reading on-chip temperature sensor

It would be most helpful if the standard AnalogRead() function could, in addition to the input pins, read the onboard temperature sensor which is a feature of pretty much every Arduino microcontroller. It's on the MUX anyways.

Start by tracing the AnalogRead() entry point and how Arduino boards expose analog MUX channels. Compare the supported microcontroller variants and their on-chip temperature-sensor details. Done would require a defined cross-board behavior and corresponding implementation and verification, but the issue does not name files or tests.

Written by the indexing model from the issue text.