
262
8155C–AVR–02/11
ATmega32A
1.
If there is no need for a Boot Loader update in the system, program the Boot Loader
Lock bits to prevent any Boot Loader software updates.
2.
Keep the AVR RESET active (low) during periods of insufficient power supply voltage.
This can be done by enabling the internal Brown-out Detector (BOD) if the operating
voltage matches the detection level. If not, an external low V
CC Reset Protection circuit
can be used. If a reset occurs while a write operation is in progress, the write operation
will be completed provided that the power supply voltage is sufficient.
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 SPMCR Register and thus the Flash from unintentional writes.
25.8.11
Programming Time for Flash when using SPM
The Calibrated RC Oscillator is used to time Flash accesses. Table 25-5 shows the typical pro-
gramming time for Flash accesses from the CPU.
25.8.12
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), 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)
call Do_spm
; re-enable the RWW section
ldi
spmcrval, (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
spmcrval, (1<<SPMEN)
call Do_spm
adiw ZH:ZL, 2
Table 25-5.
SPM Programming Time.
Symbol
Min Programming Time
Max Programming Time
Flash write (Page Erase, Page Write,
and write Lock bits by SPM)
3.7ms
4.5ms