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Question

How much current can you source/sink from the ATmega328P pins?

Answer

The microcontroller pins have a specified absolute maximum current of 40mA. The pins are not guaranteed to sink or source more than the listed test condition which is 20mA for a Vcc of 5V and 10mA for a Vcc of 3V. The output voltage specifications and test conditions for V_OL and V_OH are

Parameter	Condition	Minimum	Typical	Maximum
VOL	IOL=20mA, VCC=5V			0.9V
	IOL=10mA, VCC=3V			0.6V
VOH	IOL=-20mA, VCC=5V	4.2V
	IOL=-10mA, VCC=3V	2.3V

The other limitations on the total current per port and chip are

Maximum I_OL

• I_OL should not exceed 40mA.
• The sum of I_OL for ports C0-C5, ADC7, AD6 should not exceeed 100mA.
• The sum of I_OL for ports B0-B5, D5-D7, XTAL1, XTAL2 should not exceeed 100mA.
• The sum of I_OL for ports D0-D4, /RESET should not exceeed 100mA.

Maximum I_OH

• The absolute maximum I_OH is 40mA.
• The sum of I_OH for ports C0-C5, D0-D4, ADC7, /RESET should not exceeed 150mA.
• The sum of I_OH for ports B0-B5, D5-D7, ADC6, XTAL1, XTAL2 should not exceeed 150mA.
• Max I_CC The absolute maximum I_CC is 200mA.

References

Atmel. (2009). 8-bit AVR Microcontroller with 4/8/16/32K Bytes In-System Programmable Flash., pages 313-314. (Retrieved March 14, 2009, from doc8025.pdf)