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I was reading 17xx series` data sheet, and I had trouble understanding a specific explanation given for a major number of pins:

5 V tolerant pad providing digital I/O with TTL levels and hysteresis and analog output function. When configured as the DAC output, digital section of the pad is disabled. This pin is pulled up to a voltage level of 2.3 V to 2.6 V."

Now I understand what TTL and hysteresis are, so no need to explain them for me. I`m not certain what the last sentence means. Does it mean when those pins are configured with pull-up resistors the voltage level changes to "2.3 V to 2.6 V"?

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    \$\begingroup\$ Where's the datasheet with this explanation? \$\endgroup\$ – Matt Young Sep 8 '13 at 15:04
  • \$\begingroup\$ [here it is] (google.com/…) \$\endgroup\$ – kasra5004 Sep 8 '13 at 15:27
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If the 17xx parts are like the 13xx and 11xx devices, it means that there is an internal pullup on the general purpose I/O pins and that the default configuration at reset is to enable these pullups. What surprises many of us is that the pullup devices do not pull the pins up to the supply voltage of about 3.3V, instead they only pull the pins up to around 2.5V. The data sheet is warning you that if you really need the pins pulled all the way up to 3.3V then you will have to add an external resistor pullup.

Also, you should be able to disable the pullup or chose a pulldown instead by modifying the pin's IO configuration register.

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