Also see Does the SAMD21 have pullups on every pin?
The ATSAMD21 microcontrollers have integrated pull-up and pull-down resistors which you can enable or disable in software.
The nominal value of the pull-up and pull-down resistors is 40 kΩ but it can be between 20 kΩ and 60 kΩ.
Refer to the datasheet, section 37.9.1 for more details.