SILICON GATE MOS 8255

8255 DETAILED OPERATIONAL DESCRIPTION

CONTROL WORD

Mode Selection

There are three basic modes of operation that can be select- ed by the system software:

Mode 0 - Basic Input/Output

Mode 1 - Strobed Input/Output

Mode 2 - Bi-Directional Bus

When the RESET input goes "high" all ports will be set to the Input mode (Le., all 24 lines will be in the high im- pedance state). After the RESET is removed the 8255 can remain in the Input mode with no additional initialization required. During the execution of the system program any of the other modes may be selected using a single OUTput instruction. This allows a single 8255 to service a variety of peripheral devices with a simple software maintenance rou- tine.

The modes for Port A and Port B can be separately defined, while Port C is divided into two portions as required by the Port A and Port B definitions. All of the output registers, in- cluding the status fl ip-flops, will be reset whenever the mode is changed. Modes may be combined so that their functional definition can be "tailored" to almost any I/O structure. For instance; Group B can be programmed in Mode 0 to monitor simple switch closings or display compu- tational results, Group A could be programmed in Mode 1 to monitor a keyboard or tape reader on an interrupt-driven basis.

Mode Definition Format

~ GROUPB '"

PORT C (LOWER)

~1 = INPUT 0= OUTPUT

PORTB

1 = INPUT 0= OUTPUT

MODE SELECTION

0= MODE 0 1 = MODE 1

;I' GROUP A '"

PORT C (UPPER) 1 = INPUT

0= OUTPUT

PORTA

1= INPUT 0= OUTPUT

MODE SELECTION

00= MODE 0

01= MODE 1

1X =MODE 2

MODE SET FLAG

1=ACTIVE

 

C

 

MODE 1 --....rL.~~8~B-1/-o..J;itllfTm=;;;;:;;:::;:t:=::::;:::;:;:::;;J.i-~~:~I.J/O

Pa,.PBo CONTROL

CONTROL

PA7 ·pAo

OR I/O

OR I/O

 

MODE 2 ~"'----rrrnmi

~IA.DIRECTIONALr

I/O I

CONtROL I

 

Basic Mode Definitions and Bus Interface

The Mode definitions and possible Mode combinations may seem confusing at first but after a cursory review of the complete device operation a simple, logical I/O approach will surface. The design of the 8255 has taken into account things such as efficient PC board layout, control signal defi- nition vs PC layout and complete functional flexibility to support almost any peripheral device with no external logic. Such design represents the maximum use of the available pins.

Single Bit Set/Reset Feature

Any of the eight bits of Port C can be Set or Reset using a single OUTput instruction. This feature reduces software requirements in Control-based applications.

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Intel 8080 manual Mode Selection, Single Bit Set/Reset Feature, Detailed Operational Description, PA 7 ·pAo
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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.
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