Architecture of 8051

Architecture of 8051:

• It is a single chip

• Consists of CPU, Memory

• I/O ports, timers and other peripherals

• It is a CPU

• Memory, I/O Ports to be connected externally.

·        Small size, low power, low cost;

·        Harvard architecture with separate program and data memory;

·        No data corruption or loss of data; but with complex circuit

·        The 8051 has three very general types of memory.

·        On-Chip Memory refers to any memory (Code, RAM, or other) that physically exists on the microcontroller itself. On-chip memory can be of several types.

·        External Code Memory is code (or program) memory that resides off-chip. This is often in the form of an external EPROM.

·        External RAM is RAM memory that resides off-chip. This is often in the form of standard static RAM or flash RAM.

The 8051 is a flexible microcontroller with a relatively large number of modes of operations.

Your program may inspect and/or change the operating mode of the 8051 by manipulating the values of the 8051's Special Function Registers(SFRs).

SFRs are accessed as if they were normal Internal RAM. The only difference is that Internal RAM is from address 00h through 7Fh whereas SFR registers exist in the address range of 80h through FFh

8051 Clock and Instruction Cycle

In 8051, one instruction cycle consists of twelve (12) clock cycles. Instruction cycle is sometimes called as Machine cycle by some authors.

In 8051, each instruction cycle has six states (S ₁- S ₆). Each state has two pulses (P1 and P2)

128 bytes of Internal RAM Structure (lower address space)

The lower 32 bytes are divided into 4 separate banks. Each register bank has 8 registers of one byte each. A register bank is selected depending upon two bank select bits in the PSW register. Next 16bytes are bit addressable. In total, 128bits (16X8) are available in addressable area. Each bit can be accessed and modified by suitable instructions. The bit addresses are from 00H (LSB of the first byte in 20H) to 7FH (MSB of the last byte in 2FH). Remaining 80bytes of RAM are available for general purpose.

Internal Data Memory and Special Function Register (SFR) Map

The special function registers (SFRs) are mapped in the upper 128 bytes of internal data memory address. Hence there is an address overlap between the upper 128 bytes of data RAM and SFRs. Please note that the upper 128 bytes of data RAM are present only in the 8052 family. The lower128 bytes of RAM (00H - 7FH) can be accessed both by direct or indirect addressing while the upper 128 bytes of RAM (80H - FFH) are accessed by indirect addressing.The SFRs (80H - FFH) are accessed by direct addressing only. This feature distinguishes the upper 128 bytes of memory from the SFRs, as shown in fig 5.

Processor Status Word (PSW)         Address=D0H

PSW register stores the important status conditions of the microcontroller. It also stores the bank select bits (RS1 & RS0) for register bank selection.