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external ram and rom


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8051king

Tue Dec 08 2009, 07:15AM

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Joined: Tue Dec 08 2009, 06:53AM

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Hi,

I want to interface some external RAM and ROM to an 8051 but I am slightly confused about memory mapping?

I have attached a simple circuit I have made which uses the MSB of the address line (A15) to select between the RAM / ROM, would this work?

any more information on memory mapping would be appreciated.

Thanks

8051King

DavesGarage

Tue Dec 08 2009, 08:36AM

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Joined: Tue Jan 20 2009, 08:14AM

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Conceptually, you have the right idea, but why allow /PSEN to enable the RAM chip?

/PSEN is asserted during a code fetch cycle (only)...

Consider the following:

Code is stored in lower memory: 0x0000 - 0x0FFF
Data is stored in upper memory: 0x8000 - 0x8FFF

What happens if you read data from address 0x9000? 0xA000? 0xB000? and so on up to 0xF000?

How about if you fetch an instruction from code space 0x1000? 0x2000? and so on up to 0x7000?

-Dave
"Basic research is what I am doing when I don't know what I am doing"