Telit Wireless Solutions HE910-EU V2, Hardware User'S Manual

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The GPIO pads, when used as inputs, can be connected to a digital output of another device
and report its status, provided this device has interface levels compatible with the 1.8V
CMOS levels of the GPIO.
If the digital output of the device is connected with the GPIO input, the pad has interface
levels different from the 1.8V CMOS. It can be buffered with an open collector transistor with
a 4.7KΩ pull-up resistor to 1.8V.
NOTE:
In order to avoid a back powering effect it is recommended to avoid having any HIGH logic
level signal applied to the digital pins of the module when it is powered OFF or during an
ON/OFF transition.
The GPIO pads, when used as outputs, can drive 1.8V CMOS digital devices or compatible
hardware. When set as outputs, the pads have a push-pull output and therefore the pull-up
resistor may be omitted.