MMi203: Real Time Clock

The Real Time Clock (RTC) is an option offered on certain versions of the MMi203. It has the following features:

• Time of day (24 hour clock) and date functions.
• 16 presettable/programmable events for triggering program actions. These are programmed as events 0 to 15.
• A super-capacitor based backup power source that will keep the clock running for up to 2 days on loss of main power. Unlike batteries the super-capacitor will never wear out.
• A connection point for a user-supplied battery, in case you need more than 2 days' backup.

Connecting an external backup battery

If you need more than the 2 days' backup power provided for the RTC by the onboard super-capacitor, you can provide your own battery. Any battery voltage between 3V and 5V will work. Do not exceed 5.5V. We suggest a 3V lithium or two alkaline AA cells. Do not use rechargeable batteries as the circuit will not charge them. The battery should be connected to the small 2-pin connector near the Comms connector. Make sure you get the polarity right. The RTC chip draws less than 2uA in backup mode, so any battery will last a very long time.

Memory wear-out

The memory used to store RTC events is limited to 100,000 writes per event. After that it may become unreliable. So make sure you don't write a program that does very many RTCWriteEvent instructions.

RTC programming

Full details on programming the RTC are in the RTC documentation.