Can I connect a pullup resistor and 5-volt source to open drain output on unpowered STM32L476?

[pgregson1]

I am designing a product for a client based on the STM32L476 processor. My product requires separate processor and control power domains with a shared ground. The processor domain is 3.3 volts, the control domain is 5 volts. There are times when the processor will be turned off (Vcc = 0 volts) when the control domain still has 5 volts on it. I want to use an open-drain 5-volt tolerant output pin with a pullup resistor to the 5-volt control-domain supply to drive a signal to the control domain. My concern is whether there will be any current flowing through the ESD protection diodes and circuits in the STM32L476 diodes when that chip is unpowered while the pullup resistor has 5 volts on it.

Can anybody help? I have seen nothing about this in the data sheet.

Best regards,

Peter

[berendi]

> I have seen nothing about this in the data sheet.

It is right there under absolute maximum ratings.

so the answer is no, it could damage the MCU.

[pgregson1]

Hello berendi,

Thank you for your answer. I was aware of the absolute max values.

The Absolute Maximum values shown in Table 19 state that these are the limits for input voltage on FT_*** pins. I am not concerned with input pins. I am concerned with the maximum voltage that I can apply to the pull-up resistor for an open-drain 5-volt tolerant output pin and therefore the values in the table may not apply.

Further, I asked the question because I thought that operating at the Absolute Maximum values could cause harm to the processor. I was expecting to see a line entry in the electrical specifications that gave a maximum working value of the voltage that could be tolerated and another line item that gave current limits for the protection circuits. without detailed knowledge of the protection circuits, as would be supplied by an ESD-protection diagram, it is difficult to determine usable voltage and current limits.

Regards,

Peter

[berendi]

Where does it say that this applies to input pins only?

It applies to FT_* pins regardless of how they will be (or were the last time) configured by the software. There is even a footnote saying

2. VIN maximum must always be respected.

For me always means always, even when the part is powering down or up, and Vdd=1.5V or 0.4V or 0V.

> My concern is whether there will be any current flowing through the ESD protection diodes and circuits in the STM32L476 diodes when that chip is unpowered while the pullup resistor has 5 volts on it.

Yes. Try the following experiment:

Write a short program that does nothing else but configures a serial port and outputs something in a loop. Connect it to your PC with a 3.3V USB/Serial adapter and start a terminal program. Do not power up the MCU. It will still wake up periodically and output a few characters, because it charges up the capacitors on Vdd through the protection diode of the RX pin, which is at 3.3V when idle. I'd not repeat the experiment with a 5V USB/Serial cable.

[pgregson1]

The table refers to Vin. In every other context in the data sheet, Vin is the input voltage on an input. In the latest data sheet for the STM32L476RG (revision 8), compare the third entry in the column under the label “Symbol�? with the the 10th entry in the column labeled “Symbol�? in table 23.
More importantly, you did not address my concern with operating at voltages nearing the absolute maximum rating and the long-term implications of doing so on the reliability of the processor.
The confusion caused by these two issues warrants the making of improvements to the data sheet. Since I am designing a product that must be reliable, I asked the question.
Regards,
Peter