How many external memory interface with 8051?

Question

Examoogle · Accepted Answer

128 KB

Explanation: The 8051 microcontroller architecture uses a 16-bit program counter and a 16-bit address bus, which allows it to interface with a maximum of 64 KB of external Program Memory (ROM) and 64 KB of external Data Memory (RAM). The total external memory addressing capability is the sum of these two distinct address spaces, resulting in 128 KB of external memory interface.

Working Principle: The 8051 architecture employs a Harvard architecture where program and data memories are separated. Each memory space is addressed by the 16-bit address bus (from $A_0$ to $A_{15}$), providing $2^{16} = 65,536$ addressable locations per space. By using the PSEN (Program Store Enable) signal to access ROM and the RD/WR signals to access RAM, the controller can independently manage 64 KB for code and 64 KB for data.

Key Formulas: $Total 	ext{ } External 	ext{ } Memory = 2^{16} 	ext{ } Bytes (ROM) + 2^{16} 	ext{ } Bytes (RAM)$; $Capacity = 64 	ext{ } KB + 64 	ext{ } KB = 128 	ext{ } KB$

Key Points: The 8051 uses 16 address lines ($A_{15}-A_0$), providing a total of $2^{16} = 65536$ addresses.; Program memory access is controlled by the $\overline{PSEN}$ pin.; Data memory access is controlled by the $\overline{RD}$ and $\overline{WR}$ pins.; The Harvard architecture physically separates program and data spaces, allowing concurrent access in theory.

Advantages: Flexibility to upgrade memory modules; Separation of code and data prevents overwriting program instructions

Disadvantages: Requires external latches and decoding logic for expansion; Uses up I/O ports (Port 0 and Port 2) for multiplexed address/data bus

Applications: Embedded systems requiring large lookup tables; Data logging systems requiring extensive buffer memory

Additional Info: The question assumes the sum of external program and external data memory.. Some variations of 8051 (e.g., 8052) have more internal RAM, but the external addressing limit remains 64KB per block.. Option B (64KB) is often chosen incorrectly by students who confuse the limit of a single address space with the total system capability.

Answer: D is correct — The 8051 can interface with 64 KB of external ROM and 64 KB of external RAM, totaling 128 KB of addressable external memory.

Exam Tip: Always distinguish between 'addressable memory range' (64 KB) and 'total physical memory interface' (128 KB) in 8051-related exam questions.

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