This knowledge base article demonstrates low power operation of the EFM32TG11 using LESENSE to measure movement (rotations and direction) of a half metal disc, similar to what would be used in a water or gas meter. This article includes a software project, and schematic and layout files for an expansion board that connects to the EFM32TG11 STK (SLSTK3301A).
For more information on LESENSE and a software example for Series 0 devices, see AN0029: Low Energy Sensor Interface - Inductive Sense.

Hardware Design

This example uses the SLSTK3301A starter kit for the EFM32TG11, and a custom PCB that connects to the expansion header.  The board has two LC tanks, like the one on the STK, and a 3" half disc mounted to the board with RC car bearings and axles (Traxxas 5116 and Traxxas 6853X). There are also footprints to place two hall effect sensors (Si7204 or Si7210) under the disc, although theseare not implemented in this design.

The two tanks on the expansion board are driven by the main output of DAC0 CH0. These two tanks, and the one on the board share the excite pin (PB11). All tanks can be sampled in this configuration. To use PB11 as LESENSE_EXCITE, a wire needs to be connected to the lower half of R198, to a 100 ohm resistor. The other end of the resistor should be connected to W14. A 0.1 uF capacitor should also be connected from W14 to ground.

A third tank circuit is present on the STK. While it is not used to detect disc rotation, some tamper detection methods use a third tank, so the STK tank is measured to demonstrate that capability. When metal is detected next to this tank, a gecko symbol is drawn on the LCD screen.

The most power efficient settings are to have the DAC triggered by LESENSE. This way the DAC is only enabled when needed. Regular or alternate DAC outputs can be used. Routing the DAC signal through APORT is not recommended as the APORT has higher output impedance.

In order to reduce interference between the tanks, LESENSE pins that are not being scanned can be set to the DAC voltage. Since the tank is connected to the DAC on one side, setting the other side to the DAC voltage will result in no voltage drop across the tank. This reduces coupling between the tanks. This is accomplished by setting the channel idle mode to lesenseChPinIdleDACC. Note that this only works for pins that are an alternate output of the DAC channel being used. For DAC channel 0, this is PC0, PC1, PC2, and PC3. For DAC channel 1, this is PC12, PC13, PC14, and PC15.

Software Design

Once initialized, the main tasks of the application are handled by LESENSE without software execution. LESENSE can run in EM2 while clocked from the LFXO. LESENSE implements quadrature decoding with the decoder state machine. The decoder uses PRS to send signals to PCNT, which maintains the count of rotations. The main loop handles updating the LCD display (triggered from a periodic cryotimer interrupt), and detecting button presses to cycle through display modes.

The LESENSE module uses a sliding window to evaluate the output of the ACMP counter and convert the count into a binary result. This helps compensate for variations in the tank circuit (caused by variances in component values, for example) and removes the requirement that the tanks not be near metal when the device is calibrated.

Current Consumption

When scanning at 16 Hz and sampling 3 tanks, with the LCD off, the average current consumption is 3.5 uA. Enabling the LCD adds about 2 uA of current consumption to power the LCD as well as to do the calculations to generate the strings to display (sprintf).

In this configuration, the marginal consumption of an additional tank is about 100 nA. In cases where a tank needs to be sampled very infrequently, it may be feasible to have a periodic interrupt enable a channel and enable the decoder interrupt. During the decoder interrupt, the tank result can be measured, and the channel and decoder interrupt can be disabled until next time.

Tamper Detection

Only two tanks are required for quadrature decoding of a disc. Typically, half of the disc is made of metal and the tanks are placed 90 degrees apart. With only 90 degrees of the disc's rotation is monitored by the tanks, there is a period where the disc is not detected by either coil. Removing the disc entirely would be indistinguishable from the disc stopping in this position. This would allow unbilled use of the meter. To prevent this, a third tank can be used so that at least one tank is covered at any time. The third tank does not need to be
sampled as often as the first two. See the performance section for tradeoffs in sampling rate. It is also possible to implement a state machine that monitors more than two coils and has a state for tampering detected.

Debugging

Because most of the work this application does is handled by the LESENSE state machine, traditional debugging with breakpoints and stepping is not very effective. Components can be tested one stage at a time (LESENSE counts, scan results, decoder state, PCNT) by reading the registers, or directing the output to an IO pin (for example, PRS signals from the DECODER can be measured at IO pins to verify the DECODER is operating correctly, then the PRS signals can be sent to the PCNT module).

An oscilloscope is useful to monitor the tank oscillations, DAC output voltage, and timing of the excite signals. Note that the capacitance of the scope probe will have an influence on the circuit and may prevent it from operating normally. If a scope probe has multiple gain modes, the capacitance is typically less in the higher gain mode (this can be compensated for by the oscilloscope).

Putting the DAC into a lower power mode where it is driven infrequently can cause it to be more susceptible to noise or interference from scope probes or from pins being touched by a person. When diagnosing issues, the DAC can temporarily be set to continuous conversion mode to prevent these issues. Continuous conversion mode will have much higher current consumption.

Conclusion

This application was developed on an EFM32TG11 (Series 1). Porting to other Series 1 devices that have VDAC and LESENSE is straightforward. The firmware uses emlib for most of the configuration, ensure the availability of DAC alternate output on LESENSE pins.

The DAC peripheral has changed significantly since Series 0. For an example of LC sensing implemented on Series 0 see AN0029.  There are currently no Series 2 devices with LESENSE.

See AN0029 for a more detailed explanation of LESENSE and a software example for Series 0. See the software example SLSTK3301A_helges_demo for another software example that uses LESENSE on an EFM32TG11 STK to detect metal near the onboard tank of the STK.