ATMEGA328P PACKAGE MARKINGS
Atmel may from time to time ship product labeled ATMEGA328P-20PU in
place of ATMEGA328P-PU. ATMEGA328P-PU and ATMEGA328P-20PU are the
same device; the -20 in the manufacturer's part number designates the
Mouser will stock both ATMEGA328P-PU and ATMEGA328P-20PU under the
Mouser Stock Number 556-ATMEGA328P-20PU.