Letting Your Remote Sensor Have A Really Deep Sleep

If you can identify with the feeling of “sleeping with one eye on the clock”, you will know how it prevents you from getting that deep sleep needed to maintain your energy levels, leaving you feeling drained for most of the following day. Key to achieving that all-important deep sleep state is emptying your mind of all thoughts and worries. Achieving the deepest possible sleep state, in which the minimum of functions remain active, is also an imperative for energy conservation in battery powered electronic devices. For example, in smart agriculture, battery powered sensors are now commonly placed in remote geographical locations to monitor environmental variables such as temperature, ambient light levels or soil moisture (Figure 1).

In vast industrial complexes (e.g. pharmaceutical plants), battery powered sensors are now used at remote site locations to periodically measure humidity, temperature etc. for regulatory purposes. In these and many other scenarios, regular battery changing is either undesirable or unfeasible. Therefore maximizing battery life requires that these sensors spend the majority of their life ‘sleeping’ i.e. in a low-power standby mode of operation, only waking periodically to perform a measurement which is then either recorded on flash storage (for later reading) or transmitted on a wireless network link (if available) before returning to sleep mode until the next reading is required. In this article we review the importance of timekeeping for an electronic sensor and show how using a microcontroller to perform this function leads to unnecessary sleep mode current consumption. We then show a way to remove the timekeeping task from the microcontroller that helps reduces sleep mode power consumption while also bringing several additional benefits.

Real Time Clock

Electronic devices perform tasks in response to the periodic oscillation of an electrically varying digital voltage signal. Humans, on the other hand, measure time in seconds, minutes, hours, days, months and years (real-time). For a sensor to make a reading at a pre-set time, it must store a digital representation of real-time, which is then kept in sync by a digital clock signal. This timekeeping task is performed by an RTC circuit. For reasons of convenience, some sensors use an RTC which is already integrated within the microcontroller. However, a drawback of this approach is that it places a de-facto lower limit on how much current is drawn by the microcontroller, because even when in sleep mode, it must continue to maintain timing information. A microcontroller can draw up to 600nA of current just to keep its timing circuits active in sleep mode. Clearly, additional power savings could be achieved by delegating the timekeeping function away from the microcontroller.

Thinking Outside the Box

One way of doing this to use the discrete approach shown in Figure 2. where the timekeeping task is performed by an external RTC IC which interfaces to the micro via an I²C interface.

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Figure 2. Using an External RTC IC for Timekeeping

The primary advantage of this approach is that it allows the microcontroller to be almost completely powered down when a sensor is in sleep mode. This RTC IC typically uses only 150nA of timekeeping current, representing up to 4x power saving when compared to using a microcontroller with an integrated RTC. While delivering on power savings, it is important to consider the impact of an extra IC on system board space. Housed in an ultra-tiny, 8-pin WLP that measures only 1mm x 2mm (with 0.5mm pitch), this IC overcomes those concerns, as it can be easily located on even the most space-constrained boards. Unlike alternative discrete solutions which use an integrated crystal, this RTC uses an external crystal to generate a clock signal. This allows user selection from a range of crystals with a 6pF load capacitance and an equivalent series resistance up to 100kΩ, thereby minimizing current consumption. On space constrained boards the shape and size of the crystal can be limiting, and this extra degree of freedom is an important benefit. This IC has integrated capacitive loading to ensure greater timekeeping accuracy than other solutions that use lower-tolerance external capacitance. Operating over a wide supply voltage (1.6V to 3.6V), it also includes two time-of-day alarms and two interrupt outputs.

Summary

To maximize battery life remote sensors must spend the majority of their time in sleep mode consuming as little power as possible. In this article we demonstrated how a sensor that uses its microcontroller to perform timekeeping can waste power in sleep mode. We then showed that by using a tiny ultra-low power external RTC IC to maintain real time information, sleep mode power consumption is reduced while timing accuracy is improved. Apart from remote and portable measurement sensors, this RTC is suitable for use in a wide range of applications such as medical wearables, point-of-sale terminals, telematics, portable audio and power meters.

Maxim Integrated