Hello,
I have a problem handling the internal EEPROM feature of an Atmel chip. If
anyone could show me how this should be handled, I would be grateful.
I have been using an AT89C51ID2 chip that features 2 kbytes on chip EEPROM.
The address space of the EEPROM is shared with the on chip expanded ram as
follows.
On chip expanded RAM: Starts at 0x0000 and ends at 0x06FF
On chip EEPROM: Starts at 0x0000 and ends at 0x7FF
Both must be accessed using XRAM memory space (i.e. use either movx @dptr,a
or movx a,@dptr.) In order to select the EEPROM, the EEE bit (bit 1) of
EECON register (0x00D2) must be set.
Up to this point this is straightforward for me; however, in order to
handle the EEPROM memory I am defining variables and structures. For
example for an structure I define the following:
struct CONFIG {
uint8_t item1; // 8 bits EEPROM item
uint16_t item2; // 16 bits EEPROM item
uint32_t item3; // 32 bits EEPROM item
};
Then I define variables in XDATA memory:
__xdata CONFIG configuration;
__xdata uint8_t eeprom_variable; // Independent EEPROM variable
A set service function take the address of the variable and handles the
EEPROM writing and reading:
void eeprom_write_8 (unsigned int address, uint8_t data);
void eeprom_write_16 (unsigned int address, uint16_t data);
void eeprom_write_32 (unsigned int address, uint32_t data);
uint8_t eeprom_read_8 (unsigned int address);
uint16_t eeprom_read_16 (unsigned int address);
uint32_t eeprom_read_32 (unsigned int address);
As you may see, the problem with this approach is that the linker won't
have any clue whether any of these variables belong to the EEPROM or the
internal extended RAM. As a result, the extended RAM variables will be
mixed with the EEPROM variables. A second drawback is that the useful space
will be reduced for both devices due to the fact that their memory
addresses are overlapped.
In order to pack EEPROM variables in a contiguous space I can define the
EEPROM variables in PDATA space. This puts RAM variables just after EEPROM
variables but reduces the EEPROM address space to 256 bytes.
The specific questions would be. Is there any way to tell the linker that
the address space is shared? is there any possibility to define an
additional address space?
I have been looking for an answer online but the closest reference is this:
https://sourceforge.net/p/sdcc/mailman/message/9280573/
Thank you for taking the time to read.
Best regards,
Victor
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