Electronic Circuits and Projects Forum

Thanks..... Its a long time since I have done "movc a,@a+dptr" with tables... I'm revisiting it now.

I see EXACTLY what is going wrong. You're storing the values in code memory, and code memory cannot be directly addressed by the code.

The only way to read data values that are stored in code memory (such as you have with the db statements) is via indirect addressing by using "MOVC" instruction to read them from code memory. This is how we do look up tables on the 8051 -

Code :

 		org		0x0080
 
Value		movc		A, @A+PC
		ret
 
		db		10
		db		12

Then to call the table, you load the table line value into the accumulator, then call the subroutine -

Code :

		mov		A,0x01
		acall		Value

You can also use DPTR as the offset register for this as well -

Code :

		org		0x0080
 
Value		movc		A, @DPTR+PC
		ret
 
		db		10
		db		12

When it returns from the subroutine, the table line value will be stored in the accumulator.

In regards to your question regarding if instruction and data can fit in one byte. These are not PICs. They don't have a "long instruction word" like a PIC does. Instructions are 8 bit instructions and data is 8 bit data. 8051 uses what is called a "modified Harvard architecture". It's not a true Harvard processor like the PIC is.