SENSOR APPLICATIONS IN THE MICROAMPERE RANGE

ENERGY MICRO develops new type of sensor interface to allow the implementation of particularly energy-friendly sensor applications in deep sleep mode.

ENERGY MICRO has developed a new type of sensor interface for its Tiny and Giant Gecko ultra-low power controller families which enables the autonomous monitoring of up to 16 sensors in deep sleep mode. Both active and passive analogue sensors (such as capacitive, inductive and resistive sensors)) can be included in the applications. The name LESENSE (low energy sensor interface) is based on the feature that the monitoring and measurement of the sensors takes place on a fully autonomous basis, in other words without the intervention of the Cortex-M3. This allows particularly energy-friendly solutions to be created since the aim of the sensor measurement can be completely outsourced to the LESENSE module whose average current consumption of a few µA is unrivalled. The PRS (Peripheral Reflex System) which is already established in the Gecko family also allows the integration of other peripherals such as PCNT (Pulse Counter), RTC (Real Time Counter), LETIMER (Low Energy Timer) and GPIOs. The PRS in this case acts as a wired network which allows the exchange of trigger signals between the various peripherals, even if they are in deep sleep mode. This means that even complex measurements are possible which suppress the use of the Cortex-M3 and the change to run mode which this involves, even for lengthy periods of time.

The user has two ACMP (analogue comparators) available to measure the sensors, each of which has eight analogue inputs connected upstream of them. Several sources are available to act as reference voltage, such as two 12-bit DACs, an internal LDO (1.25V and 2.5V), a “finely” scalable supply voltage or an external reference. The measurement results from the ACMPs are sent to the LESENSE module which provides the actual intelligence for the allocation and evaluation of the results and for the entire process controller. The configurable sequencer defines in which order and duration the sensors are measured and evaluated. Passive sensors in particular require excitation before a measurement can be started. The sequencer provides various stimulus scenarios for this purpose which can be output at the measurement pin or alternatively at a different pin. The integral “count & compare” function allows pulses generated at the output of the ACMPs during a measurement cycle to be counted. The result can be compared to a reference value or saved in a buffer for later evaluation. The decoder has an integral and configurable FSM (finite state machine) with 16 states which provides additional intelligence for more complex measurement scenarios such as those required for a quadrature decoder. The function of LESENSE is described in the following on the basis of three examples of capacitive, inductive and resistive sensors.

Example: capacitive sensors
Capacitive sensors are based on a change of the capacity of a capacitor. The possible applications are very wide-ranging and include differential sensors (distance or angle), pressure sensors, level sensors or proximity and contact switches. In this example the function of LESENSE is demonstrated by the implementation of a capacitive contact switch and shown in simplified form in a block diagram. 
Depending on the output value from the ACMP (1 or 0), VDD /4 or VDD_SCALED is activated as the reference voltage at the inverting input and a capacitor is discharged or charged by the inverter. If the output value from the ACMP is 1 (inverter output = 0), the voltage at the inverting input is VDD /4 and the capacitor is discharged by the inverter until the falling voltage is less than VDD /4. This means that the ACMP output changes to 0 (inverter output = 1) which means that the reference voltage at the inverting input switches to a higher potential VDD_SCALED and at the same time a charge cycle is initiated at the capacitor. Finally the ACMP changes its output value back to 1 and the discharge and charge cycle of the capacitor repeats itself again.
The oscillation frequency caused by this depends, among other things, on the selected capacity of the capacitors. The higher the capacity the longer the discharge and charge cycle lasts and the lower the frequency becomes. Contact with a capacitive object such as a human finger increases the capacity which reduces the oscillation frequency. 
 
To calculate the frequency the integral LESENSE counter counts the number of 1s at the output of the ACMPs within a period of time specified by the LESENSE timer. The sequencer determines the order and the parameterization of each individual measurement. When the measurement at a pin has been completed, the system automatically moves to the next pin. The user can choose which event should take place in the event of detection. In the simplest case a wake up is initiated which ENERGY MICRO calls a “Wake on Touch” in this respect. The average current consumption for a measurement repetition frequency of 20Hz (20 measurements per second) is just 1.4µA, or just 1.9µA for a frequency of 100Hz.

Example: inductive sensors
Inductive sensors are based on the attenuation or frequency change of an oscillating circuit or a coil. This measurement principle generally permits non-contact and therefore wear-free angle, distance and speed measurements. The following example describes the calculation of the attenuation of an LC oscillating circuit by LESENSE. 

Within a short excitation time the capacitor of an LC oscillating circuit is precharged by the DAC. During this excitation phase the ACMPn_CHx pin is connected to earth by the sequencer. In the subsequent measurement phase the ACMPn_CHx pin is switched to tristate and DAC0_OUTx is connected to earth and the oscillating circuit starts to oscillate at its resonance frequency of

The parasitic ohmic losses of the components result in the oscillating circuit losing energy, however, and the oscillation comes to an end. With the integration of the ACMP it is possible to count the peaks above a defined threshold voltage. For this purpose the signal at ACMPn_CHx is connected to the non-inverting input of an ACMP whilst the reference voltage (threshold voltage) is provided at the inverting input by the DAC. This means that each peak above the threshold voltage generates a 1 at the output of the ACMP which can be counted by the LESENSE counter.
If there is a metallic object in the vicinity of the coil energy is transferred by the magnetic field to the object in the form of eddy currents which amplifies the decay characteristic of the oscillation. LESENSE measures the number of pulses within a period specified by the LESENSE timer and can therefore draw conclusions about the presence of a metallic object. At a measurement repetition frequency of 20Hz this process on average requires a current consumption of 1.2µA whilst at 100Hz the requirement is just 1.9Hz.
The use of more than one LC oscillating circuit means that it is possible to implement more complex measurement processes such as to record the direction and speed of rotational bodies in a flow measurement. The rotation is then tracked in this case by the integral FSM. 

Example: resistive sensors
Resistive sensors use the effect of the change in resistance caused by the deformation, elongation, irradiation and temperature change of materials. Typical applications include light, pressure, elongation, acceleration, force and temperature measurements. Like inductive and capacitive sensors, there is a whole host of possible implementation methods for LESENSE. One possible method is illustrated in the following by the measurement of a light sensor on the basis of a phototransistor. 

Throughout the entire measurement the light sensor is supplied with a voltage by LES_ALTEX0. The phototransistor acts as a voltage splitter which changes its resistance value depending on the exposure and therefore affects the voltage drop through the second resistor. This voltage drop is measured by ACMP0_CH4 at the input of the ACMP against a reference voltage which can be varied over the course of a measurement period. This allows the user to determine whether the light intensity is above or below a certain value. Depending on the measurement repetition frequency the average current consumption is 1.2µA at 20Hz and 2.3µA at 100Hz.

All the examples described above and many others can be tested using the Tiny Starter Kit. The Starter Kit contains all the peripherals required for this purpose such as a light sensor, a capacitive touch slider and an LC oscillating circuit. The source code and application notes are available at:

http://www.energymicro.com/downloads/application-notes

The launch of this new sensor interface means that ENERGY MICRO has taken an important step forwards in its efforts to reduce current consumption in sensor applications. This is made possible by the autonomous peripheral functions which require just a few µA in deep sleep mode without the use of the Cortex-M3.

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