Following their success in the microprocessor market, Intel began manufacturing microcontrollers in 1976 with the introduction of the 8048 family. This early microcontroller contains 64 bytes of RAM, 1 kB of ROM, a simple 8-bit microprocessor core, and an 8-bit timer/counter as its sole on-board peripheral. (Subsequent variants, the 8049 and 8050, include double and four times the memory of the 8048, respectively.) The microprocessor consists of a 12-bit program counter, an 8-bit accumulator and ALU, and a 3-bit stack pointer. The 8048 is a complete computer on a single chip and gained a certain amount of fame in the 1980s when it was used as the standard keyboard controller on the IBM PC because of its simplicity and low cost. The 8048 was manufactured in a 40-pin DIP and could be expanded with external memory and peripherals via an optional external address/data bus.
However, when operated as a nonexpanded single-chip computer, the pins that would otherwise
function as its bus were available for general I/O purposes—a practice that is fairly standard on
microcontrollers. Motivated by the popularity of the 8048, Intel introduced the 8051 microcontroller in 1980, which is substantially more powerful and flexible. The 8051’s basic architecture. It contains 128 bytes of RAM, 4 kB of ROM, two 16-bit timer/counters, and a serial port. Registers within the microprocessor are 8 bits wide except for the 16-bit data pointer (DPTR) and program counter (PC). Memory is divided into mutually exclusive program and data sections that each can be expanded up to 64 kB in size via an external bus. Expansion is accomplished by borrowing pins from two of the four 8-bit I/O ports. Intel manufactured several variants of the 8051. The 8052 doubled the amount of on-chip memory to 256 bytes of RAM and 8 kB of ROM and added a third timer. The 8031/8032 are 8051/8052 chips without on-board ROM. The 8751/8752 are 8051/8052 devices with EPROM instead of mask ROM. As time went by and the popularity of the 8051 family increased, other companies licensed the core architecture and developed many variants with differing mixes of memory and peripherals.
Ports 0 through 3 are each eight-bit bidirectional I/O structures that can be used as either generalpurpose signals or as dedicated interface signals according to the system configuration. In a singlechip configuration where all memory is contained on board, the four ports may be assigned freely. Some peripheral functions use these I/O pins, but if a specific function is not required, the pins may be used in a generic manner. Port 3 is the default peripheral port where pins are used for the serial port’s transmit and receive, external interrupt request inputs, counter increment inputs, and external bus expansion control signals. Port 1 is a general-purpose port that is also assigned for additional peripheral support signals when an 8051 variant contains additional peripheral functions beyond what can be supported on port 3 alone.
However, when operated as a nonexpanded single-chip computer, the pins that would otherwise
function as its bus were available for general I/O purposes—a practice that is fairly standard on
microcontrollers. Motivated by the popularity of the 8048, Intel introduced the 8051 microcontroller in 1980, which is substantially more powerful and flexible. The 8051’s basic architecture. It contains 128 bytes of RAM, 4 kB of ROM, two 16-bit timer/counters, and a serial port. Registers within the microprocessor are 8 bits wide except for the 16-bit data pointer (DPTR) and program counter (PC). Memory is divided into mutually exclusive program and data sections that each can be expanded up to 64 kB in size via an external bus. Expansion is accomplished by borrowing pins from two of the four 8-bit I/O ports. Intel manufactured several variants of the 8051. The 8052 doubled the amount of on-chip memory to 256 bytes of RAM and 8 kB of ROM and added a third timer. The 8031/8032 are 8051/8052 chips without on-board ROM. The 8751/8752 are 8051/8052 devices with EPROM instead of mask ROM. As time went by and the popularity of the 8051 family increased, other companies licensed the core architecture and developed many variants with differing mixes of memory and peripherals.
Ports 0 through 3 are each eight-bit bidirectional I/O structures that can be used as either generalpurpose signals or as dedicated interface signals according to the system configuration. In a singlechip configuration where all memory is contained on board, the four ports may be assigned freely. Some peripheral functions use these I/O pins, but if a specific function is not required, the pins may be used in a generic manner. Port 3 is the default peripheral port where pins are used for the serial port’s transmit and receive, external interrupt request inputs, counter increment inputs, and external bus expansion control signals. Port 1 is a general-purpose port that is also assigned for additional peripheral support signals when an 8051 variant contains additional peripheral functions beyond what can be supported on port 3 alone.
