WRITECODE and BLOCKSIZE on 18F26J50

[Darrel Taylor]

Changing BLOCK_SIZE to 2 will not work.
In order to write 1 word at a time, the WPROG bit must be set in EECON1, and WRITECODE does not set that bit.

For example, using the following code it tries to write 100 BYTEs to Flash. At each word location it writes the address of that word. The results were read back with a U2 programmer.
The image on the left is with BLOCK_SIZE = 64. Only 64 out of the 100 BYTEs actually gets written. This is the normal operation of WRITECODE.

The image on the right is with BLOCK_SIZE = 2.
It was still writing 64 bytes at a time, but it was doing it for every 2 bytes.
The holding registers don't get cleared, and the data gets really messed up.
It ends up writing 128 bytes with BAD Data.

; Bad program -- do not use --
; 
FlashAddr VAR WORD
Idx       VAR WORD
Wvar      VAR WORD
;----[Get address of Flash Data Area]-------------------------------------------
;
ASM
    CHK?RP _FlashAddr
    movlw  low(_FlashData)
    movwf  _FlashAddr
    movlw  high(_FlashData)
    movwf  _FlashAddr + 1
ENDASM
;
;----[The Main Program]---------------------------------------------------------
FOR Idx = FlashAddr to FlashAddr+100 STEP 2
    Wvar = Idx
    WRITECODE Idx, Wvar
NEXT Idx
STOP
;
;----[This is where the Data will be saved in Flash]----------------------------
@ org $+1023 & 0FC00h ; align to 1KB boundary
FlashData:

As stated in the other thread, you can use the assembly language routine in the datasheet.
Or, here's an adaptation that allows you to write 1 WORD at a time.
You can see in this image that it writes the correct data.

Pause 100
FlashAddr VAR WORD
Idx       VAR WORD
Wvar      VAR WORD
; 
;----[Get address of Flash Data Area]-------------------------------------------
ASM
    CHK?RP _FlashAddr
    movlw  low(_FlashData)
    movwf  _FlashAddr
    movlw  high(_FlashData)
    movwf  _FlashAddr + 1
ENDASM
;
;----[The Main Program]---------------------------------------------------------
FOR Idx = FlashAddr to FlashAddr+100 STEP 2
    Wvar = Idx
    GOSUB WRITEWORD
NEXT Idx
;
STOP
;
;-------------------------------------------------------------------------------
WRITEWORD:
    TBLPTRU = 0             ; Load TBLPTR with the base address
    TBLPTRH = Idx.HighByte
    TBLPTRL = Idx.LowByte   ; The TBLPTR must be loaded with an even address
    TABLAT  = Wvar.LowByte  ; LSB of word to be written
@   TBLWT*+
    TABLAT  = Wvar.highByte ; MSB of word to be written
@   TBLWT*                  ; The last table write must not increment the table
  ASM
    ; PROGRAM_MEMORY
    BSF      EECON1, WPROG     ; enable single word write
    BSF      EECON1, WREN      ; enable write to memory
    MOVE?TT  INTCON,GIE, R0,0  ; save state of GIE
    BCF      INTCON,GIE        ; disable interrupts
    MOVLW    0x55
    MOVWF    EECON2            ; write 0x55
    MOVLW    0xAA
    MOVWF    EECON2            ; write 0xAA
    BSF      EECON1, WR        ; start program (CPU stall)
    MOVE?TT  R0,0, INTCON,GIE  ; restore GIE state
    BCF      EECON1, WPROG     ; disable single word write
    BCF      EECON1, WREN      ; disable write to memory
  ENDASM
RETURN
;
;----[This is where the Data will be saved in Flash]----------------------------
@ org $+1023 & 0FC00h ; align to 1KB boundary
FlashData:

Keep in mind that you still have to erase 1kB at a time.
That's why you want to "Align" the data with a 1k boundary.