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Effectively, that shutdown seems to be related to a watchdog timer. By analyzing Canon's watchdog functions, it seems that the only addressed used for interfacing with the watchdog is 0xC0410000.

The first line of code is equivalent to this :

Code Select Expand

*(0xC0410000) = 0;

So I thought this might disable the watchdog. In order to try this, I used chdkptp and used the EngDrvOut function :

=call_func_ptr(0xFF025C0C, 0xC0410000, 0, 0)

It seems that this solves the issue of the camera shutting down a few seconds after entering JTAG. However the camera remains non-responsive to physical buttons or USB communication 

Hi !

A few days ago, I started a topic on the CHDK forum, about DIGIC 5 JTAG on a Canon S100 : https://chdk.setepontos.com/index.php?topic=13408.0

So far, I'm able to connect to the ARM, but I can't do more than peeking registers values. The camera immediately freezes, and eventually turns off.

Does anyone here has some experience with JTAG on DIGIC chips ?