Atmel ATmega16U4/ATmega32U4 technical doc excerpt

DC Characteristics

DC Current per I/O Pin ………………………………. max 40.0 mA
DC Current VCC and GND Pins …………………. max 200.0 mA

VOL Output Low Voltage,
IOL = 10mA, VCC = 5V, …………………………. max 0.7V
IOL = 5mA, VCC = 3V, …………………………. max 0.5V

Although each I/O port can sink more than the test conditions (20mA at VCC = 5V, 10mA at VCC = 3V)
under steady state conditions (non-transient), the following must be observed: ATmega16U4/ATmega32U4:

  1. The sum of all IOL, for ports A0-A7, G2, C4-C7 should not exceed 100 mA.
  2. The sum of all IOL, for ports C0-C3, G0-G1, D0-D7 should not exceed 100 mA.
  3. The sum of all IOL, for ports G3-G5, B0-B7, E0-E7 should not exceed 100 mA.
  4. The sum of all IOL, for ports F0-F7 should not exceed 100 mA.

If IOL exceeds the test condition, VOL may exceed the related specification.
Pins are not guaranteed to sink current greater than the listed test condition.

VOH Output High Voltage,
IOH = -10mA, VCC = 5V, …………………………. min 4.2V
IOH = -5mA, VCC = 3V, …………………………. min 2.3V

Although each I/O port can source more than the test conditions (20mA at VCC = 5V, 10mA at VCC = 3V)
under steady state conditions (non-transient), the following must be observed: ATmega16U4/ATmega32U4:

  1. The sum of all IOH, for ports A0-A7, G2, C4-C7 should not exceed 100 mA.
  2. The sum of all IOH, for ports C0-C3, G0-G1, D0-D7 should not exceed 100 mA.
  3. The sum of all IOH, for ports G3-G5, B0-B7, E0-E7 should not exceed 100 mA.
  4. The sum of all IOH, for ports F0-F7 should not exceed 100 mA.