AVR-Chat

Here is something I've learned by experience and verified with Atmel 
support. If you have an AVR MCU that has JTAG support, you can get 
into the device even if there is no MCU clock. You can't run and 
debug a program, but you can get in to repair mis-set clock selection 
fuses. Is that neat, or what?

Here's what happened. I was bringing up a new board with an 
ATmega169. I am convinced that programming without in-circuit debug 
is like tight-rope walking without a net, so I insist on bringing out 
the JTAG interface.  But, I can't use it until I get the JTAG enable 
fuse programmed, so I have to use in-system programming as well. So, 
I'm connected up to an STK500, I've read the fuses out, checked the 
JTAG enable fuse checkbox and hit write. This is when the weight of 
cables hanging off the desk pulls my very light board down and breaks 
the power connection. When I put it all together again, the in-system 
programming link is dead. I figured I was hosed, but in desperation I 
hooked up my JTAG-ICE clone and, lo and behold, it worked. The JTAG 
fuse got programmed, but the clock setting fuses got messed up too 
and the MCU was waiting for an external clock. I fixed this and 
everything is fine again. Atmel support confirms that the JTAG 
interface does *not* need an MCU clock for programming and uses its 
own. ISP, however, requires a correctly configured clock source.

In case anyone is interested, the JTAG-ICE clone is of my own rather 
unique design and you can see what it looks like here:
http://www.ecrostech.com/Other/BflyCarrier.jpg
I have been thinking about making both it and the Butterfly Carrier 
board shown in the picture into products but I can't get a reading on 
how well they might sell.  I would slightly undercut the other clones 
for the JTAG-ICE at, say, $39.95 and the Butterfly Carrier bare board 
would be $9.95.

Graham.