8051 External Data Memory Interfacing

8051 has 128K bytes of address space of which 64K bytes are set aside for program code and the other 64K bytes are set aside for data. Program space is accessed using the program counter (PC) to locate and fetch instructions, but the data memory space is accessed using the DPTR register and an instruction called MOVX, where X stands for external.

External Data ROM:
To connect the 8031/51 to external ROM containing data, we use RD. For the ROM containing the program code, PSEN is used to fetch the code. For the ROM containing data, the RD signal is used to fetch the data.

MOVX is a widely used instruction allowing access to external data memory space. To bring externally stored data into the CPU, we use the instruction “MOVX A, @DPTR”. This instruction will read the byte of data pointed to by register DPTR and store it in the accumulator. In writing data to external data RAM, we use the instruction “MOVX @DPTR, A” where the contents of register A are written to external RAM whose address is pointed to by the DPTR register.

Read from External Data ROM:

	ORG 0H
	MYDATA EQU 1000H
	COUNT EQU 30
	MOV DPTR, #MYDATA	; point to data in external data ROM
	MOV R2, #COUNT
AGAIN :	MOVX A, @DPTR
	MOV P1, A
	INC DPTR
	DJNZ R2, AGAIN
	END

Copy from External Data ROM to Internal RAM:

	ORG 0H
	TABLE EQU 000H
	RAMTABLE EQU 30H
	COUNT EQU 10
	MOV DPTR, #TABLE	; point to data in external data ROM
	MOV R5, #COUNT
	MOV R0, #RAMTABLE
AGAIN :	MOVX A, @DPTR
	MOV @R0, A
	INC DPTR
	INC R0
	DJNZ R5, AGAIN
	END

8051 with External Data and Program ROM:

Single External ROM for Program and Data:

A single external ROM chip is used for both program code and data storage. The space 0000H – 7FFFH is allocated to program code, and address space D000H – FFFFH is set aside for data. In accessing the data, we use the MOVX instruction. For accessing code, we use MOVC instruction.

External Data RAM:

Read data from Port1 and store in external RAM:

	ORG 0H
	RAMDATA EQU 5000H
	COUNT EQU 200
	MOV DPTR, #RAMDATA	; point to data in external RAM
	MOV R3, #COUNT
AGAIN :	MOV A, P1
	MOVX @DPTR, A
	ACALL DELAY
	INC DPTR
	DJNZ R3, AGAIN
HERE :	SJMP HERE
DELAY :	MOV R5, 255
LOOP :	DJNZ R5, LOOP
	RET
	END

External ROM and RAM:

Memory more than 64KB:
In some applications we need a large amount (256K. bytes, for example) of memory to store data. However, the 8051 can support only 64K bytes of external data memory since DPTR is 16-bit. To solve this problem, we connect A0 – A15 of the 8051 directly to the external memory’s A0 – A15 pins, and use some of the PI pins to access the 64K-byte blocks inside the single 256Kx8 memory chip.

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Related topics:
8051 External Program Memory Interfacing   |   8051 Memory Mapped IO   |   8051 LED Interfacing   |   8051 Switch Interfacing   |   8051 Keyboard Interfacing   |   8051 7-Segment Display Interfacing   |   8051 LCD Interfacing   |   8051 ADC Interfacing   |   8051 DAC Interfacing   |   8051 Relay Interfacing   |   8051 Sensor Interfacing   |   8051 Stepper Motor Interfacing   |   8051 DC Motor Interfacing

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List of topics: 8051

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