Bootloader questions, basic to advanced...

AdrianJ · Joined: 16 Jan 2006 Posts: 2483 Location: Queensland

A 'Bootloader' is a piece of code you write ( or copy ) into a special area of Flash called the boot block within the main Flash of an AVR - usually at the top of Flash. Various options ( fuses ) exist for setting the size. The Bootloader can contain any means you like for talking to the outside world, UART, TWI, SPI, etc. And all it has to do is *reliably* read the data in that you send, and write it into normal Flash program area. That then becomes the ( new ) main program.

Several options exist for updating a program to a new one in the field.

1> You can set a port pin, eg with a dipswitch or link, to a high ( or low ) state, detect this from within the main program, and jump into the boot loader, then wait there for the new program to be sent. That relies on having the main program running, and the user doing 'something' to fall into the bootloader.

2> You can set another fuse ( in the factory ) to force the chip to go first to the boot code after reset. Then in your boot code, you wait for some special byte sequence in your input line, and if its sent, accept the rest of the bytes as new program. If its not sent, you jump into a normal reset for your application. That way you dont need any switches, and even if the main program fails, you can still upgrade or reload it.

If you have multiple AVRS in the device, you presumably already have some sort of network so they all talk to each other. So then you can use one as a master, receive a new program from the outside, and pass it to the correct slave, with some way of invoking the bootloader in that slave.