AVR GPIO as open collector/drain

It's not possible; there are full totem-pole drivers on all pins. Use an external bipolar transistor.

Write 0 to the appropriate PORTx register bit. Use the appropriate DDRx direction register bit as the output.

VLV

By the way... why do you need it? If it is for I2C then look at the two-wire interface module. That's an exception to the "totem pole on all pins" statement.

1. Thanks, Vladimir!

2.As I've read at avrfreaks, TWI module of AVR does not work properly in multimaster mode. Under some conditions on the bus it freezes and can be released only by TWI disable/enable

You are the first one I know who does care about the I2C multimaster mode. In my ~20 years of experience, I have never seen a system with more then one master on I2C. What is it?

Vladimir Vassilevsky

DSP and Mixed Signal Design Consultant

Well, to say the truth neither have I. But I was asked to consult 'pure' software developer about the possibility of implementation of bit-banging TWI protocol on their system, which had previously worked using TWI block. They use TWI for interboard communication. Necessary condition: 'no hardware changes'. Alex.

But is this MultiMaster ? - Doing Multi-Master (with the arbitration ) in SW is not going to be easy.... Tho if you have control on BOTH ends, you can probably cobble something together.

-jg

Forget the AVR and use a PIC !