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Re: [avr-gcc-list] Lock bits


From: Graham Davies
Subject: Re: [avr-gcc-list] Lock bits
Date: Tue, 4 Jan 2005 15:26:23 -0500

"Larry Barello" wrote:

> More likely, however, is that you accidentally selected an external clock
> option which effectively kills the chip.

The OP said "[I] can also use JTAG".  Messing up the clock fuses does not
prevent the JTAG interface from working.  You can set another clock mode
with a JTAG interface and recover.  This is because it uses its own clock
and does not need the main MCU clock to run.  This is not documented, but
Atmel have confirmed that it is so.

However, the OP also talks explicitly about lock bits and not fuses.  He
does not say whether or not the fuses are messed up as well.  It should be
possible to erase the chip and clear the lock bits with either the SPI or
JTAG interfaces.  I have certainly done this, though not specifically with
an ATmega128.  He reports that this does not work.  This could be explained
by both the SPI and JTAG interfaces being disabled, in which case, yes,
parallel programming seems the only way out.

Graham.



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