What is the resistance value of the internal weak pull-up?

The internal weak pull-up is not really a resistor. It is usually a current source implemented using PMOS. Since it is PMOS, the current source can vary depending on the production-related parameter. You can check references from the web on how they implement pull-ups in CMOS.

You can calculate the effective resistance using the pull-up current specified in the datasheet. Weak pull-up current is documented on parameter D350 in the device datasheets.

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