AT90S8252
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All Flash programmable versions of the 8051 have a Flash memory space that is used for storage of the program memory. The memory is nonvolatile, meaning it does not go away when you turn off the power. But the values in this space can not be changed by the chip itself. You must use a device programmer (like our PG302) to erase these memory spaces and write new data to them.

The Atmel AT89S8252 is an 8051 compatible microcontroller that has the normal Flash space for program memory (8K) and also includes 2K of nonvolatile EEPROM space. This memory space works the same as the Flash space but can be written to and read from by the chip itself during program execution. This allows you to store data that does not get erased when the power goes away. (Good for California summers!)

The process of writing to the EEPROM can be a little complicated. That explains why there is a whole web page for it here.

First you need to define an extra register that is not included in most 8051s. The WMCON register is at memory location 96H. To simply writing to the register we assign the name WMCON to the memory location:

WMCON        .EQU 096H

The register has 3 bits that we use to read and write to the EEPROM.
 

EEMWE - Bit 4 - The EEPROM Data Memory Write Enable Bit. This needs to be set to 1 to write to the EEPROM. Set to 0 after writing to EEPROM.

EEMEN - Bit 3 - Internal EEPROM Access Enable. Set this to 1 to tell the chip to access the EEPROM when you use the MOVX instruction.

RDY/BSY - Bit 1 - Ready/Busy. This bit is normally 1. When you write data to the EEPROM this bit goes to 0. When it is 1 again you can write more data or read from the EEPROM. (You can do other stuff while you are waiting for the write operation to complete.)

So the first thing we do is enable EEPROM access with the following command:

   MOV WMCON, #00001010B

The bits go from Bit 7 on the left to Bit 0 on the right, before the B which signifies that this is a binary number. The EEMEN bit is set to 1. The RDY/BSY bit should already be 1 and we are basically leaving it at 1.

Next is our routine for writing a data value to the EEPROM. This routine assumes that the address location you are writing to has already been moved into the variable ADDRESS and the data value has been moved into the variable DATAOUT. Look at the complete example at the end of the page to see how variables are defined. First we enable writing to the EEPROM with the first MOV command. Then we move the address to the data pointer, move the data value to the accumulator (A) and write to the EEPROM using the MOVX command. The next 3 lines form a loop that continually check the RDY/BSY bit in WMCON to check if the write operation has been completed. The write operation can take as long as 2.5 milliseconds. For most programs you would probably go do something else (check football scores maybe?) rather than just sitting around waiting. Then the last line before the RETurn command disables EEPROM writing.

WRITEEEPROM:
   MOV WMCON, #00011010B      ; Enable Writing to EEPROM
   MOV DPH, #00H              ; Set up high byte of data address
   MOV DPL, ADDRESS           ; Set up low byte of data address
   MOV A, DATAOUT             ; Move Data Out value to A
   MOVX @DPTR, A              ; Write Data Value to EEPROM at address DPH:DPL
WRITEEEPROMWAIT:
   MOV A, WMCON
   CJNE A, #00011010B, WRITEEEPROMWAIT  ; Check if EEPROM write is complete (Bit 1 will go to 1)
   MOV WMCON, #00001010B      ; Reset Control Register to normal operation (Memory Write Disabled)
   RET

Reading from the EEPROM is a much simpler operation. We simply move the address to the data pointer, read from the EEPROM using the MOVX command, and transfer the byte that was read to the DATAIN variable.

READDATAIN:
;
   MOV DPH, #00H                   ; High byte of address
   MOV DPL, ADDRESS                ; Set low byte of EEPROM address
   MOVX A, @DPTR                   ; Read Data from EEPROM at Address DPH:DPL
   MOV DATAIN, A                   ; Move Data Value to DataIn
   RET

The complete, unabridged sample program is eeprom.asm.
It starts by writing a ramp function to the first 256 locations of the EEPROM. (The values go up by one in each progressive memory location.) Then it goes into an endless loop where each value is read from each memory location and output to Port 0. You can observe the data on Port 0 by hooking up LEDs to the 8 port pins.

The program can be compiled using the TASM assembler.

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