How is a logic HI MK26FN2M0VMD18 input on an unpowerd MK26FN2M0VMD18 supplying 1.6V to 3.3V logic supply.

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How is a logic HI MK26FN2M0VMD18 input on an unpowerd MK26FN2M0VMD18 supplying 1.6V to 3.3V logic supply.

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markbrinkman
Contributor II

Active USB device port powers USB-to-UART bridge (CP2102). The logic +3.3V MK26FN2M0VMD18 supply is OFF. CP2102 UART TXD and RXD are connected to MK26FN2M0VMD18 PTE1 and PTE0 respectively. UART TXD logic HI output is partially (1.6V) powering +3.3V supply through MK26FN2M0VMD18 PTE1 (ADC1_SE5a). Data sheet states there are no ESD diodes to VDD on any MK26FN2M0VMD18 digital IO or analog input. Why is +3.3V logic supply 1.6V when it is OFF?

thanks

Mark

1 Reply

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john_suchyta
NXP Employee
NXP Employee

Hi Mark,

The higher speed Kinetis devices, like the 180MHz MK26FN2M0 use a different pad design from the lower speed devices to enable higher speed GPIO. The pad design has a different gate control circuit that allows the P-channel driver to conduct when the pad voltage is greater than VDD. The 1.6V you’re seeing is leakage through the PFET drivers from the 3.3V pad to the unpowered chip VDD.

The positive ESD diode on each pin goes to an internal floating ESD bus that only reacts to fast transient conditions, like human body model ESD.

Note that the maximum input voltage on any pin is VDD + 0.3V. This includes when VDD=0V. This prevents biasing of the internal floating ESD bus and allows the ESD protection circuit to work properly. When an external voltage is applied to a GPIO pin and VDD is unpowered, the effectiveness of the transient protection circuit is compromised since the floating ESD bus is biased and has less headroom to respond to fast transients (high dV/dt). The dV is reduced in this case since the starting voltage is higher than 0V.

One way that customers effectively interface powered GPIO to an unpowered chip is by using a high speed 3-state buffer (non-inverting buffer with high active output enable). One example is the 74AHC1G126. The OE input is enabled by a GPIO pin on the MK26 when it is powered.

Regards,

John Suchyta