will be lost. This problem is overcome by the use of handshake signals, where signals are exchanged between the microprocessor and a peripheral prior to actual data transfer.

The requirements for a programmable interfacing device are as follows: The device should include:

  1. Input and output registers (a group of latches to hold data).
  2. Tri-state buffers.
  3. Capability for bidirectional data flow.
  4. Handshake and interrupt signals.
  5. Control logic.
  6. Chip select logic.
  7. Interrupt control logic.

Here is the list of Programmable Peripheral Devices that are used in a microprocessor.

  1. 8255 Programmable Peripheral Interface (PPI)
  2. 8253/8254 Programmable Interval Timer (PIT)
  3. 8259 Programmable Interrupt Controller (PIC)
  4. 8257 Direct Memory Access (DMA) Controller
  5. 8279 Keyboard/Display Controller
  6. 8251 Programmable Communication Interface (USART)
engineering subjects Microprocessor