Writing to EEPROM on PIC

Question

Are there any PIC microcontroller programmers here?

I'm learning some PIC microcontroller programming using a pickit2 and the 16F690 chip that came with it. I'm working through trying out the various facilities at the moment. I can sucessfully read a byte from the EEPROM in code if I set the EEPROM vaklue in MPLAB but I don't seem to be able to modify the value using the PIC itsself. Simply nothing happens and I don't read back the modified value, I always get the original which implies to me that the write isn't working?

This is my code for that section, am I missing something? I know I'm doing a lot of unnecessary bank switches, I added most of them to ensure that being on the wrong bank wasn't the issue.

        ; ------------------------------------------------------
        ; Now SET the EEPROM location ZERO to 0x08
        ; ------------------------------------------------------

        BANKSEL EEADR
        CLRF    EEADR           ; Set EE Address to zero

        BANKSEL EEDAT
        MOVLW   0x08            ; Store the value 0x08 in the EEPROM
        MOVWF   EEDAT

        BANKSEL EECON1
        BSF     EECON1, WREN    ; Enable writes to the EEPROM

        BANKSEL EECON2
        MOVLW   0x55            ; Do the thing we have to do so
        MOVWF   EECON2          ; that writes can work
        MOVLW   0xAA
        MOVWF   EECON2

        BANKSEL EECON1      
        BSF     EECON1, WR      ; And finally perform the write

WAIT
        BTFSC   EECON1, WR      ; Wait for write to finish
        GOTO    WAIT

        BANKSEL PORTC           ; Just to make sure we are on the right bank

Answer 1

On page 122 of the 16F690 datasheet, it details the proper way to write to EEPROM:

BANKSEL EEADR                   ;
MOVF    DATA_EE_ADDR, W;
MOVWF   EEADR          ;Data Memory Address to write
MOVF    DATA_EE_DATA, W;
MOVWF   EEDAT                   ;Data Memory Value to write
BANKSEL EECON1                  ;
BCF     EECON1, EEPGD ;Point to DATA memory
BSF     EECON1, WREN   ;Enable writes
BCF     INTCON, GIE             ;Disable INTs.
BTFSC   INTCON, GIE             ;SEE AN576
GOTO    $-2
; BEGIN REQUIRED SEQUENCE
MOVLW   55h            ;       
MOVWF   EECON2         ;Write 55h
MOVLW   AAh                     ;
MOVWF   EECON2                  ;Write AAh
BSF     EECON1, WR              ;Set WR bit to begin write
BSF     INTCON, GIE             ;Enable INTs.
SLEEP                  ;Wait for interrupt to signal write complete
; END REQUIRED SEQUENCE
BCF     EECON1, WREN   ;Disable writes
BANKSEL 0x00           ;Bank 0

I noticed that you are specifically missing this line:

 BCF     EECON1, EEPGD ;Point to DATA memory

If EEPGD is always set, then you'll try to write to program memory (aka overwrite the flash program memory) which should always fail unless you've gone out of your way to specifically enable that.

Aside from that, as far as I can tell from reading your code, everything else looks fine. It's okay that you're polling EECON1.WR instead of setting an interrupt. It will cost you more power than putting the device to sleep, but of course you should just worry about one thing at a time.