To add a bit more to what Anindo Ghosh said:
Atmel uses the following layout to their labels: PrefixDeviceOption-Suffix (there are no spaces so this is hard to read, with spaces it would look like: Prefix Device Option-Suffix)
The AT means it is an Atmel part, followed by the option (this may be missing in some devices.) Then a hyphen separates the speed and package type (and sometimes temperature range and processing.)
So for the part number you gave, ATMega168A-PU:
- AT means Atmel
- Mega168 is the part number
- A is the option, in this case smaller die and lower power usage
- PU is a DIP package
For more information take a look at Explanation of Atmel’s Part Number Code
So as a rule of thumb, the letter before the dash (-) means:
- P "Picopower" low-power parts
- A Newer Fabrication process, lower voltage, lower power, non-Atmel fabricated parts with possibly more noise on A/D inputs
- V Low-voltage, lower clock frequency parts before "A" came along
- and nothing is the default part
Also, note that the P and A are not mutually exclusive, meaning that ATMega168PA-PU is a valid part number.
ATMEGA168A-PU
you get the datasheet as the first result. This doesn't deserve to be a question on this site. \$\endgroup\$