Configuration for Unused Pins/GPIOs on Kinetis

[#1]

I got a question related to best practices to configure a GPIO if the pin is not used. To make it short, the recommendation is to leave the GPIO floating on the PCB and leave the GPIO in its "Default" state as shown in the Signal Multiplexing table in the Reference Manual. The Default state is either “Disabled” or an analog function.

Some Kinetis devices have analog only pins (PGAx/ADCx) while most have GPIO pins with analog functions (PTx/ADCx) or digital GPIO pins

Unused pins, whether analog only or GPIO, should be left floating. Analog only pins do not have input buffers that will cause shoot-through currents when the input floats. GPIO pins with analog functions default to analog functions, which disables the digital input buffer – no shoot-through current.

The digital GPIO pins default to "Disabled", which disables the input buffers - no shoot-through currents with floating inputs.

Finally, unused pins shall not be tied to VDD or VSS. Hence, when designing your board and there are some unused pins, leave them floating on the PCB and then make sure that the software leaves the GPIO in its Default state in the MUX register. 

[automotive_user]

I do understand unused pins should be left floating to prevent high currents in case of activating the pins by mistake.

I do understand that floating pins may lead to unwanted currents by rapid switching depending on influence by capacitive coupling.

So do I understand right:

The default configuration of the pins is the most stable condition of the port regarding leakage currents, ESD events or other EMC immunity as well as migration doe to humidity?

What’s about the configuration input with internal pulldown? Isn’t that more stable out of EMC and migration viewpoint?

[john_suchyta]

The default or disabled configuration of the unused pins is best for EMC emissions purposes. It doesn’t help with leakage or ESD events. For leakage, the best decision is to not connect the unused pin to anything since this eliminates the leakage path. For ESD, keeping the unused pin unconnected to any trace, including a test pad, eliminates the risk of directing the charge through the pin. Leaving an unused pin unconnected and not connected to VDD or VSS also prevents latchup in the pad area due to biasing doped structures near the pad that form the parasitic PNPN device. (I do not understand migration due to humidity – is this the increase in leakage expected due to humidity?)

Note that the main reason to avoid floating pins is to prevent the input signal from toggling the input buffer when the levels float to mid-rail. This is the main EMC emissions concern since it causes unwanted shoot-through currents which cause noise on the power rails. This is solved on the Kinetis input gating by forcing the IBE signal to 0 in the disabled configuration.

The EMC immunity concern is eliminated when the pin is not connected to anything – the path for injecting currents or voltages into the rails or logic is not there.

Selecting the internal pulldown (or pullup) resistor does not help the unused input in any way since the input buffer is already disabled. The pull resistor does nothing beneficial or harmful as far as EMC immunity is concerned when the pin is not connected on the board.

Regards,

John Suchyta

[automotive_user]

doe to humidity means for example DIN EN 60068-2-78 - Constant damp heat for several weeks while the chip is powered. Here the potential at the pin of the package is relevant. I assume the potential is defined by the ESD-protection circuit at the input to VDD/2.

What's the meaning of IBE?

Could you please put the recommendation to the reference manual.

[john_suchyta]

Hi Thomas,

I do not know how this test applies to the unbiased pins. My experience is that unbiased input pins float high or low, probably affected by the capacitive charge on the pin or package. The pins do not have a predictable bias when they float.

IBE is input buffer enable. It is the signal that disables the digital input buffer when it is 0 (when MUX=0).

The unused pin recommendations are in most datasheets. I noticed that the KW41Z and KW36 datasheets don’t have these however. We can add these recommendations.

Regards,

John Suchyta

[svyatosshiryaev]

Unused pins should be left floating to prevent activating currents.Also,the default configuration of the pins should be them most stable conditions of the port regarding Leakage currents,and also some amount of reverse current.MYBKEXPERIENCE

[john_suchyta]

Yes, this is correct.

Thanks,

John