Microprocessor & Interfaces

  8255A - Programmable Peripheral Interface The 8255A is a general purpose programmable I/O device designed to transfer the data from I/O to interrupt I/O under certain conditions as required. It can be used with almost any microprocessor. It consists of three 8-bit bidirectional I/O ports (24 I/O lines) which can be configured as per the requirement. Ports of 8255A 8255A has three ports, i.e., PORT A, PORT B, and PORT C. ·          Port A contains one 8-bit output latch/buffer and one 8-bit input buffer. ·          Port B is similar to PORT A. ·          Port C can be split into two parts, i.e. PORT C lower (PC0-PC3) and PORT C upper (PC7-PC4) by the control word. These three ports are further divided into two groups, i.e. Group A includes PORT A and upper PORT C. Group B includes PORT B and lower PORT C. These two groups can be programmed in three...

Absolute and Partial (Linear Select) Decoding  There are two kinds of address decoding techniques. These are Absolute Decoding — also known as Full Decoding.   Linear Decoding — also known as Partial Decoding. In Figure:- A all eight address line are decoded to generate one unique output pulse; the device will be selected only with the address, 01H. This is called A boslute Decoding  and is a good design practice. However, to minimize the cost, the output port can be selected by decoding some of the address lines, as figure:-B this is called  Partial Decoding . as a result, the device has multiple addresses (similar to foldback memory addresses).  Figure A:-  A boslute Decoding The  Partial Decoding  is a commonly used technique in small systems. such multiple addresses will not cause any problems, provided these addresses are not assigned to any other output ports. Figure B:-  Partial Decoding Full Address Decoding (Absolute Decoding) Partia...

THE 8085 INTERRUPTS Ø      Interrupt is a process where an external device can get the attention of the microprocessor. ·           The process starts from the I/O device ·           The process is asynchronous. Ø      Interrupt is signals send by an external device to the processor, to request the processor to perform a particular task or work. Ø      Mainly in the microprocessor based system the interrupts are used for data transfer between the peripheral and the microprocessor. NEED FOR INTERRUPTS Interrupt is a signal send by an external device to the processor, to the processor to perform a particular task or work. Mainly in the microprocessor based system the interrupts are used for data transfer between the peripheral and the microprocessor. When a peripheral is ready for data transfer, it interrupts the processor by sending an appr...