8051

The 8051, also known as the MCS-51 family, is a widely used 8-bit microcontroller originally developed by Intel and introduced in 1980. It employs a Harvard architecture with separate program and data memory spaces and has become a foundational platform in embedded systems, spawning numerous compatible variants from many manufacturers.

The core features include an 8-bit CPU with an accumulator (A) and a B register, a 16-bit data pointer (DPTR), a 16-bit program counter (PC), and a stack pointer (SP). Internal RAM totals 128 bytes, with the first 32 bytes forming four register banks and 16 bytes designated as bit-addressable. Four 8-bit I/O ports (P0–P3) provide general-purpose I/O; P0 is open-drain and requires external pull-ups, while P2 supplies high-order address bits for external memory interfacing. The instruction set includes multiply and divide operations (MUL AB, DIV AB) and a range of single- to multi-byte instructions.

Memory and addressing are flexible: program memory addresses up to 64K bytes, with most devices containing on-chip ROM or ROMless variants that rely on external code memory. Data memory can be internal RAM or external RAM accessed via MOVX instructions. The architecture supports MOVC for code memory access and MOVX for external data memory, enabling a mix of internal and external storage.

Peripherals include two 16-bit timers/counters, a full-featured UART/serial port, and a set of interrupts (including external, timer, and serial sources). The standard timing model uses a clock divided into machine cycles, traditionally 12 clocks per machine cycle, with most instructions completing in one to three machine cycles.

Since its inception, the 8051 has been extended with higher speeds, larger memories, on-chip flash, and additional peripherals. It remains influential in education and industry, with a broad ecosystem of tools, compilers, and compatible cores.