miniOptiboot/HowTo at master · bitflipser/miniOptiboot · GitHub

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miniOptiboot is an assembler rewrite of Optiboot to match the minimum bootloader size of ATmega88/168 of 128 words

it is written for 'assembler2' and refers to the 'MCU definitions'.inc files and to Atmels 'macros.inc' for accessing SFR

beside the assumptions and tricks of Optiboot it needs some more tricks to fit

so miniOptiboot

- does not blink LED upon start

- does not respond a valid SW_MINOR, SW_MAJOR value to the appropriate STK_GET_PARAMETER command (see below)

to set up your configuration step through 1 to 5 then do an assembler run

if the build does not fit into 128 words then go to steps 6 and 7

6 - memsaving specials:

- allPARAM_0x14: Optiboot replies a standard 0x03 to all STK_GET_PARAMETER commands beside SW_MINOR, SW_MAJOR ("..enough to keep Avrdude happy"),
save 1 word by changing this dummy value to 0x14

- noSW_MINOR_MAJOR: it is Optiboot convention to place the version numbers SW_MINOR, SW_MAJOR into the very last FLASH location,
save 1 word by circumventing this convention

- allFUSE_LOCK_0x14: (mini)Optiboot does not provide 'read FUSE' and 'read/write LOCK bits' but replies 0x00 to all of these commands,
save 1 word by changing this dummy value to 0x14

- overrideSIGNATURE: (mini)Optiboot replies the preset SIGNATURE to the STK_READ_SIGN command,
you can save 3 words by sending a '0x14 0x14 0x14' SIGNATURE, but be careful with this, because it requires some 'adjustments' to avrdude!!!!!
a) run avdude in force mode '-u -F', be cautious to place the right MCU type ('-m...') !!!
or
b) modify your MCU's 'part' entry in 'avrdude.conf' with 'signature       = 0x14 0x14 0x14;'
or
c) create a new 'part' entry to your MCU by taking the original one and modifying 'signature       = 0x14 0x14 0x14;' and 'id              = "m???";


7 - check baud rates

you can save memspace by selecting a special baud rate (in the range provided by your computer)

- a baud rate that generates a 'Baud = 0x0000' value will save 2 word

- a baud rate that generates a 'doubleSpeed = 0' value will save 1 word