12C fails on first attempt, then works fine...

[brid0030]

birg0030, Do you have the requisite resistor pull-ups on the SCL and SDA lines?

I have 22K pull-ups in place. I know that value is a bit high, but slowing down the transmission rate does not help.

[Scampy]

I'm no expert, and don't understand the formula outlined in the datasheet http://www.ti.com/lit/an/slva689/slva689.pdf but it seems to suggest that lower value resistors are required. The example given for fast transmission at 3.3v is typically 1K, or around 5K (4.7K are traditionally the norm) for 5v operation

[brid0030]

I don't think it is the resistors. I've used 100k before and it's been OK. I also tried different values in parallel with the 22k resistors to bring the pullup values down. Issue remains.

I've done some more tweaking and I don't seem to have the same problem with two other I2C devices on the same bus. So the issue might be something particular to the RTC.