Q&A: Does AM0805 register access via the I2C interface still work if the XT oscillator is not mounted?

Full question(s):

Is I2C communication possible in a system that does not have the XT oscillator?  During I2C access from the host to the RTC, the ACK signal does not respond.  The internal timer circuit does not operate because the crystal oscillator is selected (OSEL: 0) immediately after reset.

1.) If the XT oscillator is not mounted, does register access via the I2C interface work?

2.) Even though high accuracy is not necessary, is the XT oscillator essential for RTC?

3.). If RTC operation is possible without the XT oscillator and the XI and XO pins are left open, is it a problem?

4.) Please indicate if there is a method or a procedure to operate RTC with RC oscillator only.

Answer(s):

1.) Yes, there can still be register access over I2C without an XT oscillator.

2.) AM0805 can be operated without a crystal, but accuracy will be low (min 89Hz, max 220Hz). The largest variation occurs at high temp (> 60C). The most common usage of the RTC is "RC oscillator and Autocalibration" which operates from RC, but uses a crystal to auto-calibrate the RC. This mode provides extremely low power combined with high accuracy. Only reason to leave off crystal would be cost/size.

3.) Yes, XO and XI can be left unconnected if no crystal is used.

4.) Set OSEL bit to 1 to select RC oscillator mode (default is "0"), and set ACAL to 0 to disable auto-calibration (which is default). Power consumption specs for RC oscillator mode are included in the data sheet.