Hi, We are using AT91SAM9G25 MPU and its hangs now and then. We use a 2.8V super capacitor to power VDDBU. Just resetting the processor is not helping at all.
Hanging issue can be recreated by power-cycling the board many times in a second. Once the CPU enters into a hang state, NRST is held low and SHDN pin will have the VDDBU rail voltage, here 2.8V.
In other forums I have seen that another single power-cycling makes the CPU come out of the hang condition. But in this case it was not happening, it took long time, say 1 hr to get out of hang condition. So we suspected the VDDBU rail which still has power from the Super-Cap.
We discharged the Supercap while the board was off with a 5ohm resister for 2 sec, during which period the voltage on VDDBU went below 1V level and came back to 2.8 V after removing the discharge path. This time the cpu came out of hang condition the moment VDDBU was cycled. we were able to recreate the case many times. Every time only when we bring the voltage on VDDBU low, along with all other power rails off, the CPU came out of hang condition.
In practical scenario, in our case, VDDBU is supposed to be there virtually all the time (to save RTC data). Is there any explanation for this behavior of CPU? How can I prevent this hang conditions? ( the board has enough capacitors and transorbs). Is it a bug in POR section of ATMEL devices? I have seen in forums, many people reporting same hang cases with different Atmel devices.