The PI/T has three 8-bit ports in total, two of which (ports A and B) are general purpose I/O. The third port (port C) can be configured to present some of the Timer/Counter internals to the outside world.
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On the sbc-3, two pins of port C (0 and 1) are used to implement a bit-banged I2C bus to talk to the on-board real time clock, a Dallas Semiconductors ds1307.
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On the sbc-3, two pins of port C (0 and 1) are used to implement a bit-banged $I^2C$ bus to talk to the on-board real time clock, a Dallas Semiconductors ds1307.
### Links
- [Good Example](https://people.engr.tamu.edu/rgutier/web_courses/ceg453_s00/l2.pdf) of how to program the device and handle the interrupts in C.