It's about the MEGA328P reset circuit. I try to get the Makeblock plotter V2.0 Me-Orion control board (12V external powersupply) working with Atmel Studio 7 through ISP. On the board is a Mega328P. Flashing the program, there is no problem. Setting de Debug Wire fuse, no problem. Donwloading a small led blinking program in debug mode after a few times makes the Mega328P inaccessible. On the reset pin of the atmel there's an oscillating signal of app. 1KHz (0V-3V) even if all wires are removed and only the powersupply is connected to the board. Pressing and releasing the reset button restarts this oscillation. I tried everything: pullup 10K/4K7, Capacitor to GND, rectifier to VCC, to no avail. The uC VCC is stable at 5V. I discovered that shorting the 12V powersupply capacitor (during poweroff) makes the reset circuit stable again and I can only once download a new program. I'v seen this problem on another different kind of board with an MEGA328PB.
What I today discoverd is this:
(1) keep the reset button pressed and poweron the board: my blinking led program runs!!! (reset signal monitored by scoop)
(2) release reset button: the blinking program still runs
(3) press the reset button: the blinking program stops
(4) release the reset button: reset pin oscillates and the blinking program does not start
It seems as if the reset circuit of the atmel likes to see a low-going edge to activate the internal reset circuit.
Can someone explain this point (1) to (4) behaviour?
I use the Atmel Dragon programmer/debugger.