How do you read internal temperature on the PIC18F?

The temperature diode is internally connected to the ADC. There is no pin access. ADC setting of CHS = 0b11100 simply selects the temperature diode as the ADC channel. The connection is internal. The idea is that the ADC is used to make the conversion on the internal diode as the input. The result will change over temperature.

Please read Application Note AN1333, which discusses about calibrating this temperature indicator. This note refers to PIC12 and PIC16, but can be extended to PIC18 as well.

© 2018 Microchip Technology, Inc.
Notice: ARM and Cortex are the registered trademarks of ARM Limited in the EU and other countries.
Information contained on this site regarding device applications and the like is provided only for your convenience and may be superseded by updates. It is your responsibility to ensure that your application meets with your specifications. MICROCHIP MAKES NO REPRESENTATIONS OR WARRANTIES OF ANY KIND WHETHER EXPRESS OR IMPLIED, WRITTEN OR ORAL, STATUTORY OR OTHERWISE, RELATED TO THE INFORMATION, INCLUDING BUT NOT LIMITED TO ITS CONDITION, QUALITY, PERFORMANCE, MERCHANTABILITY OR FITNESS FOR PURPOSE. Microchip disclaims all liability arising from this information and its use. Use of Microchip devices in life support and/or safety applications is entirely at the buyer's risk, and the buyer agrees to defend, indemnify and hold harmless Microchip from any and all damages, claims, suits, or expenses resulting from such use. No licenses are conveyed, implicitly or otherwise, under any Microchip intellectual property rights.