Hi everyone,

I have searched through the forum for previous threads about this question but none have addressed exactly what I would like to know.

I am wondering if it is a safe idea to connect a port pin to the /EA pin on an 89S52 in order that the MCU can control which program address space it uses on the fly.

I would like to put a simple boot program in the internal program space and use an external RAM device for combined xcode and xdata (by anding /RD and /PSEN) to achieve a von-neumann model after the boot program is complete.

Let's say I connect P1.0 to /EA. My assumption is that at power on, because the port pins on an 8x52 initialise to 1, the internal code memory will be used to start with. The boot program will then copy program data from a serial EEPROM into the xdata space before clearing P1.0 to 0 in order to start using the external RAM as the code space. If necessary, the code that clears the port pin could actually be put in the xdata space at an address above the end of the 8k internal code and the boot program can jump there so it would already be running code from the external space when it switches /EA.

I am planning to use an AT89S52 and having looked through the data sheet, from what I can see, as long as none of the lock bits are set I think this idea *should* work.

I wonder if anyone has tried this sort of thing before and can offer some advice before I try it - or rather before I get one of my undergraduate students to try it in their final year project...

Best wishes

Chris