RTCs detect the exact time

From GPS receivers through portable multimedia players to implanted blood sugar monitors, electronic devices require integrated time functions. Miniature RTCs supply the exact time yet require minimal power.

Independent of the Internet and battery technology, regular PC users have long been used to seeing the exact time displayed as soon as they switch on their computers. The reason this works without an external power supply is down to built-in real-time clocks (RTCs). Real-time clocks are hardware clocks that keep track of the time even when a device is switched off.

The hardware clock is a chip, which like a quartz clock basically comprises a counter and a quartz crystal as the clock-pulse generator (32 kHz). The counter is increased on every clock pulse of the frequency generator. An energy store is integrated in the device so that the clock does not stop when the device is switched off. In modern PCs this is usually a Lithium battery (3 V) on the main board.

By contrast there are also separate real-time clocks that have a very low power requirement and utilise the main power supply of the device in which they are integrated. These are often found in applications for portable devices and security systems, which require the time to be recorded for specific tasks. In electronic access systems with token solutions for buildings or IT systems it is possible to record when somebody enters a building or accesses specific applications, for instance. In addition, real-time clocks are also used in building automation solutions with wireless sensors which control the lighting or temperature.

Many wireless sensor nodes are often in sleep mode most of the time. The only subsystem that is awake is the real-time clock. As soon as the sensor node needs to measure a sensor input the real-time clock wakes it up. When it comes to energy balance, it makes sense to adopt this method even for GPS applications, where the processor is woken every second to detect a new satellite position.

If a usable power supply is available, real-time clocks can to some extent handle similar tasks as a software solution. After all, a precise time base can also be programmed with the main quartz crystal of the microcontroller. However, when it comes to extreme precision and the lowest possible power consumption, separate chip-based hardware solutions are advantageous. They require less power than the real-time function of a processor. They are therefore also of interest for other high-tech applications such as smart meter and energy harvesting systems.

Current real-time clock ICs have dimensions in the millimetre range and operate in a voltage range of a few volts. Here current consumption is in the nano to microamp range. The quartz crystals are optimally tailored to the RTC functions thus ensuring a high level of precision. Additional functions such as applying timestamps on power-down events, detecting insufficient battery capacity or switching to a backup battery are already being implemented in high-quality RTC ICs. Additional components are therefore not required.

State-of-the-art RTCs simplify the design of portable devices and increase the performance capabilities of systems whose functions rely on data being recorded, timestamps being added, intervals being monitored or synchronisation. In future these applications, which currently include digital cameras, GPS receivers, portable multimedia players, card readers, and portable medical monitoring equipment such as blood sugar monitors, will account for ever-increasing market volumes.