Current Injection into IMX7 nPad

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Current Injection into IMX7 nPad

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Contributor I

I have a mixed system of +5V and +3.3V logic. I have pins multiplexed between the logic systems. IMX7 @ +3.3V and Atmega32U @ +5V. I protect the IMX7  pad with a series 1K resistor and low Vf diode to +3.3V rail.

There is also a series 1K resistor on the Atmega signal to the same diode.

IMX7 pad - 1K - Diode - 1K - Atmega pad

If an ERROR occurs and both the Atmega and IMX7 are outputs, a current of 200-400uA  will be injected into IMX7 pad.

I have not seen any specification for injected current in the IMX7 documentation.

Does this condition violate any IMX7 electrical specifications?

Your feedback is greatly appreciated.

David

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NXP TechSupport
NXP TechSupport

Hi David

unfortunately injected current value is not specified in datasheet, however

value below 1mA may be considered as safe.

Best regards
igor
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Contributor I

Hi Igor,

Does this injected current value of 1mA apply when in both cases where the pad is either input or output?

This value may or may not change relative to the fabrication of the integrated circuit.

Your response is greatly appreciated.

David

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NXP TechSupport
NXP TechSupport

Hi David

could you clarify how current can be injected to pad configured as output?

Best regards
igor

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Contributor I

Here is an article on the current injection relative to my issue.

https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=10&ved=2ahUKEwixgrHqnYjjAhXZW80KHYawBD... 

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NXP TechSupport
NXP TechSupport

one can increase resistor to 2K, so current will not exceed 1mA,

formally design should respect Table 9. Operating ranges i.MX7D Datasheet

https://www.nxp.com/files-static/32bit/doc/data_sheet/IMX7DCEC.pdf 

Best regards
igor

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Contributor I

Hi Igor,

For example, an error occurs in a system, where two connected pads are configured as outputs.  Normally outputs are coupled to inputs unless they are open drain or open collector. Both pads are driving high with one at +5V and the other at +3.3V. Even if there are external protection diodes there will exist a potential to the lower voltage pads ,lets assume 0.4V, forward diode voltage,  with a series limiting resistor which gives 400uA. The +3.3V output pad, a source, is being driven by the +5V output pad, another source,  and in this condition current can be injected into the internal +3.3V rail through its source path rather than pushing the current through the protection diode. Internal protection diodes usually have ultra low leakage current in the nano amp range. However injecting any current into a source path when in the high state can cause issues with the chip.

I have a complicated design where several pads are connected to each other in a mixed voltage system. Protection diodes and series limiting resisters are in circuit. I can physically take any pad out of the circuit by eliminating the series resistors however I am looking into the design where I can leave all pads connected if there are no electrical violations.

If this is not the case please explain.

I greatly appreciate your assistance.

David

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