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i.MX8 Mini GPIO Behavior During Reset

Question asked by Grant Kilmer on May 11, 2020
Latest reply on Jun 15, 2020 by igorpadykov

Hi there,

 

I am working on a project using the i.MX8 Mini and we are running into an issue regarding the GPIO's behavior during reset. It seems as though the GPIO's are being actively driven high by the CPU before POR_B goes high and the CPU comes out of reset. This is a problem for our project because we do not want these GPIO's to trigger components down the line. I have attached a scope picture, SAI1_TXD7.png, and in this picture the blue is VDD_3V3, purple is POR_B and yellow is SAI1_TXD7. As you can see, the SAI1 line goes high before the CPU comes out of reset. We use this signal as a strap option for boot up so I know that the datasheet calls this signal out as an input on reset, see SAI1_TXD7_Datasheet.png. We have a 2K2 Ohm external pull down resistor on this pin so knowing this, it looks as though the CPU is actively driving this pin high during reset. I have attached another picture, GPIO7.png, that shows this behavior on GPIO1_IO07 and is one of the GPIO's that we really do not want to trigger during reset. Finally, I have attached yet another scope shot of GPIO1_IO05 in the picture GPIO5.png. This GPIO stays high for 1.3 seconds before it finally comes down and it seems the CPU finally takes over and makes it a low signal. Both GPIO7 and GPIO5 have 100K Ohm external pull down resistors on our board.

 

As I mentioned, this behavior is extremely undesired as the GPIO toggling is causing issues down the line on our board. Is this behavior expected on this chip? I could not find anything in the datasheet or reference manual that discussed in detail what to expect of the pins during reset before POR_B goes high.

 

Thank you and I'd be happy to answer any questions regarding our board to help with this problem,

Grant

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