What is the behavior of the IO when the device is unpowered?

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What is the behavior of the IO when the device is unpowered?

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leehonaker
Contributor I

I have a board with an MK60FX512VLQ15 on it with IO connected as control inputs to an LTM4609 DC/DC Regulator. When the uC is not powered, the IO appear to be logic '0' to the regulator. Unfortunately this regulator supplies power to the uC and the active low state is preventing it from turning on.

Does anyone know if the IO of an unpowered device are supposed to behave this way?

Thanks,

Lee

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Jorge_Gonzalez
NXP Employee
NXP Employee

Hello Lee Honaker:

Not sure I understand your setup.

- Does the MK60 control de regulator to turn off its own power but then you want the regulator to enable on its own?

- Can you share a schematic of your connections so we understand better?

When the MCU is not powered it does not drive the pins in either high or low level, so the behavior is unpredictable, you cannot rely on the pin to use it as a control signal. Maybe what you need is some kind of gate or a double pole switch to turn on the board and once the MCU started you can control the regulator with a pin.

Regards!

Jorge Gonzalez

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Jorge_Gonzalez
NXP Employee
NXP Employee

Hello Lee Honaker:

Not sure I understand your setup.

- Does the MK60 control de regulator to turn off its own power but then you want the regulator to enable on its own?

- Can you share a schematic of your connections so we understand better?

When the MCU is not powered it does not drive the pins in either high or low level, so the behavior is unpredictable, you cannot rely on the pin to use it as a control signal. Maybe what you need is some kind of gate or a double pole switch to turn on the board and once the MCU started you can control the regulator with a pin.

Regards!

Jorge Gonzalez

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leehonaker
Contributor I

Thanks Jorge. I agree, you can't predict the behavior of the IO when the MCU is unpowered. I recommended placing a pulldown on the control signal which would enable the regulator (active low enable) until the MCU was powered and configured. Sorry I didn't get back to you sooner, but I took an extended Christmas break!

God Bless,

Lee

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