Electronic – Two wire vs. three wire serial interface for RTC

In my [in]experience, 99% of serial communication errors are due to timing mismatches; the other* 99% is due to wiring hoopla.

• It works for short wire but not a 30ft CAT-5 cable
• It works for normal serial wire upto 10ft but not more than that.

CAT-5 cable is twisted pair (also, all of what I have used is also shielded, but apparently this is not common); I assume the "normal serial wire" is two loose wires. As wiring gets longer, your nice digital rise times stretch out, effectively shrinking the available window for sampling incoming data. Twisted pair helps this a bit. What this really means is that your UART clock must be prescaled more accurately to the baud rate being used. System clocks for perfect USART communications should be multiples of 1.8432MHz. Short-range communications can tolerate a few percentage error, but as your sampling window shrinks it must be more accurate. What your prescaler (int), and baud error?

prescaler = (int)(F_CPU / (USART_baudrate * 16) - 1)
actual_baudrate = F_CPU / (prescaler * 16)
baud_error = 100% * (1 - actual_baudrate / USART_baudrate)

• It works for 6V battery but not for 12 volt battery or a 12V adapter.

This sounds fishy! This may be a completely different problem, perhaps causing your widget to reset due to a dirty supply or poor regulation. There are stability issues with LDO linear regulators and varying capacitive loads; there may be dissipation issues from a 12V supply; if one isn't using a LDO regulator, then a 6V battery will really be giving you 4.5V to 5V, and won't be well regulated. Power supply issues are in a class all their own, and must be rectified before peripherals can be effectively debugged.

Other than this, there is always the issue with environmental interference, or EMI. RS-485 is made for this, but should be used with twisted pair[, shielded wire,] like the CAT-5 cable you've used.

**(What, my math is wrong? Do you know who I am!? ;)*

You are correct. Multimaster must be handled by you. i2c Multi-Master description. The Main thing for a proper multimaster system is that it understands arbitration logic, and can tell when the system is busy. The first is more difficult than the latter. To tell when the system is busy, each multi-master must be able to check for a start condition, and wait for a stop condition before trying to use the bus. Arbitration logic is for situations where both masters might try to take over the bus at the same time.

But there are other ways to do it, outside of the i2c standard. You can use an extra pin each of the masters, and make a simple busy signal standard.

Check if Busy pin is On.

If Off, set a status variable to On and Busy pin to On, then talk to the rtc. When done, set variable and pin to Off.

If On, check if status variable is On (Did "I" set status variable to On?). If no, loop until busy pin is Off.