Hello,

Can anybody provide some links on using 8051 with external memory. The Bibles have very little info. on this topic.

Well after going through the replies and bible, I have come to the following 2 conclusions:

First Conclusion...

1> The EA of AT89S53 will be permanently held high, so that after reset only the internal code is executed first.

2>The External Code memory(NVRAM) will be mapped above 12k - say 4000H to C000H (ie AT89S53 - 0000H to 2FFFH ). CS of External NVRAM is permanently grounded. and PSEN directly connected to RD of NVRAM.

3>After all the initialization codes are executed(internal memory), a jump to 4000H is made(ext. memory). This will automatically select external memory and start executing code from external memory.

Question:
---------
In the above mentioned condition, when the code is being executed from the external memory, if there is a instruction which says call a function from location 0100H(Internal memory), will the control go to internal ROM execute it and then again come back to external ROM.


Second Conclusion...
Bascically in the above configuration the external NVRAM will be used only as external ROM(Which is of no much help to me). Here is what I plan to do...

1> The EA of AT89S53 will be permanently held high, so that after reset only the internal code is executed first.

2> Depending on certain condition(a port pin P1.0 is held low), I start rewriting the same external NVRAM - but this time I will and the PSEN AND RD, the output of the AND gate will be given to RD of NVRAM.

3> Now using MOVX commands I will update the external NVRAM - after all the updating the uC enters a endless loop. To come out of this endless loop a reset is required. A reset is generated - But now the the port pin P1.0 is held high, so what happens is the updating of external NVRAM is not performed but all the initialization codes are executed and then at the end the control jumps to EXTERNAL NVRAM(mapped above 12k)...

I am sorry if you people are confused...the fact is even I am :-(.

Regards
Raj