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	<title>Jkx@home &#187; jtag</title>
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	<description>Titanium Exposé</description>
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		<title>Howto recover wrong fuses settings on a AVR</title>
		<link>http://www.larsen-b.com/Article/260.html</link>
		<comments>http://www.larsen-b.com/Article/260.html#comments</comments>
		<pubDate>Wed, 30 Nov -0001 00:00:00 +0000</pubDate>
		<dc:creator>admin</dc:creator>
				<category><![CDATA[Electronics]]></category>
		<category><![CDATA[avr]]></category>
		<category><![CDATA[diy]]></category>
		<category><![CDATA[jtag]]></category>

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		<description><![CDATA[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 [...]]]></description>
			<content:encoded><![CDATA[<p>Yesterday, I ran into a issue with an <a class="reference" href="http://www.atmel.com/dyn/products/product_card.asp?part_id=2014">ATMega32</a>. 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 <a class="reference" href="http://palmavr.sourceforge.net/cgi-bin/fc.cgi">Avr Fuse Calculator</a>, but I messed my ATMega with the wrong clock source. I don&#8217;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.</p>
<p>After a little googling, I found there is 3 ways to fixe this kind of issue:</p>
<ul class="simple">
<li>Use a Jtag (as it doesn&#8217;t need a clocked micro)</li>
<li>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&#8217;t really easy to find. The only one I know right now is the <a class="reference" href="http://www.atmel.com/dyn/products/tools_card.asp?tool_id=2735">STK500</a></li>
<li>Use an external clock (This is what I used, and describe here)</li>
</ul>
<p>As, I don&#8217;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 (&#8221;rue_mohr&#8221;) 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.</p>
<p>Of course, you need a external clock at something like 1Mhz. Of course I don&#8217;t have this kind generator at home, but this can be easily done with another AVR micro this something like this:</p>
<pre class="literal-block">void main()
{
DDRA = 0xFF;
while (1)
{
PORTA = ~PINA;
nop();  // added some nop, to slow the clock a bit
nop();
}
}</pre>
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