;*******************************************************************************
;* Subroutine: WriteEE (with compare) and WriteEE_CONFIG (no compare)
;* Programmer: Tony Papadimitriou <tonyp@acm.org>
;* Purpose   : Self-writing EEPROM code using stack RAM temporarily
;* Language  : Motorola/Freescale/NXP 68HC11 Assembly Language (aspisys.com/ASM11)
;* Status    : FREEWARE Copyright (c) 2016 by Tony Papadimitriou <tonyp@acm.org>
;* History   : 02.04.19 v1.00 Original
;*           : 02.04.21       Protect X from being destroyed
;*           : 02.04.22       Shortened RAM code to absolute minimum
;*           : 02.04.23       Changed RAM code for 811E2 compatibility
;*           : 03.05.23 v1.01 Made smarter: A single call erases and writes
;*           :                depending on new and current values
;*           : 11.01.31       Changed some exits to :AnRTS
;*******************************************************************************

#ifmain ;-----------------------------------------------------------------------
                    #ListOff
                    #Uses     mcu.inc
                    #ListOn

                    #ROM

Start               proc
                    lds       #STACKTOP
                    clr       BPROT               ;needed to allow writes to EEPROM

                    clrb                          ;(for simulator only)
                    clry                          ;         -//-

          ;Example calling sequences

                    ldx       #$FF00
                    lda       #$AA
                    bsr       WriteEE             ;use WriteEE_CONFIG for CONFIG register

                    bra       *

                    @vector   Vreset,Start        ;to run the demo code

                    end       :s19crc

                    #ROM
#endif ;------------------------------------------------------------------------

;*******************************************************************************
; Action: Subroutine that does the actual write/erase to EEPROM is first
;       : copied to RAM and then executed from there until the EEPROM
;       : write/erase cycle finishes.
; Input : X -> EEPROM location to write
;       : A =  value to write (if A=$FF, an erase will be performed)
;       : B = $02 to program, $16 to erase
; Output: All registers except CCR are preserved by parent procedure
; Notes : Uses 18 bytes of stack RAM

?RAM_Code           proc
                    stb       PPROG
                    sta       ,x                  ;for erase, any value will do
                    inc       PPROG               ;Turn on programming voltage

                    ldx       #DELAY@@            ;10 msec delay constant
                              #Cycles
Loop@@              dex
                    bne       Loop@@
DELAY@@             equ       10*BUS_KHZ/:cycles
                    clr       PPROG               ;Turn off programming voltage
                    rts

                    #size     ?RAM_Code           ;number of RAM bytes needed

;*******************************************************************************
; Action: Write to EEPROM if value is different from current value
;       : (not good for CONFIG but good for anything else) preventing
;       : unnecessary wearout of EEPROM.
; Input : X -> EEPROM location to write
;       : A =  value to write
; Output: All registers except CCR are preserved
; Notes : Call once with actual value, erase is automatic if required
;       : Interrupts must be disabled before calling, if so required.

WriteEE             proc
                    cmpa      ,x                  ;is it the same as written?
                    beq       :AnRTS              ;yes, no need to write again
;                   bra       WriteEE_CONFIG      ;no, so we need to write

;*******************************************************************************
; Action: Write to EEPROM without comparing current value
;       : This is good mostly for changing the CONFIG register,
;       : prefer WriteEE (above) for writing anything else.
; Input : X -> EEPROM location to write
;       : A =  value to write
; Output: All registers except CCR are preserved
; Notes : Call once with actual value, erase is automatic if required
;       : Interrupts must be disabled before calling, if so required.

WriteEE_CONFIG      proc
                    push                          ;save all registers

                    ldy       #?RAM_Code+::?RAM_Code-1 ;routine to stack
Loop@@              ldb       ,y                  ;create a RAM copy by
                    pshb                          ;  pushing each byte to the
                    dey                           ;  stack in reverse order
                    cmpy      #?RAM_Code          ;  so it turns out correctly
                    bhs       Loop@@              ;do for all bytes

                    tsy                           ;Y -> RAM copy of routine

                    ldb       #$16                ;to erase a byte in EEPROM

                    cmpa      #$FF                ;Are we erasing instead of writing?
                    beq       DoIt@@

                    pshx                          ;protect write address
                    jsr       ,y                  ;first erase (v1.01) when programming
                    pulx                          ;restore write address

                    ldb       #$02                ;to program a byte to EEPROM
DoIt@@              jsr       ,y                  ;call RAM routine

                    givex     #::?RAM_Code        ;de-allocate stacked routine

                    pull                          ;restore all registers
                    rts