A
programmable thermostat for controlling
space conditioning equipment receives power from the equipment. The
thermostat has an interactive interface for user
programming, a temperature sensor and a processor including: a
central processing unit; a memory for storing program and
data information, a
real time clock and a
non volatile random access memory. A control program stored in the memory causes the
real time clock to periodically read its
current time and date information into the non-volatile RAM. Upon restart after an outage, the time and date information stored in the non-volatile RAM is read into the
real time clock to provide an acceptable time and date basis for resuming operation. Accordingly, the need for providing a
backup power source, such as a battery or a supercap, is eliminated. In one exemplary embodiment, the
thermostat also includes a
receiver for accessing precision time and date signals from, for example, WWVB. The output of the
receiver is used to periodically update the time and date in the real time
clock such that, even after an outage, the real time
clock will be accurate. If the
receiver is not operating for some reason after an outage, then the real time
clock will still be set from the non-volatile RAM.