The manufacturer website is where you usually want to go to find the documentation. The product datasheet will specify the peripherals and register set for the chip you're looking at, including a description of each register and the fields within them. Some companies have a different name for this document. E.g. ST usually define their registers in a Reference Manual.
To add, ST has a 'datasheet' and a 'reference manual' for each chip and chip family. The datasheet is usually under 200 pages and is focused more on electrical information and is focused on a few chips. The ST reference manuals are generally over 1000 pages and cover all of the software/register information for a family of parts.
At least in the UK, smart meters don't use radio-based time switching as they have their own clocks which are synchronized with their remote system. All commands to these smart meters (including the ability to remotely disconnect them) are cryptographically protected to ensure integrity and authenticity, and also have anti-replay protections.
Disclaimer: I've worked for a smart metering manufacturer.
Here's the datasheet for the ATMega328: http://ww1.microchip.com/downloads/en/DeviceDoc/Atmel-42735-...