ChibiOS Free Embedded RTOS

Are you constrained to using the DS1307? Something like the DS1340 is intended for 3V3 operation, and will handle fast mode (although I don't think its got the extra battery backed RAM). I've been using it successfully in non-ST systems for some years, and have certainly had it working with the 32F051. I've got a little PCB with a DS1340 and a FRAM on it that I use for I2C testing generally.
(Software-wise, the DS1307 and DS1340 look pretty similar)