ATMEGA640 datasheet(318/436 Pages) MICROCHIP

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ATmega640/V-1280/V-1281/V-2560/V-2561/V [DATASHEET]

DS40002211A

Flash corruption can easily be avoided by following these design recommendations (one is sufficient):

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 VCC reset protection circuit can be used. If a reset occurs while a write operation is in prog-

ress, 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 VCC. This will prevent the CPU from

attempting to decode and execute instructions, effectively protecting the SPMCSR Register and thus the

Flash from unintentional writes.

29.6.12

Programming Time for Flash when Using SPM

The calibrated RC Oscillator is used to time Flash accesses. Table 29-6 shows the typical programming time for

Flash accesses from the CPU.

29.6.13

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:

r0, Y+

r1, Y+

ldi

spmcrval, (1<<SPMEN)

call Do_spm

adiw ZH:ZL, 2

sbiw loophi:looplo, 2

;use subi for PAGESIZEB<=256

brne Wrloop

Table 29-6.

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

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