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3. Keep the AVR core in Power-down sleep mode during periods of low V
CC
. This will pre-
vent the CPU from attempting to decode and execute instructions, effectively protecting
the SPMCSR Register and thus the Flash from unintentional writes.
24.7.11 Programming Time for Flash when Using SPM
The calibrated RC Oscillator is used to time Flash accesses. Table 24-5 shows the typical pro-
gramming time for Flash accesses from the CPU.
24.7.12 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), spmcsrval (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
spmcsrval, (1<<PGERS) | (1<<SPMEN)
call Do_spm
; re-enable the RWW section
ldi spmcsrval, (1<<RWWSRE) | (1<<SPMEN)
call 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 spmcsrval, (1<<SPMEN)
call 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
Table 24-5. SPM Programming Time
Symbol Min Programming Time Max Programming Time
Flash write (Page Erase, Page Write, and
write Lock bits by SPM)
3.7 ms 4.5 ms
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