3 Reasons to Use a Discrete Real-Time Clock in Your Wearable Design
August 01, 2019
| By: Binay Bajaj
Director; Core Products Group; Maxim Integrated
Sure, a real-time clock (RTC) is considered a commodity product. But not all RTCs are made the same. In fact, you don’t even have to use a discrete RTC in your design. After all, microcontrollers have the function built in. But I’d like to argue that using a discrete RTC can actually be more beneficial in terms of your overall solution size and power management efficiency, especially if you are developing space-constrained applications like wearables.
Here are three reasons why designing with a discrete RTC can be beneficial:
- Keep track of time while saving power. Microcontrollers with integrated RTCs do go into sleep and low-power modes, yet the clock needs to keep running in order to provide accurate time-keeping and alarm functions. Keeping the RTC on results in leakage current from the microcontroller, which does waste some energy. An external RTC allows you to keep the clock running while the microcontroller is powered down.
- Save on other external components when using a small RTC with integrated functionality. By choosing a highly integrated RTC in a small form factor, you can avoid having to design in specialized components for those functions. For example, some external RTCs have a built-in power management feature that triggers the RTC to wake up the microcontroller when the feature senses a change in the analog or digital inputs. Another useful built-in feature is battery backup, which kicks in when the battery voltage drops below a certain threshold.
- Extend battery life when using an RTC with ultra-low quiescent current. By keeping the quiescent current in, say, nanoampere levels, you can get always-on time-keeping without draining the battery, a particularly important benefit for designs powered by small coin-cell batteries.
RTCs provide clocking capabilities to a variety of applications, such as wearables. The right discrete RTC can deliver power management and solution size benefits to compact designs like smartwatches.
Accurate Time-Keeping Without Draining the Battery
Wearables are just one example of an application that can benefit from a discrete RTC. Medical devices are another area. For example, insulin pens for diabetics are powered by coin-cell batteries. If the microcontroller in these pens must stay on all the time for the built-in RTC to run, the battery would drain very quickly. The same goes for power relays in a power grid—if the power goes off, you’ll still want the RTC to keep running, and an RTC with a built-in battery backup can provide that assurance. So there’s really a wide variety of applications that can benefit from a discrete RTC.
Maxim’s new MAX31341B ultra-small nanoPower RTC is an example of a highly integrated time-keeping solution that demonstrates the advantages of using a discrete RTC. The IC operates at less than 180nA of timekeeping current and is available in a 2mm x 1.5mm 12-pin WLP (35% smaller than the smallest RTC alternatives currently available). For accuracy and flexibility, the MAX31341B features external clock synchronization with an external crystal of your choice.
The part’s battery backup feature allows you to program the part so that if the battery voltage drops below, say 2V, it would switch to the backup battery with a built-in trickle charger. Having this function is useful wherever accurate timing is needed, such as in set-top boxes or even AM/FM radios. It also has a power management feature where an external switch can be hooked up to interrupt output of the RTC. If the RTC senses a change in either the analog input versus an established programmable threshold or the digital Schmitt Trigger input, the IC can be programmed to trip the switch and wake up the microcontroller. These built-in functions eliminate the need to use external circuitry, simplifying the design and reducing the bill of materials (BOM).
Let’s do a simple calculation to see (at least hypothetically) how the MAX31341B supports long battery life. Consider a typical 3V CR1216 Lithium coin-cell battery with a rating of 34mAh and a self-discharge rate of 1% a year (to drop from 3V down to 2V). If the battery was sitting on a shelf and the device it powers was unplugged, this battery would presumably have 2V remaining after 100 years. Given the MAX31341B time-keeping current at 3V of under 180nA, the battery would still last 26 years before dropping to 2V. Imagine, timekeeping for 26 years! This, of course, does not include powering the circuitry required to display the time, but does account for its use as an internal clock.
To assess the MAX31341B for your next application, buy the MAX31341EVKIT evaluation kit. The kit operates from a single supply and its onboard crystal provides a 32.768kHz clock signal.