AVR-Chat

According to the datasheet, the value is latched from the EA pin at 
reset...so the only way to do it on the fly is to have the CPU put 
everything in memory first...then reset.   If it looks like the 
hardware to change the EA pin and reset the CPU (on the fly) is goint 
to take too much space you might do well to simply use a 2k 
flash/eeprom chip.  That's enough for a boot loader.



--- In AVR-Chat@yahoogroups.com, "Srdjan Ridanovic" <ridanovic@p...> 
wrote:
> Hi, I'm working on a boot loader project with some SRAM, EEPROM and 
AT89C52 and I want to ask you guys some things that I coudn't find in 
any tehnical spacifications. For instance, if the  /EA-Vpp  pin is 
on "1", procesor will execute program from internal code memory ( in 
this case 8Kb ) and then countinu using external memory up to 64Kb ! 
If the /EA-Vpp is on "0" procesor will read program code from 
external memory only ( 0x0000 - 0xFFFF ). The question is: What will 
heppn if I change /EA-Vpp pin from "1" to "0" while the program is 
executing comands from internal code memory? ( If I change it while 
internal program is running ) 
> My presupposition is that the program will continue executing code 
from the PC ( program counter ) address of the external memory code 
and if it's not right sincronized it will gone mad. Or it will 
countinue doing it's program from internal 8Kb code memory and afther 
next reset it will start from 0x0000 of external code memory.
> 
> If enyone has some experiance with these things I would be more 
than grateful.
> Sincerly Srdjan Ridanovic