Simple Assembly Code Example for a Boot Loader

;-the routine writes one page of data from RAM to Flash
; the first data location in RAM is pointed to by the Y pointer
; the first data location in Flash is pointed to by the Z-pointer
;-error handling is not included
;-the routine must be placed inside the boot space
; (at least the Do_spm sub routine). Only code inside NRWW section can
; be read during self-programming (page erase and page write).
;-registers used: r0, r1, temp1 (r16), temp2 (r17), looplo (r24),
; loophi (r25), spmcrval (r20)
; storing and restoring of registers is not included in the routine
; register usage can be optimized at the expense of code size
;-It is assumed that either the interrupt table is moved to the Boot
; loader section or that the interrupts are disabled.

.equ PAGESIZEB = PAGESIZE*2 ;PAGESIZEB is page size in BYTES, not words

.org SMALLBOOTSTART

Write_page:
 
      ; Page Erase
 
      ldi spmcrval, (1<<PGERS) | (1<<SPMEN)
 
      rcall Do_spm


 
      ; re-enable the RWW section
 
      ldi spmcrval, (1<<RWWSRE) | (1<<SPMEN)
 
      rcall Do_spm
 
 

      ; transfer data from RAM to Flash page buffer
 
      ldi looplo, low(PAGESIZEB)  ;init loop variable
 
      ldi loophi, high(PAGESIZEB) ;not required for PAGESIZEB<=256

Wrloop: 
 
      ld r0, Y+
 
      ld r1, Y+
 
      ldi spmcrval, (1<<SPMEN)
 
      rcall Do_spm
 
      adiw ZH:ZL, 2
 
      sbiw loophi:looplo, 2 ;use subi for PAGESIZEB<=256
 
      brne Wrloop

 

      ; execute Page Write
 
      subi ZL, low(PAGESIZEB) ;restore pointer
 
      sbci ZH, high(PAGESIZEB) ;not required for PAGESIZEB<=256
 
      ldi spmcrval, (1<<PGWRT) | (1<<SPMEN)
 
      rcall Do_spm

 

     ; re-enable the RWW section
 
     ldi spmcrval, (1<<RWWSRE) | (1<<SPMEN)
 
     rcall Do_spm

 

     ; read back and check, optional
 
     ldi looplo, low(PAGESIZEB) ;init loop variable
 
     ldi loophi, high(PAGESIZEB) ;not required for PAGESIZEB<=256
 
     subi YL, low(PAGESIZEB) ;restore pointer
 
     sbci YH, high(PAGESIZEB)

Rdloop: 
 
     lpm r0, Z+
 
     ld r1, Y+
 
     cpse r0, r1
 
     rjmp Error
 
     sbiw loophi:looplo, 1 ;use subi for PAGESIZEB<=256
 
     brne Rdloop

 
 
     ; return to RWW section
 
     ; verify that RWW section is safe to read

Return: 
 
     in temp1, SPMCR
 
     sbrs temp1, RWWSB ; If RWWSB is set, the RWW section is not ready yet
 
     ret; re-enable the RWW section
 
     ldi spmcrval, (1<<RWWSRE) | (1<<SPMEN)
 
     rcall Do_spm
 
     rjmp Return 



Do_spm:
 
     ; check for previous SPM complete

Wait_spm:
 
     in temp1, SPMCR
 
     sbrc temp1, SPMEN
 
     rjmp Wait_spm
 

     ; input: spmcrval determines SPM action
 
     ; disable interrupts if enabled, store status
 
     in temp2, SREG
 
     cli; check that no EEPROM write access is present

Wait_ee: 
 
     sbic EECR, EEWE
 
     rjmp Wait_ee 
 
     ; SPM timed sequence
 
     out SPMCR, spmcrval
 
     spm; restore SREG (to enable interrupts if originally enabled)
 
     out SREG, temp2
 
     ret