I am trying to write to the eeprom in a 16F876, but although using the
procedure detailed in the datasheet (DS30292C page 43) my routine will not
write. I am attempting to have a 10 bit number in two adjacent locations,
and have default values specified in my source code, which get written by
the programmer, and I can read ok. However any attempt at writing to these
locations fails in my code, and seems to set the corresponding locations to
all ones.

Source code below for the write routine, using Olin's macros

;*  Subroutine "a2d_write_default"
;*  Write a default value into the correct EEPROM location for
;*  the a2d channel and upper or lower limit selected.
;*
;* Inputs
;*  Reg1  - 0 for low limit, non-zero for high limit value
;*  Req3(high byte)/Reg2(low byte) - value to save
;* Reg4 - a2d channel address (becomes eeprom offset)
;*
;* Outputs
;*  Reg1  - 0 good save, non-zero save failed or invalid channel
;*  Req4 - corrupted



a2d_write
; start by ensuring a write is not in progress
  dbankif EECON1
a2d_write_check
  btfsc EECON1,WR
  goto a2d_write_check

; get channel address *4 into REG4
; This will overflow if warning above is not heeded

  bcf  status,c ; clear carry before rotate
  rlf  Reg4,F  ; move up by three bits (*2)
  rlf  Reg4,F  ; (*4 ready here for eeprom access)


  dbankif EEADR
  getf Reg4  ; mov offset into EEADR
  movwf EEADR
  movlw a2d0_high ; now allow for address to be adjusted
  testfz Reg1   ; check  to see which it is
  skip_z
  addlw 2   ; adjust address for low limit to be set
  addwf EEADR,F

  dbankif EEADRH
  clrf EEADRH  ; clear high address for safety.

  dbankif EEDATA
  getf Reg3  ; load high byte
  movwf EEDATA  ; into the data register

  dbankif EECON1
  clrf EECON1
  bsf  EECON1,WREN ; allow eeprom write

  intr_off   ; disallow interrupts through critical portion
  movlw 0x55  ; see data sheet for machinations performed here
  movwf EECON2
  movlw 0xAA
  movwf EECON2
  bsf  EECON1,WR
  intr_on    ; and allow interrupts again

a2d_wr_wait1
  btfsc EECON1,WR ; allow write to complete
  goto a2d_wr_wait1

  bcf  EECON1,WREN ; remove write enable to stop corruption

  dbankif EEADR
  incf EEADR,F  ; now write other half

  dbankif EEDATA
  getf Reg2
  movwf EEDATA

  dbankif EECON1
  clrf EECON1
  bsf  EECON1,WREN ; allow eeprom write

  dbankif EECON2
  intr_off   ; disallow interrupts through critical portion
  movlw 0x55  ; see data sheet for machinations performed here
  movwf EECON2
  movlw 0xAA
  movwf EECON2
  bsf  EECON1,WR
  intr_on    ; and allow interrupts again

a2d_wr_wait2
  btfsc EECON1,WR ; allow write to complete
  goto a2d_wr_wait2

  bcf  EECON1,WREN ; remove write enable to stop corruption

; Now we check that the write happened OK.
; first check that the second byte wrote

  clrf EECON1
  bsf  EECON1,RD ; start read operation

  dbankif EEDATA
  getf EEDATA  ; get the byte
  subwf Reg2,W  ; compare with what should have been written
  movwf Reg0  ; and save result as a flag

  dbankif EEADR
  decf EEADR,F  ; now do other byte


  dbankif EECON1
  clrf EECON1
  bsf  EECON1,RD ; start the read

  dbankif EEDATA
  getf EEDATA  ; get this byte
  subwf Reg3,W  ; compare with what should have been written
  iorwf Reg0,F  ; and OR it with previous result

a2d_wr_exit     ; and exit

--
http://www.piclist.com hint: PICList Posts must start with ONE topic:
[PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads