Howto recover wrong fuses settings on a AVR

Yesterday, I ran into a issue with an ATMega32. In fact, I wanted to disable the Jtag support on the chips because I need to use the pins used by the Jtag (PC2-PC5). To disable the Jtag, you simply need to change some fuses. I changed the fuses with the help of the Avr Fuse Calculator, but I messed my ATMega with the wrong clock source. I don’t know how, but I choose the low freq clock. So my ATMega was totally frozen, as the clock settings was wrong, I was unable to reflash it, and worst, unable to change the fuses.

After a little googling, I found there is 3 ways to fixe this kind of issue:

  • Use a Jtag (as it doesn’t need a clocked micro)
  • Use a high voltage programmer (you can erase the fuse, with a programmer that do a parallel programming with RESET plug to a 12v PSU). This kind of programmer aren’t really easy to find. The only one I know right now is the STK500
  • Use an external clock (This is what I used, and describe here)

As, I don’t have any Jtag, or STK500, the only way to fix that is to use an external clock. In fact, I decided to ask on the #avr channel on freenode. And a really kool guy (”rue_mohr”) give me this tips. On every AVR micro, you can use a external clock if you messed the fuses settings. Simply plug a external clock on the clock pin (XTAL1 on the Mega32), and changes the fuses by this way.

Of course, you need a external clock at something like 1Mhz. Of course I don’t have this kind generator at home, but this can be easily done with another AVR micro this something like this:

void main()
{
DDRA = 0xFF;
while (1)
{
PORTA = ~PINA;
nop();  // added some nop, to slow the clock a bit
nop();
}
}

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admin April 15th, 2007


9 Responses to “Howto recover wrong fuses settings on a AVR”

  1. # madeon 26 Jul 2007 at 5:05 am

    How about my problem?
    i have the same problem but i use atmega8535, assembly.
    thank’s.

  2. # Jkxon 09 Jan 2008 at 7:40 pm

    Moses just me this link on his weblog, but due to baysien filters, the url have been filtered. This post fix that issue.

    Evil Mad Scientist

    Thanks Mose

  3. # kalandaon 01 Feb 2008 at 12:44 am

    This really works, Thanks. I tried with an attiny13 and this code to simulate 1Mhz clock. Only i needs to set the ISP Freq. on the programmer to lowest rate (51.1hz) that is more slower but works.

    // Library includes
#include 
#include 
int main()
{

  DDRB = 0xFF;
  while (1)
  {

   PORTB = ~PINB;
   asm volatile("nop");
   asm volatile("nop");
  // _delay_ms(1000); // Uncomment this line to probe if something is live :)
  }
return 0;
}

  4. # nmcon 22 May 2008 at 4:24 am

    Thanks a lot for the tip. It really helped!

  5. # Santiagoon 09 Jul 2008 at 2:55 am

    i’d like to know if I need only set the external clock and the default values back, or I need make anything else

  6. # adminon 09 Jul 2008 at 9:14 am

    Santiago, you need to set the right fuse (clock, reset flag etc ..)

  7. # Danielon 23 Aug 2008 at 3:26 am

    I was wondering.. can I use a similar method if I can only get a 32hz instead of 1mhz input? I have 3 atmega168 AVR that are unusable due to some fuse setting.. We switched tabs between fuse writes & now we can’t program/erase the chips any more.

  8. # Danielon 23 Aug 2008 at 3:27 am

    I meant 32khz not 32hz..

  9. # adminon 23 Aug 2008 at 1:11 pm

    I don’t know .. You should give it a try.

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