The CDP1879 real-time clock supplies time and calendar information from seconds to months in BCD format. It consists of 5 separately addressable and programmable counters that divide down an oscillator input. The clock input can have any one of 4 possible frequencies, allowing flexibility in the choice of crystal or external clock sources. Using an external 32kHz clock source, timekeeping can be performed down to 2.5V (see Standby (Timekeeping) Voltage Operation). The device can be memory-mapped for use with any general-purpose microprocessor and has the additional capability of operating in the CDP1800 series input/output mode. The real-time clock functions as a time-of-day/calendar with an alarm capability that can be set for combinations of seconds, minutes or hours. Alarm time is configured by loading alarm latches that activate an interrupt output through a comparator when the counter and alarm latch values are equal.