Problem with AVR Reset

[NeeKoo]

Hello everyone

There is a unreliable (reset) situation here with my project using a XMega device :

I have a system with main process and control unit (MPCU) which controls 20 smaller units. MPCU includes one ATxmega128A1 micro and after reset it switches power supply for other 20 units and then control them . In each unit also there is an ATxmega128A1 micro too so there are 21 micro working in this system all together.

* Problem 1 : There is an USB to serial converter which connected to the MPCU. This converter powered by PC USB port and it's serial output connected to one of the Xmega UART pins at MPCU. When I connect this converter to MPCU and MPCU power is off there is voltage about 1.5V on XMega VCC (I think this is because of protection diodes in Xmega inputs which makes this voltage appear on VCC). In this situation when I power MPCU some times Xmega won't start at all (Something like hang at power-up) but if I power MPCU first and connect USB to serial converter next then everything is fine. I think there is a problem with clock but I am not sure and need your ideas. I am using external crystal oscillator at 16MHz and then using PLL to provide XMega 32 MHz working clock.

* Problem 2 : This problem I think similar to the Problem 1. As I said before MPCU provides DC supply of other 20 units by controlling 20 MOSFET switches. I mean power source of each unit which is a +12V DC switched by a MOSFET by supervising of the MPCU. There is almost 50-50 chance to fail in reset of units when MPCU try to OFF and then ON the unit power switch. I investigate unit circuits and realized that when unit's power switch is off there is a voltage about 1.2V on unit's Xmega VCC because there is some control signals from MPCU which connected to other units (Like Problem 1).



So.... I have a serious situation here.

Thank you for helping.

 

[rjenkinsgb]

You should never design any circuit so you have a "high" signal going in to a powered-off device, unless it's using some form of interface specifically intended to handle that situation, like RS232, CAN or RS485

Use open-collector (active low) for basic logic signals, so the only "high" level is generated in the target device.
Just use schottky diodes to emulate open-collector, with pullup resistors on the target inputs.

For the serial input, just connect a fairly high value resistor between the USB adapter and MCU pin. That should have no effect on the data but limit the possible current feed.

(That's also a kludge way of limiting interaction between the powered and unpowered boards).

 

[NeeKoo]

You should never design any circuit so you have a "high" signal going in to a powered-off device, unless it's using some form of interface specifically intended to handle that situation, like RS232, CAN or RS485

Use open-collector (active low) for basic logic signals, so the only "high" level is generated in the target device.
Just use schottky diodes to emulate open-collector, with pullup resistors on the target inputs.

For the serial input, just connect a fairly high value resistor between the USB adapter and MCU pin. That should have no effect on the data but limit the possible current feed.

(That's also a kludge way of limiting interaction between the powered and unpowered boards).

Thank you for replying .
I will try serial resistor for USB Converter and see what is happening next.
Of course all signals going to powered-off modules are RS-422 and RS-485 and there is one single signal which is open collector But still the problem exist in AVR reset.