We are working on a design containing a SAM4S2A working as a slave controller to a larger processor system.
On the board we have the erase pin of the SAM4S2A wired to a gpio of the main processor via a 0 ohms resistor.
When we leave the 0 ohms resistor on, and keep the IO pin on the main processor side high impedent, we were expecting the
SAM4S2A to start up properly, as the pull down in the SAM4S2A is pulling the erase pin low. Also, when we measure the level on the pin with the scope, we can confirm the pin is low.
However, we see that the SAM4S2A controller is not even starting up: we can erase flash, program it, verify it, program the NVM bits, confirmed that the GPNVM bit is set to 1, so all looks perfect, but the controller does not start up.
When we remove the 0 ohms resistor, such that the erase pin is connected to nothing, the SAM4S2A controller starts up normally.
We do not really need the connection to the erase pin, as we can set the GPNMV in software and then use the Samba boot loader.
But we would like to explain the observations.
So can anyone explain why connecting a high-impedant IO pin of another device to the erase pin is causing the controller not to start up?