The PCF2123 is an ultra-low power RTC with SPI interface. The device consumes as low as 100 nA, typically, at 2 V supply voltage. In addition, the device has an electronic tuning register which can be used to increase the time-keeping accuracy in a system. The electronic tuning register is used to compensate for aging of the quartz crystal, temperature drift, and initial accuracy tuning. The correction is made by adding pulses to, or subtracting pulses from, an internal 64 Hz clock. The correction is made once every two hours in the normal mode and once every hour in the course mode.