We are using the AT91SAM7X512 for a project. The hardware has been stable for many months; we are currently working on firmware updates. Recently, a few circuit boards stopped working with the same problem.

For some reason the NRST pin is held low on the microcontroller. When the JTAG programmer is connected it is not able to reprogram the device.

I broke the connection between NRST and the JTAG programmer and the JTAG reset signal was able to float high (as expected). Therefore, I am reasonably sure the microcontroller itself it holding the line low.

So far my guess is either there is a firmware bug that switches NRST to an output, or ESD during the connection of the JTAG programmer caused latchup of NRST.

I am not all that familiar with the SAM7 family, does anyone know of any known issues that could cause the problems we are seeing?