The ISL12058 is a low power real time clock with alarm functions. The oscillator uses an external 32.768kHz crystal, and separate registers for tracking hours, minutes, and seconds. The ISL12058 also has a calendar with registers for date, month, year and day of the week. The calendar functions are accurate through 2099 and will automatically correct for leap years. Like the SHT21, the ISL12058 utilizes the I2C interface and has a 400kHz data transfer rate. With a supply voltage range of 1.4 to 3.6 volts, the ISL12058 fits nicely into our 3.3 volt power scheme. Couple that with the fact that it is only 2x2 mm and it easily fits into our size constraints.