General, Data Bus Buffer, Read/Write and Control Logic

SILICON GATE MOS 8255

8255 BASIC FUNCTIONAL DESCRIPTION

General

The 8255 is a Programmable Peripheral Interface (PPI) de- vice designed for use in 8080 Microcomputer Systems. Its function is that of a general purpose I/O component to inter- face peripheral equipment to the 8080 system bus. The functional configuration of the 8255 is programmed by the system software so that normally no external logic is nec- essary to interface peripheral devices or structures.

Data Bus Buffer

This 3-state, bi-directional, eight bit buffer is used to inter- face the 8255 to the 8080 system data bus. Data is trans- mitted or received by the buffer upon execution of INput or OUTput instructions by the 8080 cPU. Control Words and Status information are also transferred through the Data Bus buffer.

Read/Write and Control Logic

The function of this block is to manage all of the internal and external transfers of both Data and Control or Status words. It accepts inputs from the 8080 CPU Address and Control busses and in turn, issues commands to both of the Control Groups.

Chip Select: A "low" on this input pin enables the com- munication between the 8255 and the 8080 CPU.

8255 Block Diagram

(RD)

Read: A II10w" on this input pin enables the 8255 to send the Data or Status information to the 8080 CPU on the Data Bus. In essence, it allows the 8080 CPU to II read from"

the 8255.

(WR)

Write: A "low" on this input pin enables the 8080 CPU to write Data or Control words into the 8255.

(AO and A1)

Port Select 0 and Port Select 1: These input signals, in con- junction with the RD and WR inputs, control the selection of one of the three ports or the Control Word Register. They are normally connected to the least significant bits of the Address Bus (A o and A 1 ).

8255 BASIC OPERATION

A1	AO	RD	WR	CS	INPUT OPERATION (READ)
0	0	0	1	0	PORT A ~ DATA BUS
0	1	0	1	0	PORT B => DATA BUS
1	0	0	1	0	PORT C=> DATA BUS
					OUTPUT OPERATION
					(WRITE)
0	0	1	0	0	DATA BUS => PORT A
0	1	1	0	0	DATA BUS => PORT B
1	0	1	0	0	DATA BUS~ PORT C
1	1	1	0	0	DATA BUS => CONTROL
					DISABLE FUNCTION
X	X	X	X	1	DATA BUS=> 3-STATE
1	1	0	1	0	ILLEGAL CONDITION

5-114

8080 specifications

The Intel 8085 and 8080 microprocessors were groundbreaking innovations in the world of computing, paving the way for future microprocessor development and personal computing.

The Intel 8080, introduced in 1974, was an 8-bit microprocessor that played a fundamental role in the early days of personal computing. With a 16-bit address bus, it had the capability to address 64 KB of memory. Running at clock speeds of 2 MHz, the 8080 was notable for its instruction set, which included 78 instructions and 246 opcodes. It supported a range of addressing modes including direct, indirect, and register addressing. The 8080 was compatible with a variety of peripherals and played a crucial role in the development of many early computers.

The microprocessor's architecture was based on a simple and efficient design, making it accessible for hobbyists and engineers alike. It included an 8-bit accumulator, which allowed for data manipulation and storage during processing. Additionally, the 8080 featured registers like the program counter and stack pointer, which facilitated program flow control and data management. Its ability to handle interrupts also made it suitable for multitasking applications.

The Intel 8085, introduced in 1976, was an enhancement of the 8080 microprocessor. It maintained a similar architecture but included several key improvements. Notably, the 8085 had a built-in clock oscillator, simplifying system design by eliminating the need for external clock circuitry. It also featured a 5-bit control signal for status line management, which allowed for more flexible interfacing with peripheral devices. The 8085 was capable of running at speeds of up to 3 MHz and had an extended instruction set with 74 instructions.

One of the standout features of the 8085 was its support for 5 extra instructions for stack manipulation and I/O operations, which optimized the programming process. Additionally, it supported serial communication, making it suitable for interfacing with external devices. Its 16-bit address bus retained the 64 KB memory addressing capability of its predecessor.

Both the 8080 and 8085 microprocessors laid the groundwork for more advanced microprocessors in the years that followed. They demonstrated the potential of integrated circuits in computing and influenced the design and architecture of subsequent Intel microprocessors. Their legacy endures in the way they revolutionized computing, making technology accessible to a broader audience, and their influence is still felt in the design and architecture of modern microprocessors today.

Intel 8080 General, Data Bus Buffer, Read/Write and Control Logic, Basic Functional Description

Models: 8080