Hi,

(Copy Protection in Modern Microcontrollers).

See this link:
http://www.cl.cam.ac.uk/~sps32/mcu_lock.html

The latest paper on this subject:
http://www.cl.cam.ac.uk/techreports/UCAM-CL-TR-630.html

Semi-invasive attacks – A new approach to hardware security analysis
Sergei P. Skorobogatov
April 2005, 144 pages
http://www.cl.cam.ac.uk/techreports/UCAM-CL-TR-630.pdf

Best regards,

Luciano