Why does the Uno have two microcontrollers?

Using Arduino IDE 1.x
1

The Arduino Uno schematic (http://arduino.cc/en/uploads/Main/arduino-uno-schematic.pdf) shows a chip (U1) labeled ATMEGA8U2-MU, and another (ZIC1) labeled ATMEGA8. I see these on the board: ATMEGA8U2-MU in the TQFP32 package and ATMEGA8 in the PDIP28 package.

Could someone please tell me why the Uno employs two microcontrollers? How are tasks divided between the two?

2

The ATMEGA8U2-MU does the USB to serial bit and replaces the previous dedicated USB chip. The other is the arduino processor that you program.

3

Thank you Grumpy. Thank you very much for satisfying my curiosity.