Understanding 8051 External Data Memory Interfacing

The question asks about the total external data memory that can be interfaced to the 8051 microcontroller. This capability is directly related to the 8051's memory addressing scheme and the size of its address bus when accessing external memory.

8051 Memory Addressing

The 8051 microcontroller has separate address spaces for internal memory (RAM and SFRs) and external memory (Program and Data). When accessing external memory, the 8051 uses a 16-bit address bus.

Calculating Maximum Addressable Memory

The number of unique memory locations that a processor can access is determined by the number of bits in its address bus. If an address bus has 'n' bits, it can address $2^n$ distinct locations.

For the 8051 accessing external memory, the address bus is 16 bits wide. Therefore, the total number of locations it can address is:

\( \text{Total locations} = 2^{\text{Address bus width}} \)

In this case:

\( \text{Total locations} = 2^{16} \)

Let's calculate the value of \(2^{16}\):

\( 2^{16} = 65,536 \text{ locations} \)

Converting to Kilobytes (K)

In computer memory contexts, 'K' (Kilobyte) typically represents \(2^{10}\) bytes (1024 bytes), although in some data transfer contexts, it can mean \(10^3\). For memory capacity, \(1 \text{K} = 1024\).

To convert 65,536 locations (bytes) into Kilobytes, we divide by 1024:

\( \text{Memory in K} = \frac{65,536}{1024} \)

\( \text{Memory in K} = 64 \text{ K} \)

External Memory Spaces

The 8051 can access external program memory and external data memory. Both share the 16-bit address bus, meaning the total addressable external memory space is 64 K. The 8051 uses different instructions (like MOVX for external data and fetching opcodes from external program memory) to distinguish between accesses to the external data space and the external program space. Each of these spaces can be up to 64 K in size, but typically the question implies the maximum capacity of one of these spaces accessible via the 16-bit address bus.

Thus, the maximum size for external data memory addressable by the 8051 is 64 K.

Analyzing the Options

• Option 1: 256 K (Greater than the maximum 64 K addressable)
• Option 2: 64 K (Matches the calculated maximum addressable memory)
• Option 3: 32 K (Less than the maximum 64 K addressable)
• Option 4: 128 K (Greater than the maximum 64 K addressable)

Based on the 8051's 16-bit external address bus, the maximum external data memory that can be interfaced is 64 K.

Revision Table: 8051 Memory Basics

Additional Information: 8051 External Memory Access

Accessing external memory in the 8051 involves dedicated pins and control signals:

• Port 0 (P0): Used for both the lower 8 bits of the address bus (AD0-AD7) and the 8-bit data bus (D0-D7). It is time-multiplexed.
• Port 2 (P2): Used for the upper 8 bits of the address bus (A8-A15) when accessing external memory.
• \(\overline{\text{ALE}}\) (Address Latch Enable): A signal used to latch the lower 8 bits of the address from P0 into an external latch, separating the address and data.
• \(\overline{\text{PSEN}}\) (Program Store Enable): An active-low signal used to enable external program memory when fetching instructions (EA pin is low).
• \(\overline{\text{RD}}\) (Read): An active-low signal used to enable external data memory for read operations.
• \(\overline{\text{WR}}\) (Write): An active-low signal used to enable external data memory for write operations.
• EA (External Access): A pin that determines if instruction fetches are from internal ROM (EA=1) or external memory (EA=0). When EA=0, all program fetches are external. When EA=1, fetches are internal until the program counter exceeds the internal ROM size, then they become external. External data memory access using MOVX is always possible regardless of the EA pin state.

The 16-bit address generated using P0 (multiplexed) and P2 allows the 8051 to point to any location within a 64 K address space for both external program and external data memory.