The DS1603 is a real time clock/elapsed time counter designed to count seconds when VCC power is applied and continually count seconds under battery backup power with an additional counter regardless of the condition of VCC. The continuous counter can be used to derive time of day, week, month, and year by using a software algorithm. The VCC powered counter will automatically record the amount of time that VCC power is applied. This function is particularly useful in determining the operational time of equipment in which the DS1603 is used. Alternatively, this counter can also be used under software control to record real time events. Communication to and from the DS1603 takes place via a 3wire serial port. A 1-byte protocol selects read/ write functions, counter clear functions and oscillator trim. The device contains a 32.768 kHz crystal which will keep track of time to within 2 min/mo. An internal lithium energy source contains enough energy to power the continuous seconds counter for over 10 years.