Reset, Group a and Group B Controls, Ports A, B, and C

SILICON GATE MOS 8255

(RESET)

Reset: A "high" on this input clears all internal registers in- cluding the Control Register and all ports (A, B, C) are set to the input mode.

Group A and Group B Controls

The functional configuration of each port is programmed by the systems software. In essence, the 8080 CPU "out- puts" a control word to the 8255. The control word con- tains information such as "mode", "bit set", "bit reset" etc. that initializes the functional configuration of the 8255.

Each of the Control blocks (Group A and Group B) accepts "commands" from the Read/Write. Control Logic, receives "control words" from the internal data bus and issues the proper commands to its associated ports.

Control Group A - Port A and Port C upper (C7-C4) Control Group B - Port B and Port Clower (C3-CO)

The Control Word Register can Only be written into. No Read operation of the Control Word Register is allowed.

8255 BLOCK DIAGRAM

Ports A, B, and C

The 8255 contains three 8-bit ports (A, S, and C). All can be configured in a wide variety of functional characteristics by the system software but each has its own speci al featu res or "personality" to further enhance the power and flexi- bil ity of the 8255.

Port A: One 8-bit data output latch/buffer and one 8-bit data input latch.

Port B: One 8-bit data input/output latch/buffer and one 8-bit data input buffer.

Port C: One 8-bit data output latch/buffer and one 8-bit data input buffer (no latch for input). This port can be di- vided into two 4-bit ports under the mode control. Each 4- bit port contains a 4-bit latch and it can be used for the control signal outputs and status signal inputs in conjunc- tion with Ports A and B.

PIN CONFIGURATION

PA3	1			PA4
PA2	2		39	PA5
PAl	3		38	PA6
PAO	4		37	PA7
AD	5		36	WR
cs	6		35	RESET
GNO	7		34	DO
A1	8		33	0,
AO	9		32	O2
PC7	10	8255	31	0 3
PC6	11	8255	30	0 4
			30	0 4
PC5	12			Os
				Os
PC4	13			0 6
				0 6
PCO	14			0,
				0,
PC1	15		26	V
				CC
PC2	16			P87
PC3	17		24	P86
P80	18		23	P85
P81	19		22	P84
P82	20		21	P83
		PIN NAMES
0 7 - 00		DATA BUS (BI-DIRECTIONAL)
RESET		RESET INPUT
CS		CHIP SELECT
RD		READ INPUT
WR		WRITE INPUT
AO,A1		PORT ADDRESS
PA7·PAO		PORT A (BIT)
PB7·PBO		PORT B (BIT)
PC7·PCO		PORT C (SfT)		j
VCC		+5VOL1S		1
GND		"VOLTS

5-115

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 manual Reset, Group a and Group B Controls, Ports A, B, and C, PIN Configuration

Models: 8080