276
8271D–AVR–05/11
ATmega48A/PA/88A/PA/168A/PA/328/P
26.2.5 Simple Assembly Code Example for a Boot Loader
Note that the RWWSB bit will always be read as zero in ATmega 48A/48PA. Nevertheless, it is
recommended to check this bit as shown in the code example, to ensure compatibility with
devices supporting Read-While-Write.
;-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<<SELFPRGEN)
rcallDo_spm
; re-enable the RWW section
ldi spmcrval, (1<<RWWSRE) | (1<<SELFPRGEN)
rcallDo_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<<SELFPRGEN)
rcallDo_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<<SELFPRGEN)
rcallDo_spm
; re-enable the RWW section
ldi spmcrval, (1<<RWWSRE) | (1<<SELFPRGEN)
rcallDo_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
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