This s/w writes reliably (AFAICT) to the 628's EEPROM.
Unfortunately, like Jennifer Lopez, I'm more than a little
behind and haven't time for post mortems on the other
write code variants, but I still have one or two things I'd
like to get clear in my own mind at some point

The deal with the EEPROM locations erasing was that
MPLAB writes FF to the whole block when you upload new
code to the PIC. Unticking the EEPROM Data box before
programming stopped that and so left previous contents
as they were. I should've spotted that, but I'll take the Homer
stance - "It's everybody's fault but mine" ;-)

Thanks for the input chaps

==========================================

;ee_628.asm
;
;7th February 2002
;
         list p=16F628
         #include <p16F628.inc>

led    equ 0x04       ;indicator - lights for short time during write
go     equ 0x05       ;push button, start write

start  equ 0x00       ;program start vector
ram    equ 0x20       ;start of RAM in bank0

       __config _intrc_osc_noclkout & _wdt_off & _pwrte_on & _lvp_off &
_boden_off

         org 0x00
         goto entry

entry    clrf   porta
            movlw  0x07     ;comparators off
            movwf  cmcon
            clrf   status        ;set rp0, rp1 = 0 = Bank0

         bsf    status,rp0
         bcf    status,rp1

         movlw  b'00000000'   ;oooo oooo
         movwf  trisa

         movlw  b'00100000'   ;ooio oooo
         movwf  trisb

         movlw  0x80
         movwf  option_reg

         bcf    status,rp0
         bcf    status,rp1

         clrf   porta
         clrf   portb

wait     btfsc  portb,go   ;look for PB press/release on b.5
            goto   wait
waitl    btfss  portb,go
            goto   waitl

STEE  bsf    PORTB,LED
            bsf    STATUS,RP0     ;
            bcf    STATUS,RP1     ;bank 1

      movlw  0x10
      movwf  EEADR
      movlw  0xd2
      movwf  EEDATA         ;load address and data

WRITE bcf    INTCON,GIE
             bsf    EECON1,0x2     ; eecon1,wren
             movlw  0x55
             movwf  EECON2         ; eecon2
             movlw  0xAA
             movwf  EECON2         ; eecon2
             bsf    EECON1,0x1     ; eecon1,wr

WREND btfsc  EECON1,0x1     ;wait for WR bit clear
                goto   WREND

      bcf    EECON1,0x2     ; eecon1,wren
      bcf    STATUS,RP0     ; bank 0

      bcf    PORTB,LED

ENDWR goto   ENDWR

  end

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