AVR bootloaders: reboot into bootloader

When dealing with firmware updates, you need to make the AVR reboot into bootloader. And there are lots of solution when you try to do that:

  • either you force a reboot using the ‘reset’ button, and try to launch the avrdude command just 42ms after hitting the switch, or
  • you can use the not always wired up and not always working properly DTR line pushed down, or
  • you can wire a pin to the reset button — which is the worst solution, as avr pins are in an undefined state at startup time) or
  • you can use the watchdog reset to reboot.

My preferred solution would be the DTR line pulled down, though it’s mostly an electronic solution and it does not work with newer atmega USB MCUs. So, let’s have fun hacking the bootloader.

As I hacked this on the stk500 bootloader from Peter Fleury, I’m going to show the modifications based on that bootloader, which is known to work on all AVR from the atmega168 to the 2560. But that kind of hack could be easily patched on optiboot, the newer smaller (less than 1k) bootloader.

In that latest version of the firmware, patched by Mark Sproul, he fixed an issue with the watchdog timer, which we can thank him for, as we won’t have to do write it. Indeed, when you set up a watchdog in your firmware for something like 1 second, that for a reason your code gets reboot, and then you have a 2 seconds timeout in bootloader mode, the watchdog will get triggered during boot time and won’t be reset unless you hard reset the AVR.

So his code in the _FIX_ISSUE_181_ preprocessor block fixes that:

uint8_t mcuStatusReg;
mcuStatusReg    =   MCUSR;

__asm__ __volatile__ ("cli");
__asm__ __volatile__ ("wdr");
MCUSR   =   0;
WDTCSR  =   0;
__asm__ __volatile__ ("sei");
// check if WDT generated the reset, if so, go straight to app
if (mcuStatusReg & _BV(WDRF))

This code stores the reboot status register in mcuStatusReg, clears the watchdog reset registers, and if a watchdog caused the reboot, jump straight to address 0x00 (which app_start() points to).

Though the only minor modification we need to do here is:

if (mcuStatusReg & _BV(WDRF))
    boot_timeout =  200000; // sixteen LED blinks
} else {
    boot_timeout =  50000; // four LED blinks

So there’s a really long timeout at boot time in order for the flashing to be done.

And voila \o/

Don’t forget to merge the bootloader with your firmware before uploading using your favorite flasher and now you can reboot into bootloader using the following code in your firmware:

#include <avr/wdt.h>

void reboot_into_bootloader(void) {